Equal cash today, not equal contributions
The IRS contributions page says pre-tax elective deferrals are generally excluded from income now, and that designated Roth contributions are included in gross income now and are tax-free when distributed. If you defer $1,000.00 pre-tax at a 22 percent marginal rate, the tax you do not pay today is $220.00, so the out-of-pocket cost is $780.00. A Roth contribution that costs the same $780.00 out of pocket is a smaller deposit, because the tax was paid before the money went in. Comparing a $1,000.00 Roth deposit with a $1,000.00 traditional deposit compares two different sacrifices. This page refuses that comparison. The Roth side invests the after-tax cost of the traditional contribution.
With no investment growth and the same 22 percent rate in retirement, the traditional $1,000.00 is taxed down to $780.00. The Roth balance is the $780.00 that was invested, and the model does not tax it again. They match. That match is the algebra of equal rates and a zero return. It is not a discovery that the accounts are the same in a plan. Change the future rate and they stop matching. A higher future rate makes the traditional after-tax balance smaller. A lower future rate makes it larger. The page will not tell you which future rate is likely. You type it.
Where 22 percent comes from, and where it does not
Revenue Procedure 2025-32 keeps the 10, 12, 22, 24, 32, 35, and 37 percent statutory rates for 2026. Twenty-two percent is one of those rates. It is not your rate unless your next dollar of taxable income falls in that bracket. The tax-bracket page shows the bracket for a taxable income and a filing status, and it separates the rate on this income from the rate on the next dollar when you are sitting on a boundary. A marginal rate is the rate on the next dollar, not the tax divided by income. The effective rate is lower than the marginal rate whenever income passes through lower brackets. Using the effective rate in this comparison would understate the tax saved by a traditional deferral. The form asks for marginal rates for that reason.
Future brackets are not the 2026 brackets. Congress can change them. The model multiplies the traditional ending balance by one minus the percent you type. It does not run the 2026 tables in a future year and pretend the thresholds still exist. If you want to see today's bracket, use the tax page. If you want a guess about retirement, type the guess here and remember that it is a guess.
What the IRS limit does, on the other page
The 2026 elective deferral limit is $24,500.00 for a 401(k) and similar plans, from the IRS news release and Notice 2025-67. This comparison does not enforce it. A typed contribution of more than that limit is not a legal deferral. The retirement calculator does enforce the cap, including the general age-50 catch-up if you check that box, and it compounds employee and employer dollars at a return you type. It does not split traditional from Roth. Use the two pages for the two questions. Do not add their balances.
A match changes the story. If the employer matches traditional and Roth deferrals the same way, the match is extra money this equal-out-of-pocket model does not include. If the match is pre-tax even when your deferral is Roth, the tax treatment of the match is another line this page does not draw. The IRS says a plan may match elective deferrals, for example at 50 cents per dollar. It does not say your plan does. Leave the match on the retirement page, where you can type it.
Qualified is a legal word
The model treats the Roth ending balance as untaxed. That is the result the IRS describes when a designated Roth distribution is qualified. Qualification depends on the account and on the timing. A distribution that is not qualified can include taxable earnings. This page does not ask your age or how long the account has been open, so it cannot know. Read the Roth result as "if the distribution is qualified." Read the traditional result as "if the entire balance is taxed at the single rate you typed," which ignores a retirement withdrawal that itself spans several brackets. Both simplifications are labeled by the inputs: one rate in, one rate out, one return, no match, no penalty, no required distribution.
The growth, when you type a return above zero, uses the same monthly compound function as the compound-interest page, with no monthly deposit. The contribution sits there from the start. A contribution spread through the year would grow a little less. The paycheck page is where a traditional deferral reduces wages for federal income tax and stays inside Social Security and Medicare wages, which is how Publication 15 treats elective deferrals. This page does not recompute FICA. Giuseppe Ilarda, the site owner, writes and checks the comparison. It is not advice to choose Roth or traditional. The disclaimer says so in the site's ordinary words: educational estimates, not financial advice.