What this comparison answers
A traditional IRA gives you a tax deduction now and taxes the money when you withdraw it. A Roth IRA gives no deduction now and, if the withdrawals qualify, no tax later. The practical question is which leaves you with more to spend. This calculator compares the two on the same pre-tax budget, so neither account starts with an advantage, using the tax rates you think apply now and in retirement.
The default example sets aside 5,000 of pre-tax income a year for 30 years, with a 6% return and a 22% tax rate now and in retirement. The traditional account receives the full 5,000. The Roth receives 3,900, which is what is left after 22% tax. The traditional balance reaches 395,291 before tax and 308,327 after the 22% retirement tax. The Roth reaches 308,327. They end level, and that is not a coincidence.
Why equal tax rates give equal results
Both accounts apply the same growth factor to the contributions, so the only difference is when the tax is taken. The traditional balance is multiplied by (1 minus the retirement rate) at the end. The Roth contributions were multiplied by (1 minus the current rate) at the start. Multiplication is order-free, so when the two rates are equal the results match exactly. Take 1,000 a year at 20% in both periods and a 0% return for one year: each account leaves 800. Over 10 years at 5% both leave 10,062.31.
This is the central lesson of the comparison. The choice is not about the accounts being better or worse, it is about whether your tax rate in retirement will be higher or lower than your rate today. The calculator makes that explicit with a table that varies the retirement rate around your entry.
When the rates differ
If you expect to pay more tax in retirement than now, the Roth comes out ahead. With the retirement rate at 32% instead of 22%, the traditional account is worth 268,798 after tax against 308,327 in the Roth, a gap of 39,529. If you expect to pay less, the traditional account wins: at 12% it leaves 347,856 against the same 308,327. Your future rate is the biggest unknown, and it depends on your income, the law at that time and how you take withdrawals, none of which this tool can predict.
The return and the length of time scale both results in the same proportion, so they change the size of the gap and not who wins. Lengthening the default plan from 30 to 40 years raises the Roth balance to 603,572. A negative return shrinks both: at -3% for 10 years with 24% tax in both periods each ends at 33,260.
Assumptions and limits of the model
The model assumes the traditional contribution is fully deductible, that you invest the same pre-tax amount each year, that both accounts start at zero and that contributions are added at the end of each year at a constant return. Roth withdrawals are assumed to be qualified. It uses one tax rate for each period, whereas real income is taxed in brackets, and it ignores the tax effects of required distributions, state tax and the interaction with other income.
For 2026 the IRS limits total contributions to all IRAs to 7,500, or 8,600 at 50 or over. By default the calculator checks the pre-tax budget against the highest limit and stops if you exceed it, because a traditional contribution of that size would not be a like-for-like eligible scenario. You can untick the check if you are modelling a different year. The deduction for a traditional contribution can be reduced if you or your spouse is covered by a retirement plan at work, and a Roth contribution can be limited by income; the calculator does not test either.
To see how a Roth balance grows by itself, use the Roth IRA calculator. To see how tax affects growth in other account types, try the tax-adjusted return calculator, and for a workplace plan use the 401k calculator.
Frequently asked questions
Which is better, a Roth or a traditional IRA?
In this model it depends on the tax rates. A higher rate in retirement favours the Roth, a lower rate favours the traditional account, and equal rates leave them level. The answer is only as good as the rates you enter.
Why does the Roth get a smaller deposit?
Because the comparison fixes your pre-tax income. The traditional account takes all of it and defers the tax. For the Roth you pay tax first, so less is left to deposit.
What if I can afford to put the same after-tax amount in both?
Then the Roth effectively holds more of your income than the traditional account, and a comparison on equal deposits would not be equal in cost. This calculator keeps the pre-tax budget equal for that reason.
Does the calculator check whether I can deduct or contribute?
No. It checks only the yearly budget against the 2026 limit. Deductibility and Roth eligibility depend on your income, filing status and workplace plan coverage.
Is this tax advice?
No. It is an illustration under your assumptions and promises no return. See the disclaimer.
Sources
Contribution limits: IRS, Retirement topics: IRA contribution limits (page reviewed by the IRS on 23 September 2026, read on 2 November 2026). The 2026 limits are labelled for that year only and are not assumed for any other year.