Roth vs Traditional 401(k) Calculator

Introduction

Choosing between a Roth 401(k) and a traditional 401(k) is one of the most important tax decisions many workers make while saving for retirement. Both account types let your investments grow inside a tax-advantaged workplace plan, but they apply taxes at different times. A traditional 401(k) usually gives you a tax break now and taxes withdrawals later. A Roth 401(k) does the opposite: you pay tax on the money before contributing, but qualified withdrawals in retirement are generally tax-free. This calculator helps you compare those two paths using the same contribution amount, the same growth rate, and the same time horizon so you can see which option produces the larger after-tax value under your assumptions.

The purpose of the tool is not to predict the future with certainty. Instead, it gives you a structured way to test a few key variables that drive the decision. Those variables are your annual contribution, your expected annual return, the number of years until retirement, your current marginal tax rate, and your expected retirement tax rate. Once you enter those values, the calculator estimates the after-tax value of a Roth contribution and the after-tax value of a traditional contribution, then shows the difference. Because the math runs directly in your browser, the tool is fast, simple, and easy to rerun as your income, tax outlook, or retirement timeline changes.

This comparison is especially useful because the Roth-versus-traditional question is really a tax-timing question. If your tax rate in retirement will be lower than it is today, the traditional 401(k) often looks stronger because you deduct contributions at a higher rate now and pay tax at a lower rate later. If your retirement tax rate will be the same or higher, the Roth 401(k) can become more attractive because you lock in today’s tax rate and avoid tax on future withdrawals. The calculator turns that broad idea into concrete numbers, which can make the trade-off easier to understand.

How to Use This Calculator

Start by entering your Annual Contribution, which is the amount you plan to put into the account each year. In this calculator, that contribution is treated as the same stated dollar amount for both account types. Next, enter the Expected Annual Return as a percentage. This is your assumed average yearly investment growth rate. Then enter Years Until Retirement, which tells the calculator how long the contribution has to compound. Finally, enter your Current Tax Rate and your Retirement Tax Rate. These should be entered as percentages, such as 24 for 24%.

After you click Calculate, the results area shows three values. The first is the estimated after-tax value of the Roth 401(k) contribution after growth. The second is the estimated after-tax value of the traditional 401(k) contribution after growth and retirement taxation. The third line shows the difference between the two results, calculated as Roth minus Traditional. A positive difference means the Roth result is larger under your assumptions. A negative difference means the traditional result is larger.

When using the calculator, it helps to think carefully about what each input represents. The tax rates are not average lifetime tax rates; they are simplified stand-ins for the marginal tax effect that matters most in this comparison. The return assumption is also a planning estimate, not a guarantee. If you are unsure what to enter, try a few scenarios. For example, you might run one case with your current federal marginal bracket, another with a lower retirement bracket, and a third with a higher retirement bracket to see how sensitive the outcome is to taxes. That kind of scenario testing is often more useful than relying on a single estimate.

It is also worth noting that some people prefer to compare equal reductions in take-home pay rather than equal stated contribution amounts. This calculator uses equal contribution amounts because that keeps the comparison straightforward and matches the formulas shown below. If you want to compare equal out-of-pocket cost instead, you can still use the tool by adjusting the contribution amount manually and running multiple scenarios. That flexibility makes the calculator useful both for quick checks and for more thoughtful retirement planning discussions.

Formula

The calculator uses a simple compound-growth framework. Let P represent the annual contribution amount, r the annual rate of return, n the number of years until retirement, t_c the current tax rate, and t_r the retirement tax rate. For the Roth 401(k), taxes are applied before the money is invested, so the contribution is reduced by the current tax rate and then allowed to grow. For the traditional 401(k), the full contribution grows first and taxes are applied when the money is withdrawn.

The Roth after-tax value is shown below:

The traditional after-tax value is:

In plain language, the Roth formula says: reduce the contribution by today’s tax rate, then grow the remaining amount for the number of years entered. The traditional formula says: grow the full contribution first, then reduce the final amount by the retirement tax rate. The calculator converts your percentage entries into decimals, applies the compound-growth factor, and then displays the two after-tax outcomes side by side. This makes it easy to isolate the effect of tax timing without adding unnecessary complexity.

The table below summarizes the same formulas in a compact format. It can be helpful if you want a quick visual reminder of where taxes enter the calculation.

Example

Suppose you contribute $10,000 per year, expect a 7% annual return, and have 30 years until retirement. Assume your current marginal tax rate is 24% and your retirement tax rate will be 22%. Under those assumptions, the Roth side of the calculation first reduces the contribution by 24%, leaving $7,600 of after-tax money to grow. The traditional side allows the full $10,000 to grow, but taxes the ending value at 22% when withdrawn.

Using the formulas above, the Roth after-tax value comes out to about $76,122, while the traditional after-tax value comes out to about $81,019. The difference is roughly -$4,897 when calculated as Roth minus Traditional, meaning the traditional 401(k) produces the larger after-tax result in this scenario. That outcome makes intuitive sense because the retirement tax rate is lower than the current tax rate, so the traditional account benefits from taking the deduction at a higher rate now and paying tax at a lower rate later.

Now imagine changing only one assumption: instead of expecting a 22% retirement tax rate, you expect a 28% retirement tax rate. In that case, the traditional account loses some of its advantage because more of the final balance is taxed at withdrawal. The Roth result does not depend on the retirement tax rate in this simplified model, so the gap narrows and may eventually reverse. This is why the calculator is most useful when you test several plausible tax scenarios rather than relying on a single forecast.

A worked example like this is valuable because it shows how sensitive the decision can be to taxes. Many savers focus only on whether they get a deduction today, but the real question is whether paying tax now or later leaves more spendable money in retirement. By changing one input at a time, you can see which assumptions matter most for your own situation.

Limitations and Assumptions

Like any retirement calculator, this one simplifies reality. It assumes a constant annual rate of return, even though real investment returns vary from year to year. It also assumes a single current tax rate and a single retirement tax rate, while actual tax systems are progressive and may involve federal, state, and local taxes. In practice, your retirement withdrawals could span multiple tax brackets, and future tax law may differ from today’s rules. The calculator is therefore best used as a planning aid, not as a precise forecast.

The tool also compares equal stated contribution amounts rather than equal effects on take-home pay. That is a reasonable and common way to frame the decision, but it is not the only valid approach. If your real question is, “Should I put the same amount of salary into each account?” then this calculator fits well. If your question is, “Which option leaves me better off if I am willing to reduce my paycheck by the same amount?” then you may want to run additional scenarios with adjusted contribution amounts.

Another limitation is that the calculator focuses only on the direct tax treatment of your contribution and withdrawal. It does not model employer matching contributions, required minimum distributions, Social Security taxation, Medicare premium thresholds, early withdrawal penalties, or the possibility that Roth withdrawals may provide more flexibility in retirement income planning. Those factors can matter a great deal. For example, employer matches are often deposited into a pre-tax source even when you choose Roth contributions, which means your actual retirement savings may end up split between tax treatments.

Even with those limitations, the calculator remains useful because it highlights the core trade-off clearly. If you expect lower taxes later, traditional contributions often look stronger. If you expect similar or higher taxes later, Roth contributions often become more appealing. Many households ultimately choose a mix of both to create tax diversification. Running this calculator periodically can help you decide whether your current contribution strategy still matches your income level, career stage, and retirement expectations.

Finally, remember that retirement planning is not only about maximizing a formula. Cash-flow needs, behavioral preferences, estate goals, and uncertainty about future law all matter. Some people value the immediate tax deduction because it makes saving easier today. Others prefer the simplicity of knowing that qualified Roth withdrawals should be tax-free later. This calculator gives you a clean numerical starting point, and that is exactly what a good planning tool should do: clarify the decision, reveal the assumptions, and help you ask better questions.