How much does a 401(k) contribution actually cost out of pocket?

Less than the contribution amount itself, because pre-tax contributions escape your marginal tax bracket. At a 22% marginal rate, a $10,000 contribution reduces take-home pay by only about $7,800 — the difference (~$2,200) is the tax you would have otherwise owed on that income.

What's the 2025 401(k) employee contribution limit?

$23,500 for employees under 50, per IRS 2025 figures. A catch-up contribution is available for those 50 and older, though the exact catch-up mechanics can vary by plan and specific age bracket.

Why does a small contribution percentage increase matter so much?

Because the effect compounds on two timescales at once — this year's tax saving is real and immediate, but the same dollar amount also grows through the remaining working years before retirement. A bump from 10% to 12% on a $100,000 salary looks like a small paycheck change but compounds into a substantial retirement balance difference over a multi-decade career.

How is the out-of-pocket cost of a contribution calculated?

Out-of-pocket cost equals the contribution amount minus the tax saving it generates. The tax saving is calculated as the difference between tax owed on gross income minus the standard deduction, versus tax owed on gross income minus the contribution minus the standard deduction — walked through the actual 2025 progressive federal brackets.

A $10,000 401(k) Contribution at a 22% Marginal Rate Only Costs About $7,800 of Real Take-Home Pay

Most people evaluate a 401(k) contribution by asking one question: “how much can I afford to put away each month?” There’s a second question the IRS answers automatically in the background, and most people never see it explicitly: how much does that contribution actually cost after the tax benefit is accounted for?

The gap between sticker price and real cost

	Value
401(k) contribution	$10,000
Marginal tax rate	22%
Tax saved	~$2,200
Real out-of-pocket cost	~$7,800

A $10,000 pre-tax contribution doesn’t reduce take-home pay by $10,000 — it reduces it by roughly $7,800, because that contribution escapes taxation at the marginal rate it would otherwise have been taxed at. The government effectively co-pays the remaining $2,200 in the form of tax not collected this year. This is the mechanism behind pre-tax retirement contributions, but it’s easy to lose sight of when looking only at the number leaving a paycheck.

Two timescales, one decision

The tax saving described above happens this year — a real, immediate, and calculable reduction in current tax liability. But the same contributed dollar also exists on a second timescale: growing, invested, through however many years remain until retirement. A modest contribution-rate increase — say, from 10% to 12% of a $100,000 salary — represents only $2,000 more per year, a change that barely registers on a monthly paycheck. Projected forward through decades of compound growth before retirement, that same $2,000/year increase compounds into a substantially larger retirement balance difference — the kind of asymmetric tradeoff that’s easy to miss when evaluating a contribution decision purely by its immediate paycheck impact.

The 2025 limits that set the ceiling

The IRS 2025 employee deferral limit is $23,500 for those under 50, with catch-up provisions available for older savers. Employer matching, where offered, typically has its own separate percentage-based cap layered on top of the employee’s own contribution — meaning the true maximum benefit isn’t just about hitting the employee limit, but also capturing the full employer match available, since an unmatched employer contribution left on the table is effectively free money forfeited regardless of the employee’s own contribution strategy.

Why the two-halves-at-once framing matters

Evaluating a contribution decision by looking only at “what does this cost me this month” undersells the decision; evaluating it only by looking at “what will this be worth at retirement” makes the near-term cost feel abstract and easy to postpone. Seeing both numbers together — the real (tax-adjusted) out-of-pocket cost this year, and the projected future value of that same contribution by retirement — gives a more complete picture than either number alone, and tends to make the case for a modest contribution increase more concrete than either framing does in isolation.

Where this framework doesn’t apply

Roth 401(k) contributions work differently. This describes the tax mechanics of pre-tax (Traditional) 401(k) contributions specifically — Roth 401(k) contributions are made with after-tax dollars and don’t generate this immediate tax saving, though they offer tax-free growth and withdrawal instead.
State tax treatment varies. This calculation focuses on federal tax savings — many states also allow pre-tax 401(k) contributions to reduce state taxable income, which would increase the real tax saving beyond the federal-only figure shown here, while a few states have different rules.
You’re already maximizing the employer match and hitting the annual limit. Once the full employer match is captured and the annual contribution limit is reached, this specific “should I increase my contribution rate” framework no longer applies — the next decision becomes whether to direct additional savings elsewhere (IRA, HSA, taxable brokerage).
Your marginal rate will change significantly this year. A large one-time change in income (a bonus, a job change, a business sale) partway through the year changes the effective marginal rate the contribution saves at — this simplified model assumes a stable marginal rate across the full year.

What to actually do

Check your current contribution rate against your marginal tax bracket to see your actual real out-of-pocket cost, not just the sticker-price percentage.
Confirm you’re capturing your full employer match before considering any other retirement savings vehicle — an unmatched employer contribution is forfeited value.
Project a modest contribution-rate increase forward to retirement to see the compounded difference, not just the near-term paycheck impact.
Check the current year’s IRS contribution limit ($23,500 for under-50 in 2025) before assuming you have unlimited room to increase contributions.
Revisit your contribution rate whenever your salary or marginal tax bracket changes materially, since both the tax-saving and future-value sides of the calculation shift with those changes.

Open the 401(k) Tax Savings Simulator → and see both the real cost this year and the projected value at retirement for your own numbers.