Which bucket, and in what proportion

Why balance and return cancel

Put one dollar of pre-tax income to work. Pre-tax route: the whole dollar goes in, grows by a factor g, and is taxed on the way out at your future rate. Roth route: you pay tax at today's rate first, and the remainder grows by the same g, untaxed at withdrawal.

The g appears on both sides and divides out. So does the amount. What remains is (1 − t_ret) versus (1 − t_now) — the entire decision. This is why a chart of this question has tax rates on both axes and nothing else, and why "Roth is better because it grows tax-free" is a non-argument: both routes grow at the same rate.

Where age actually enters — four indirect channels

• Visibility, not mathematics. At 25 you're guessing your retirement bracket across forty years. At 57 you can nearly read it off. Age doesn't move the break-even line; it narrows your uncertainty band around it — which is exactly what Fig. 3 prices.
• Career shape. Most people trace a path across Fig. 1: low earnings early (Roth territory), peak earnings mid-to-late career (pre-tax territory), then a low-income window between retiring and starting Social Security or RMDs — the classic opening for Roth conversions. Toggle the career path on to see it.
• Contribution limits are nominal. $24,500 of Roth shelters more real value than $24,500 of pre-tax, because the Roth dollars are already taxed. This only bites if you're contributing at the cap — below the cap you could simply contribute more pre-tax instead. If you max out, tilt the reading toward Roth by a few points.
• RMD horizon. Pre-tax balances are forced out starting in your seventies whether you need the money or not, and can drag Social Security into taxation and raise Medicare premiums. Roth 401(k) balances have had no lifetime RMDs since 2024. The longer you expect to live past 75 without needing the money, the more this matters.

The bracket-filling argument (Fig. 2), stated plainly

Almost everyone compares their marginal rate today against their marginal rate in retirement. That's the wrong comparison for the first slice of pre-tax money. Withdrawals fill the brackets from the bottom: the standard deduction is taxed at nothing, then 10%, then 12%. So the effective rate on a modest pre-tax withdrawal is far below any headline bracket.

The practical consequence is that a pure all-Roth strategy is almost never optimal. You want enough pre-tax balance to generate withdrawals that fill the cheap brackets, and Roth above that. Fig. 2 finds where that crossover sits; the readout converts it into a target balance.

What this sheet cannot see

• State income tax — can swing the answer several points, especially if you'll retire in a different state than you work in.
• Future changes in tax law. Rates are set by legislation, not physics, and the current schedule is not a promise.
• Employer match, which is pre-tax in most plans regardless of what you elect — so your pre-tax balance grows even on an all-Roth election.
• The extra standard deduction at 65+, ACA premium subsidies before Medicare, Social Security taxation thresholds, and IRMAA cliffs — all of which reward keeping taxable income low in specific years.
• Whether you can actually afford the higher out-of-pocket cost of Roth contributions today.