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The per-check math
per check = salary × % ÷ pay periods take-home drop ≈ per check × (1 − marginal rate) · max % = limit ÷ salary
The gap between "contribution" and "take-home drop" is the quiet pitch for traditional 401(k)s: the tax man funds part of every deposit now. A $288 contribution that only shrinks the check by $219 means $69 of it was going to withholding anyway.
A worked example
$75,000 biweekly at 10%: $288 per check, $219 of real take-home impact at a 24% marginal rate, $7,500 for the year. Maxing a $23,500 limit needs 31.3% — steep, which is why most people ratchet 1% per raise instead: the check never feels smaller, and five raises later the rate is real.
The match-timing footnote that's worth real money
If you can max out, spread it: plans that match per-paycheck stop matching once your contributions stop, and front-loaders without a true-up forfeit those months of match. Set the percent so you hit the limit in the final period — this calculator's max-% line divided evenly does exactly that. Check what your match is worth with the 401(k) match calculator.
Frequently asked questions
How much does 10% to my 401(k) take from each paycheck?
On a $75,000 salary paid biweekly, 10% is $288 per check — but traditional contributions come out pre-tax, so take-home only drops about $220 at a typical marginal rate. The paycheck impact is always smaller than the contribution.
What percentage maxes out my 401(k)?
Annual limit ÷ salary. The limit changes most years (check irs.gov — it was $23,500 for 2025), so this calculator makes it an input: on $100,000 with a $23,500 limit, 23.5% maxes exactly.
Is maxing out early in the year bad?
It can cost match money: many plans match per-paycheck, so if you hit the limit in September, October–December checks contribute nothing and earn no match. Unless your plan has a 'true-up' provision, spreading contributions across all periods captures the full match.
Roth or traditional for these contributions?
Traditional shrinks today's paycheck less (pre-tax); Roth costs full dollars now for tax-free later. Common shorthand: higher earners lean traditional, early-career lean Roth. Either way the per-check math here shows the cash-flow side of the choice.