401(k) Plans: The Complete Guide (2026)

What a 401(k) Actually Is

A 401(k) is an employer-sponsored retirement plan that lets you divert part of your paycheck into a tax-advantaged account before the money ever reaches your bank. The name comes from the section of the tax code that authorizes it. In IRS language the arrangement is a cash or deferred arrangement, or CODA, and the money you send into it is called an elective deferral.

Two things separate a 401(k) from an IRA, and almost every rule difference traces back to one of them.

First, your employer owns the plan. You do not open a 401(k) the way you open an IRA. Your employer adopts a plan document, hires a recordkeeper, and decides what the plan will and will not allow. The tax code sets outer limits. Your plan document sets the actual rules you live under, and it is often more restrictive. Loans, hardship withdrawals, Roth contributions, after-tax contributions, and in-service distributions are all optional features. If your plan does not offer one, the fact that the IRS permits it does not help you.

Second, the contribution limits are far higher. For 2026 you can defer $24,500 of your own pay, and total contributions from every source can reach the lesser of your compensation or $72,000. A Traditional IRA caps out at $7,500. That gap is the entire reason the 401(k) is the primary retirement vehicle for most working Americans.

A 401(k) is a defined contribution plan. What you get at the end is whatever your account is worth, not a promised monthly benefit. If you want the promised-benefit version, that is a pension or a cash balance plan.

If you are self-employed with no employees, the plan you want is a Solo 401(k). It is the same section of the code with most of the compliance stripped out. Nearly everything on this page applies to it, with the testing and coverage rules removed.

2026 Contribution Limits

Every figure below comes from IRS Notice 2025-67, which set the 2026 limits.

The $24,500 limit is yours, not your plan's. It applies across every 401(k) and 403(b) you participate in during the year. Change jobs in June and you get one $24,500 limit for the year, not two. Nobody catches this for you. Your old employer does not tell your new employer what you already deferred, and neither recordkeeper can see the other. Excess deferrals are yours to find and yours to fix, and the deadline to pull them out is April 15 of the following year.

The $72,000 limit is per employer, not per person. If you genuinely work for two unrelated employers, each plan gets its own annual additions limit. The $24,500 deferral limit still applies once across both.

The $72,000 is a ceiling, not a floor. Section 415(c) caps annual additions at the lesser of 100 percent of your compensation or $72,000. If you earn $50,000, your ceiling is $50,000, and every calculation that assumes you can fill the gap up to $72,000 is wrong for you.

The 60 to 63 catch-up is not a bonus for turning 60. It applies in the calendar years you turn 60, 61, 62 and 63, and it stops in the year you turn 64. In that year you drop back to the ordinary $8,000. Nothing about turning 64 makes you less able to save. The statute simply ends the window.

For a year by year history of these numbers going back to 1978, see the historical contribution limits guide. For every 2026 limit across every account type, see the 2026 contribution limits page.

The 2026 Mandatory Roth Catch-Up Rule

This is the single biggest 401(k) change taking effect in 2026, and most people affected by it will find out from a payroll notice rather than from planning for it.

Section 603 of the SECURE 2.0 Act says that if your prior-year wages from the employer sponsoring your plan exceeded a set threshold, your catch-up contributions must be Roth. Not may be. Must be. The threshold for determining your 2026 catch-ups is $150,000 of 2025 FICA wages from that same employer.

Cross that line and the $8,000 or $11,250 you were deferring pre-tax becomes after-tax money. Your taxable income for 2026 goes up by that amount. Everything downstream of taxable income moves with it.

Several details matter more than the headline.

It is wages from that employer, not total income. The test looks at FICA wages reported by the plan sponsor for the prior year. Investment income, rental income, and wages from a different employer do not count toward the threshold.

Self-employment income is not FICA wages. A partner whose only compensation from the business is self-employment income has no FICA wages from that employer, and is therefore outside the rule no matter how large the number is. This is one of the few places where the tax code quietly favors the self-employed.

A new job resets you. The measurement is prior-year wages from the employer sponsoring the plan. Start with a new employer in 2026 and you had no 2025 wages from that employer, so you are not subject to the requirement in that plan for 2026.

Your plan is not required to add Roth. If your plan allows catch-up contributions but has no Roth option, the plan does not have to create one. What it can do instead is stop letting affected high earners make catch-up contributions at all. If you are over the threshold in a plan with no Roth feature, the practical result is that your catch-up disappears.

The transition relief is over. IRS Notice 2023-62 gave plans an administrative grace period, and that period ran only through taxable years beginning before January 1, 2026. Final regulations issued in September 2025 did not extend it. Those regulations formally apply to taxable years beginning after December 31, 2026, and for the 2026 gap year plans may implement the requirement using a reasonable, good faith interpretation of the statute. The statutory requirement itself is in force for 2026.

Traditional vs. Roth 401(k)

Traditional deferrals reduce your taxable income now and are taxed when you withdraw. Roth deferrals give you no deduction now and come out tax free in retirement if the distribution is qualified. The contribution limit is shared. You get $24,500 total across both, not $24,500 each.

The honest version of the comparison is that it turns on one question: is your marginal tax rate higher now or in retirement? If you knew that, the math would be simple. Nobody knows it, so people use rules of thumb, and the rules of thumb are where the mistakes live.

Three things are actually true regardless of your rate guess.

Roth 401(k) dollars have no income limit. Unlike a Roth IRA, there is no phase-out. A person earning $500,000 can make Roth 401(k) contributions directly, which is why the Roth 401(k) matters far more to high earners than the Roth IRA does.

Employer contributions are pre-tax unless the plan says otherwise and you elect otherwise. SECURE 2.0 lets plans offer Roth employer contributions, but the plan has to offer it and you have to elect it. If you do, that money is included in your income for the year it is contributed. Most plans still default employer money to pre-tax.

Roth 401(k) accounts no longer require lifetime distributions. Starting in 2024, designated Roth accounts inside a 401(k) are exempt from lifetime RMDs, matching how a Roth IRA has always worked. Before that change, the standard advice was to roll the Roth 401(k) to a Roth IRA before RMD age purely to escape distributions. That reason is gone.

If you want to model the tradeoff with your own numbers, the Roth vs. Traditional comparison tool runs it.

Employer Matching, Vesting, and the True-Up

The match is the reason to participate at all. A 50 percent match on the first 6 percent of pay is an immediate 50 percent return on that slice of your salary, which no investment reliably produces.

Vesting

Your own deferrals are always 100 percent vested from the first dollar. That is a plan qualification requirement, not a courtesy. Employer money is different. A plan may impose a vesting schedule on employer contributions, and the tax code caps how aggressive that schedule can be under section 411(a)(2)(B):

• Three-year cliff. Nothing vests until you complete three years of service, then 100 percent vests at once.
• Six-year graded. 20 percent at two years, then 20 more each year, reaching 100 percent at six.

Those are the most restrictive schedules permitted. A plan can be more generous, and many are. Safe harbor and SIMPLE 401(k) plans must vest employer contributions immediately. A qualified automatic contribution arrangement, or QACA, may require up to two years.

The front-loading trap

Most plans match per pay period. If you max your $24,500 by August because you increased your deferral rate, the plan has no deferrals to match in September through December, and you lose four months of match. Some plans have a true-up provision that reconciles this at year end. Many do not, and the ones that do not rarely advertise it.

Check your summary plan description for the word true-up before you accelerate deferrals. If there is no true-up, spread contributions across all pay periods instead of finishing early.

Automatic Enrollment and Escalation

Section 101 of the SECURE 2.0 Act added section 414A to the tax code, requiring most newly established 401(k) and 403(b) plans to enroll employees automatically. The requirement applies to plan years beginning after December 31, 2024.

Under an eligible automatic contribution arrangement, the plan sets a default deferral rate of at least 3 percent and no more than 10 percent, then increases it by 1 percentage point per year to a minimum of 10 percent and a maximum of 15 percent. You can opt out or pick your own rate at any time, and a permissible withdrawal window lets you pull back automatic contributions shortly after they start.

The exceptions matter as much as the rule. A plan established before December 29, 2022 is grandfathered. That grandfather is not permanent, though. It can be lost in a corporate transaction, since a pre-enactment plan that merges with a post-enactment plan generally does not keep the exemption unless the merger falls inside a section 410(b)(6)(C) transition period and the pre-enactment plan is the designated ongoing plan. Church plans, governmental plans, and SIMPLE 401(k) plans are also excepted, and the statute excepts employers that normally employ 10 or fewer employees and businesses that have existed for less than three years. A starter 401(k) deferral-only arrangement is not automatically excepted, contrary to a common assumption. It is subject to the mandate unless it independently qualifies for one of the statutory exceptions.

Eligibility and the Part-Time Rule

A plan can generally require you to be 21 and to complete a year of service before you may defer. What changed recently is the treatment of long-term part-time workers, who used to be excluded indefinitely by hours requirements they could never meet.

The original SECURE Act required plans to let in employees who completed at least 500 hours of service in each of three consecutive 12-month periods. SECURE 2.0 cut that to two consecutive 12-month periods of 500 or more hours, effective for plan years beginning after December 31, 2024.

Two limits on that right are worth knowing. Employers are not required to make matching or nonelective contributions for long-term part-time employees even while making them for everyone else, and employers may exclude those employees from coverage and nondiscrimination testing. So the rule buys you the ability to defer your own money. It does not buy you a match.

IRS Notice 2024-73 stated that final regulations for 401(k) long-term part-time employees will apply no earlier than plan years beginning on or after January 1, 2026.

After-Tax Contributions and the Mega Backdoor Roth

There is a third contribution type that most participants never hear about. Separate from pre-tax deferrals and Roth deferrals, some plans allow plain after-tax employee contributions. These are not Roth. They go in after tax, and their earnings are taxable when distributed.

What makes them interesting is where they sit in the limit structure. After-tax employee contributions are their own category of annual additions under section 415(c), distinct from elective deferrals. That means they are constrained by the $72,000 total, not by the $24,500 deferral limit.

The arithmetic works like this. Say you earn well above $72,000, defer the full $24,500, and your employer contributes $10,000. That is $34,500 of the $72,000 ceiling. In a plan that permits after-tax contributions, you could add up to $37,500 more. If your compensation is below $72,000, your own pay is the ceiling and the room is smaller than the headline number suggests.

The mega backdoor Roth is the move that converts those after-tax dollars into Roth dollars, either through an in-plan Roth rollover or by rolling them out to a Roth IRA. IRS Notice 2014-54 confirms the out-of-plan version: you can direct pre-tax amounts to a Traditional IRA and after-tax amounts to a Roth IRA in the same distribution, and distributions sent to multiple destinations at the same time are treated as one distribution for allocating pre-tax and after-tax money.

Three conditions have to hold, and all three are plan-level:

1. The plan permits after-tax employee contributions.
2. The plan permits in-service distributions or in-plan Roth rollovers of those contributions.
3. You move the money promptly, because earnings on after-tax dollars are pre-tax and get taxed on conversion.

Most plans fail at least one. And note the catch that makes this a high-earner problem: after-tax employee contributions are tested under the ACP test, so in a plan where mostly highly compensated employees use the feature, the contributions can be limited or refunded.

401(k) Loans

If your plan permits loans, you may borrow the lesser of $50,000 or 50 percent of your vested account balance. Where half the vested balance is under $10,000, a plan may allow borrowing up to $10,000.

There is a lookback that catches people who have borrowed before. A new loan, added to the outstanding balance of all your other plan loans, cannot exceed the plan maximum, and in applying that maximum the $50,000 is reduced by the difference between your highest outstanding loan balance during the 12 months ending the day before the new loan and your outstanding balance on the date of the new loan. Repay a $40,000 loan this month and you cannot turn around and borrow $50,000 next month.

Repayment is generally required within five years with payments at least quarterly. The five-year term does not apply to a loan used to buy your principal residence, which may run longer.

You pay interest to yourself, which sounds like a free lunch and is not. The money you borrowed is out of the market, and you repay with after-tax payroll dollars that will be taxed again when distributed in retirement.

The real risk is separation from service. Plan sponsors may require full repayment of the outstanding balance when you leave or when the plan terminates. If you cannot repay, the unpaid balance becomes a qualified plan loan offset, reported on Form 1099-R, taxable, and subject to the 10 percent additional tax if you are under 59 and a half. There is a rescue: you may roll over all or part of the offset amount to an IRA or eligible plan by the due date, including extensions, for filing that year's return.

A loan that misses required payments or fails the rules becomes a deemed distribution, which is taxable immediately. Repayments made after a deemed distribution become basis in the plan.

Hardship and Emergency Withdrawals

Hardship distributions are permitted only if the plan offers them, only for an immediate and heavy financial need, and only up to the amount necessary to satisfy it. The IRS safe harbor list of qualifying expenses covers six categories:

• Medical care expenses for you, your spouse, dependents, or beneficiary
• Costs directly related to buying a principal residence, excluding mortgage payments
• Tuition, related educational fees, and room and board for the next 12 months of postsecondary education
• Payments necessary to prevent eviction or foreclosure on your principal residence
• Funeral expenses
• Certain expenses to repair damage to your principal residence

Two things surprise people. A hardship distribution is taxable and is not itself an exception to the 10 percent penalty. The hardship gets you access to the money. It does not get you out of the tax. The one carve-out is that amounts consisting of designated Roth contributions are not taxed the same way. And a hardship distribution cannot be repaid or rolled over. Once it leaves, the contribution room is gone permanently.

The newer emergency options

SECURE 2.0 created several distributions that are penalty free, which hardship distributions are not. Each one is optional at the plan level, so your plan has to have adopted it.

• Emergency personal expense distribution. One per calendar year, up to the lesser of $1,000 or your vested balance over $1,000. Available for distributions after December 31, 2023.
• Domestic abuse victim distribution. Up to the lesser of $10,000, indexed, or 50 percent of the account. Available for distributions after December 31, 2023.
• Qualified disaster recovery distribution. Up to $22,000 per disaster, aggregated across all your plans and IRAs, for federally declared major disasters occurring on or after January 26, 2021. This one is per disaster, not per year.

A terminal illness distribution, certified by a physician as an illness reasonably expected to result in death within 84 months, is penalty free for distributions made after December 29, 2022.

New for 2026: section 334 of SECURE 2.0 created a qualified long-term care distribution, effective for distributions made after December 29, 2025. The 2026 limit is $2,600, calculated as the least of premiums paid, that dollar cap, or 10 percent of vested benefits. It applies to defined contribution plans including 401(k), 403(b) and 457(b) plans, and expressly does not apply to IRAs. It is optional, so your plan has to adopt it.

The Rule of 55 and Other Penalty Exceptions

The rule of 55 is the most valuable 401(k) feature that does not exist in an IRA, and the one most often destroyed by a well-meaning rollover.

If you separate from service during or after the calendar year you turn 55, distributions from that employer's plan are exempt from the 10 percent additional tax. For qualified public safety employees the age is 50, or 25 years of service under the plan if earlier.

The trap is in the words "that employer's plan." The exception applies to qualified plans and not to IRAs. Roll the balance into a Rollover IRA and the exception does not travel with it. A 56-year-old who separates, rolls to an IRA out of habit, and then needs money is back to the 10 percent penalty until 59 and a half.

If there is any chance you will need the money before 59 and a half, leave it in the plan.

Exceptions that apply to plans but not IRAs

• Separation from service at 55 or later, described above
• Payments under a qualified domestic relations order
• Corrective distributions of excess contributions
• Dividends passed through from an ESOP

Exceptions that apply to IRAs but not plans

• Qualified higher education expenses
• First-time home purchase, up to $10,000
• Health insurance premiums while unemployed

Exceptions that apply to both

• Reaching age 59 and a half
• Death or total and permanent disability
• Unreimbursed medical expenses above 7.5 percent of AGI
• IRS levy
• Substantially equal periodic payments
• Qualified military reservist called to active duty
• Qualified birth or adoption, up to $5,000 per child
• Rollover completed within 60 days

Claiming an exception is not automatic. You report it on Form 5329 unless the plan already coded the 1099-R correctly.

Required Minimum Distributions

The government let the money grow untaxed. Eventually it wants its cut, and required minimum distributions are how it takes it.

The applicable age is 73 for people born on or after January 1, 1951 and before January 1, 1959, and 75 for people born on or after January 1, 1960.

The still-working exception

This is the other 401(k) advantage that does not exist in an IRA. If you are still employed by the company sponsoring the plan, you may delay RMDs from that plan until the year you retire, provided the plan allows it and you are not a 5 percent owner of the business. Your required beginning date becomes April 1 of the year following the later of the year you reach the applicable age or the year you retire.

Three limits. It does not apply to IRAs, where you must begin regardless of employment. It does not apply to plans from former employers, only to the one you still work for. And the plan may require distributions anyway, since the exception is permissive rather than mandatory.

Timing and the two-distribution year

Your first RMD is due April 1 of the year after your required beginning year. Every RMD after that is due December 31. Using the April 1 grace period means taking two distributions in the same calendar year, which stacks both into one year of taxable income. That is often worse than taking the first one on time.

Roth 401(k) accounts

Designated Roth accounts inside a 401(k) are no longer subject to lifetime RMDs, effective for taxable years beginning after December 31, 2023. The old advice to roll a Roth 401(k) to a Roth IRA purely to escape distributions no longer applies.

Missing one

The excise tax on an amount not withdrawn is 25 percent, reduced to 10 percent if corrected within the two-year correction window. SECURE 2.0 cut this from 50 percent effective 2023. See RMD mistakes and fixes for the correction procedure, and the RMD planner to project the distributions themselves.

Rollovers and the 20 Percent Withholding Trap

When you leave a job you have four options: leave the money in the old plan, move it to the new employer's plan, roll it to an IRA, or cash out. The fourth is usually the worst and is chosen more often than it should be.

How you move it matters more than where you move it. A direct rollover, where the plan sends the money straight to the receiving account, has no withholding. An indirect rollover, where the check comes to you, triggers mandatory 20 percent federal withholding on the taxable portion, and it applies even if you tell the plan you intend to roll it over.

The consequence is arithmetic. Move $100,000 indirectly and you receive $80,000. To complete a full rollover within 60 days you must deposit $100,000, which means finding the missing $20,000 somewhere else. You get the withheld amount back as a credit when you file, but not until then. Whatever you fail to make up is treated as a taxable distribution and can carry the 10 percent additional tax.

Ask for a direct trustee-to-trustee transfer and this problem does not exist.

Two more things to weigh before rolling to an IRA. You give up the rule of 55, covered above. And a pre-tax IRA balance created by a rollover contaminates the pro-rata rule, which can make future backdoor Roth conversions mostly taxable. If backdoor Roth is part of your plan, rolling a large 401(k) into a Traditional IRA can be an expensive habit.

Net Unrealized Appreciation on Company Stock

If your 401(k) holds appreciated employer stock, rolling everything to an IRA can be the most expensive default choice available to you.

Net unrealized appreciation is the difference between what the plan paid for the shares and what they are worth when distributed. When employer securities are distributed as part of a lump-sum distribution, the NUA is generally not taxed until you sell the shares, at which point it is taxed as long-term capital gain regardless of holding period. Only the cost basis is income in the year of distribution. Deferral is the default treatment, and the affirmative election runs the other way: you may elect to include the NUA in income in the year the securities are distributed if you would rather pay now.

Roll the shares to an IRA instead and the entire value eventually comes out as ordinary income. On a position with a $20,000 basis and a $200,000 value, that is $180,000 taxed at ordinary rates rather than capital gain rates.

The election requires a lump-sum distribution: the entire balance from all of the employer's qualified plans of one kind, distributed within a single tax year, triggered by one of four events. Death of the participant. Reaching 59 and a half. Separation from service, for employees. Total and permanent disability, for a self-employed participant.

The plan reports NUA in box 6 of Form 1099-R. This is a one-shot, easy-to-blow election, and it is worth professional advice before you move the shares.

When Your Plan Fails Its Testing

Most participants never think about nondiscrimination testing until a refund shows up in February with no explanation.

The ADP test compares the elective deferrals of highly compensated employees against everyone else. The ACP test does the same for matching and after-tax contributions. An HCE for 2026 testing is generally someone who received more than $160,000 of compensation in 2025, or who owns more than 5 percent of the business.

When a plan fails, the usual correction is to refund excess contributions to the highly compensated employees. That refund is taxable to you in the year received, and you had no say in it. Two deadlines govern the employer. Distributing or recharacterizing the excess within two and a half months after the plan year ends, or six months for certain eligible automatic contribution arrangements, avoids a 10 percent excise tax on the employer. Missing that window triggers the excise tax. Missing the 12-month correction period is worse: the cash or deferred arrangement is no longer qualified and the entire plan can lose its tax-qualified status.

A plan is top-heavy when key employees hold more than 60 percent of plan assets. Key employees are officers above an indexed compensation threshold, more-than-5-percent owners, and more-than-1-percent owners earning over $150,000, a figure that is not indexed. A top-heavy plan generally must make a minimum contribution of 3 percent of compensation for non-key employees.

A safe harbor 401(k) buys an exemption from ADP and ACP testing by committing to one of three formulas: a basic match of 100 percent on the first 3 percent of pay plus 50 percent on the next 2 percent; an enhanced match at least as favorable at every deferral rate, with no match above 6 percent of pay; or a 3 percent nonelective contribution for every non-highly compensated employee. Traditional safe harbor contributions are fully vested when made.

If you are the employer rather than the participant, the small business retirement plans guide compares the 401(k) against the SEP IRA and SIMPLE IRA, and the plan selector narrows it down.

Common Mistakes

Deferring past the limit across two jobs

Nobody is tracking your combined 402(g) total but you. Excess deferrals not withdrawn by April 15 of the following year are taxed twice, once in the year deferred and again when distributed.

Front-loading contributions in a plan with no true-up

Hitting the limit in August in a per-pay-period matching plan forfeits four months of match. Check for a true-up first.

Rolling to an IRA at 55

The rule of 55 dies at the moment of the rollover. If you separated at 55 or later and might need the money, leave it in the plan.

Taking an indirect rollover

Twenty percent gets withheld, and you must replace it from other funds within 60 days to complete the rollover. Ask for a direct transfer instead.

Rolling out company stock without considering NUA

A one-shot election that converts ordinary income into long-term capital gain, gone the moment the shares land in an IRA.

Assuming your plan allows what the code allows

Loans, hardship distributions, after-tax contributions, in-service distributions, Roth deferrals, and the still-working RMD exception are all optional. Your summary plan description is the authority, not the tax code.

Cashing out a small balance at job change

A $15,000 cash-out at 35 in the 24 percent bracket costs roughly $5,100 in tax and penalty, plus decades of compounding on the rest.

Ignoring the 2026 Roth catch-up change

If your 2025 wages from your employer exceeded $150,000, your 2026 catch-up is Roth. The deduction you planned around is not there.

FAQ

Related Knowledge Blasts

Short, plain-English breakdowns of the rules behind this guide:

Catch-Up Contributions in 2025: Ages 50+ and the New 60-63 Rule →

The Rule of 55: The Most Misunderstood Early Withdrawal Exception →

Contribution Limits vs. Income Limits vs. Compensation Limits →

Mega Backdoor Roth Confusion →

What Actually Happens in an Indirect Rollover →

Rollover vs. Transfer: They Are Not the Same Thing →

The Pro Rata Rule in Roth and Traditional IRAs: Why It Ruins "Clean" Moves →

Required Minimum Distribution Penalty Relief →