The same take-home cost
To compare fairly, start with the same slice of pay. Set aside $10,000 of pre-tax pay. In a traditional 401(k), all $10,000 goes in. In a Roth 401(k), income tax comes out first: at 22%, $7,800 goes in. Either way, your take-home drops by the same $7,800.
Why it's a tie
Grow both for 30 years at a hypothetical 7%, a growth factor of about 7.61. Traditional: $10,000 × 7.61 = $76,123, then 22% tax on withdrawal leaves $59,376. Roth: $7,800 × 7.61 = $59,376, no tax on qualified withdrawals. Same answer, because multiplication order doesn't matter: $10,000 × 7.61 × 0.78 equals $10,000 × 0.78 × 7.61.
So what decides it?
The tax rate now versus the tax rate when you withdraw. If your rate later is lower (say 12%), traditional comes out ahead: about $66,988 versus $59,376. If it's higher (say 24%), Roth wins: traditional leaves about $57,853, so Roth is ahead by about $1,522. The chart shows the difference across future tax rates; it crosses zero exactly where the later rate equals today's.
Nobody knows the future rate
Future tax rates depend on your income in retirement, where you live, and tax law decades from now. That's why the choice is genuinely uncertain. Some people split contributions between both to hedge. This page doesn't pick one; it shows why the math comes down to that one comparison.
Simplifications and rules
Real withdrawals are taxed through brackets, not at one flat rate. Both account types share one employee limit ($24,500 in 2026), not every plan offers a Roth option, and Roth 401(k) accounts no longer have lifetime required minimum distributions for the original owner. Employer matching contributions have their own tax treatment set by the plan.
Assumptions
Yearly compounding at a steady hypothetical rate, flat tax rates, no fees, and qualified withdrawals.
Sources
Last reviewed: Sep 28, 2026