401(k) Paycheck Impact Calculator (2026)

The Tax Arbitrage Effect: Why $1 Contributed Costs Less Than $1

When you contribute $1 to a traditional 401(k), your paycheck doesn't drop by $1 — it drops by roughly $1 minus your marginal tax rate. At a 22% federal bracket plus a 5% state rate, a $1 contribution only reduces take-home pay by about $0.73. That gap is an immediate, guaranteed "return" before any investment growth happens, and it's the single most misunderstood number on a pay stub.

Scenario 1: $85,000 salary, 6% contribution, 50% match up to 6%

The $3,978 given up from the paycheck becomes $7,650 in the account — a 92% immediate return, before any market performance.

Scenario 2: $130,000 salary, 10% contribution, 100% match up to 4%

Here the higher bracket and richer match combine: a $9,230 sacrifice turns into $18,200 saved — a 97% immediate multiplier.

Scenario 3: $55,000 salary, 3% contribution, no match

Without a match, the only lever left is the tax bracket — at 12% the arbitrage is modest, so cash-flow tradeoffs matter more here than in higher brackets.

Comparison: contribution rate vs real cost per dollar

Is it worth it? (Czy to się opłaca)

Whether contributing beyond the match is worth it depends on three variables working together, not any single one: your current marginal bracket, your expected bracket in retirement, and what else the money would otherwise do. Contributing up to the match is essentially never a bad decision — it's an instant, risk-free return no market investment can match. Beyond the match, the calculus shifts to a bracket comparison: contributing is favorable when today's marginal rate is meaningfully higher than the rate you expect to pay on withdrawals. It becomes less favorable when you're already in a low bracket, carry high-interest debt, or need the cash flow within the next few years.

A useful sanity check: take the "real cost per dollar" figure from the calculator above. If it's $0.73, you're giving up $73 of take-home pay to save $100 for retirement. Ask whether that trade beats your next-best use of $73 — paying down a 22% APR credit card almost always wins; building a 3-6 month emergency fund from zero usually wins; a routine budget squeeze on discretionary spending usually loses to the 401(k).

Interpreting your calculator result

The "real cost per dollar" figure is the single most informative output above. A value near $0.65–$0.75 means you're in a middle-to-upper bracket where tax savings are doing real work; a value near $0.85–$0.90 means the tax shield is modest and the decision should weigh cash-flow comfort more heavily. The "effective ROI from match" figure is separate and should always be evaluated first — even a low-bracket earner with a 100% match up to 4% is getting a 100% instant return on that first 4%, regardless of tax bracket.

Watch the gap between "paycheck reduction" and "total going to your 401(k)." That gap is money created out of tax policy and employer generosity — it exists whether or not the market goes up next year, which is why maximizing it before adding investment risk elsewhere is generally the correct order of operations.

Long-term financial impact

Contribution decisions compound. At a 7% average annual return, $500/month sacrificed today becomes roughly $260,000 after 20 years and $610,000 after 30 years — from contributions alone, before any employer match. Add a 50% match on that same $500/month and the account crosses $900,000 by year 30. The paycheck reduction you feel this month is fixed and immediate; the wealth impact is exponential and back-loaded, which is exactly why the decision feels harder than the math actually is.

The flip side matters too: money not contributed doesn't just stay in a bank account — for most households, it gets spent. The realistic comparison isn't "401(k) vs. a taxable brokerage account with the same discipline," it's "401(k) vs. whatever your actual spending habits do with an extra $300–$500 a month." That reframing is why automatic payroll deduction is one of the most effective savings mechanisms available to a W-2 employee.

Traditional vs. Roth 401(k): the bracket-timing decision

Traditional tends to win for anyone in the 24%+ bracket today who expects a lower bracket in retirement, or who lives in a high-tax state now but may relocate later. Roth tends to win for early-career earners in the 10–12% bracket, or anyone who expects higher future income, pensions, or wants to avoid RMDs. Splitting contributions between both is a reasonable hedge against not knowing future tax policy.

2025 vs. 2026: what changed

The One Big Beautiful Bill Act (signed July 2025) locked in the current seven-bracket structure through at least 2026, so the bracket math above is stable year over year. The bigger year-to-year shift is the rising Social Security wage base, which slightly raises the ceiling on how much salary is FICA-taxable — a factor that interacts with, but is separate from, your 401(k) contribution decision since 401(k) deferrals never reduce FICA wages.

Common mistakes that cost real money

Overestimating the paycheck hit. Most people assume a 6% contribution cuts take-home pay by 6%. After tax savings it's closer to 4.5–5%, which is why the calculator's side-by-side comparison often surprises people into contributing more than they expected they could afford.

Leaving employer match unclaimed. Roughly 1 in 4 employees doesn't contribute enough to capture the full match. Missing a 3% match on an $80,000 salary for 30 years, at 7% growth, leaves an estimated $227,000 unclaimed at retirement.

Front-loading contributions without a true-up. Maxing out by September on a plan without a true-up provision can forfeit employer match for the remaining months. Check your plan document before front-loading.

Ignoring FICA. 401(k) contributions reduce federal and state income tax but never Social Security or Medicare tax — those apply to gross wages regardless of retirement deferrals.

Edge cases worth knowing

Mid-year salary changes. A raise or bonus mid-year can push your effective contribution rate above or below your intended percentage relative to annual income — recalculate after any pay change.

Job changes mid-year. The $23,500 deferral limit is per person, not per employer. If you switch jobs, contributions at both employers count toward the same annual cap — over-contributing across two W-2s is a common and correctable-but-annoying filing-season surprise.

Highly compensated employee (HCE) limits. Employees earning above roughly $160,000 may face additional nondiscrimination testing limits that cap contributions below the standard IRS maximum, depending on plan participation rates among lower earners.

Decision guide: who this is (and isn't) right for