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Same dollars, two tax schedules, one winner.
The Roth-or-Traditional question reduces to one comparison: your marginal tax rate today versus your rate in retirement. Contribute pre-tax and every dollar is taxed at withdrawal; contribute Roth and the tax is paid now at today's rate. This calculator runs both sides on equal out-of-pocket dollars and shows the after-tax retirement value each way — including the honest case where the rates are equal and the two are mathematically identical.
Savers use it every time income changes: the first job (low bracket — Roth), the peak-earning years (high bracket — Traditional), and the in-between years where splitting is the honest answer. It converts a tax-law guessing game into one rate comparison.
Whichever tax rate is lower decides. $6,000 a year for 30 years at 7%: at a 22% rate both today and in retirement, both sides end at exactly $35,625 after tax — identical. If your retirement rate is 32%, Roth wins by $4,567; if it drops to 12%, Traditional wins by $4,567. Higher rate later → Roth; lower rate later → Traditional.
Hedge by splitting contributions — most savers end up with some of both anyway because employer matches are always pre-tax. Rules of thumb: early-career low brackets favor Roth; peak-earning years favor Traditional; and Roth has structural bonuses (no RMDs, income-tax-free to heirs) that break close ties.
Roth IRAs have none during the owner's lifetime, and since 2024 Roth 401(k)s are RMD-free too. Traditional accounts force taxable RMDs starting at age 73 whether you need the money or not — a real argument for holding at least some Roth in case tax rates rise.