Introduction
Choosing between Roth and traditional 401(k) contributions is one of the most common retirement planning decisions, and it sounds simple at first: Roth means you pay tax now, while traditional means you delay that tax until retirement. In practice, the comparison is not just about whether taxes are lower today or lower later. It also depends on how long your money compounds, how much of your employer match is added on top, what returns you earn, and whether you actually reinvest the tax savings created by traditional contributions instead of spending them along the way.
This calculator is built to answer a more precise question than a generic Roth-or-traditional article usually does. Rather than giving a one-size-fits-all rule, it estimates the future after-tax value of both strategies under a shared set of assumptions. It also calculates a break-even future marginal tax rate: the retirement tax rate at which the Roth path and the traditional path would produce the same spendable value according to this model. That threshold is useful because it turns the decision into a sensitivity test. If your likely retirement tax rate is meaningfully above the break-even rate, Roth tends to look stronger. If it is meaningfully below, traditional tends to look stronger.
Just as important, this page keeps the real decision in plain language. A traditional contribution usually lowers today’s tax bill. That creates cash-flow room. If you invest that tax savings in a taxable side account, the traditional strategy gets an additional compounding engine that many rough comparisons ignore. If you do not invest those savings, Roth can look relatively better because the traditional strategy loses a major part of its advantage. This calculator lets you model that behavior directly instead of pretending every saver behaves the same way.
How to use this calculator
Start by entering the amount you contribute each year as an employee and the amount your employer contributes through match or profit sharing. Then enter your current marginal tax rate and your expected retirement marginal tax rate. Those two rates drive the core tax comparison: current tax affects the size of the tax savings from a traditional contribution, while future tax determines how much of traditional retirement balances you keep after withdrawal.
Next, enter your time horizon and expected returns. The calculator assumes end-of-year contributions and annual compounding, which keeps the math consistent and easy to compare across strategies. Then decide how realistic your savings behavior is. The field for the percent of tax savings you invest matters a great deal. If you choose traditional contributions and immediately spend the tax reduction, the long-term outcome is very different than if you capture that tax reduction and invest it in a taxable account every year.
Finally, click Compare Strategies. The result box will show the estimated after-tax retirement value for each strategy, identify which one produces more spendable wealth under the assumptions you entered, and report the estimated break-even future tax rate. The scenario table below the form then varies the future retirement tax rate slightly lower and higher so you can see whether the winner is stable or whether a small tax-rate change flips the result. If the answer changes with only a small assumption shift, that usually means the decision is close and a split contribution strategy may be worth considering.
How this calculator works (and what break-even means)
This calculator compares two ways to make 401(k) contributions: traditional (pre-tax) contributions that reduce today’s taxable income, and Roth contributions that are taxed today but can be withdrawn tax-free later. The output is an estimate of after-tax retirement wealth under each strategy, not just the raw balance shown on an account statement.
The key feature is that it also models what you do with the tax savings created by traditional contributions. If you invest some or all of those savings in a taxable brokerage account, that side account can compound for years and materially change the comparison. If you spend the tax savings instead, the traditional strategy gives up an important source of long-run value.
Why reinvested tax savings can change the answer
A lot of Roth-versus-traditional comparisons stop too early. They compare a Roth 401(k) balance against a traditional 401(k) balance and say the lower retirement tax rate wins. That shortcut misses a behavior difference that matters in real life. Traditional contributions usually reduce current taxes. If that lower tax bill leaves you with extra cash and you invest it consistently, the traditional path gains an extra taxable account that Roth does not create in the same way.
That does not mean traditional always wins. The taxable side account may face capital gains taxes, and the eventual tax rate on traditional 401(k) withdrawals still matters. The point is simply that a fair comparison should ask what happens to the tax savings. This calculator includes that piece on purpose so that you can test both disciplined behavior and less disciplined behavior. In many households, that single assumption explains why two people with similar salaries can reach different conclusions about the better contribution type.
All contributions are treated as end-of-year deposits for simplicity. Returns are assumed constant and compounded annually. Tax rates are entered as marginal rates, meaning the tax rate applied to the next dollar of income, not the average rate across your whole tax return.
- Annual employee contribution (USD): the amount you contribute each year. This is the amount that is either Roth or traditional.
- Employer contribution each year (USD): match or profit sharing. This is modeled as traditional money in both strategies.
- Current marginal tax rate (%): used to estimate the annual tax savings from traditional contributions.
- Expected retirement marginal tax rate (%): applied to traditional balances at retirement to estimate after-tax value.
- Years until retirement: how long contributions and investments compound.
- Expected annual investment return (%): return for 401(k) investments in both Roth and traditional accounts.
- Percent of tax savings you invest (%): how much of the traditional tax savings you actually invest in a taxable account.
- Expected annual return on the taxable account (%): growth rate for the reinvested tax savings.
- Capital gains tax rate at retirement (%): applied to the gains portion of the taxable account when liquidated at retirement.
The calculator uses the future value of an annuity factor for end-of-year contributions. For an annual contribution A, annual return r, and n years:
Formula: FV = A × ((1+r)^n - 1) / r
Traditional strategy after-tax value at retirement is modeled as (traditional 401(k) balance × (1 − future tax rate)) + after-tax taxable side account. Roth strategy after-tax value is modeled as (Roth employee balance) + (employer traditional balance × (1 − future tax rate)). That setup keeps employer dollars consistent because employer contributions are generally pre-tax regardless of whether your own employee deferral is Roth or traditional.
The calculator also reports a break-even future tax rate based on the model’s algebra. In plain terms, it estimates the retirement marginal tax rate where the extra after-tax value from reinvested tax savings would offset the Roth advantage on employee contributions. You should treat this as a planning threshold rather than a forecast of what tax law will actually be decades from now.
Worked example (using the default inputs)
Suppose you contribute $19,500 per year, receive $5,000 per year from your employer, have a current marginal tax rate of 24%, expect a retirement marginal tax rate of 22%, and retire in 25 years. Assume the 401(k) earns 6% annually. You invest 75% of the tax savings into a taxable account earning 5%, and you pay 15% capital gains tax on the taxable growth at retirement.
Under those assumptions, the calculator estimates the future value of repeated annual contributions in the tax-advantaged account, the future value of the taxable side account funded by reinvested tax savings, and the after-tax amount you would actually keep once retirement taxes are applied to traditional balances. It then compares that traditional outcome with the Roth outcome, where employee contributions are already tax-paid but employer contributions still create a taxable traditional balance.
If the traditional result is higher, the model is effectively saying that the up-front deduction plus reinvested tax savings more than compensate for paying tax later. If the Roth result is higher, the model is saying that paying tax today leaves you with more spendable retirement wealth under the stated assumptions. The scenario table adds context by showing whether that conclusion holds at a somewhat lower and somewhat higher retirement tax rate.
How to interpret the results
- Traditional after-tax balance includes the taxable side account if you reinvest tax savings.
- Roth after-tax balance still includes taxes on the employer match because employer dollars are modeled as traditional.
- Break-even future tax rate is a planning threshold: if your actual retirement marginal rate is above it, Roth tends to look better in this model; below it, traditional tends to look better.
Use the scenario table to stress-test your assumptions. If a small change in future tax rate flips the winner, the decision is sensitive. In that case, some savers prefer to split contributions between Roth and traditional accounts so they are not making a single large bet on one future tax outcome. A split strategy is not modeled directly here, but the table makes it easier to spot situations where flexibility may be valuable.
When a split contribution strategy may make sense
Some households do not need one perfect answer. If you are early in your career and expect income growth, Roth contributions can be appealing while your current tax rate is still moderate. Later, as your income rises, traditional contributions may become more attractive. Some people also split contributions because future tax law is uncertain, not because they are indecisive. Holding both Roth and traditional money can make future withdrawals more flexible. In retirement, that flexibility can help you manage taxable income, Medicare surcharges, and bracket creep year by year.
This calculator does not force a split-strategy recommendation, but it helps you recognize when the math is close enough that tax diversification deserves a look. When the break-even rate lands near your own retirement estimate, the better strategy may depend less on one hard prediction and more on preserving options.
Limitations and assumptions (important)
This is a simplified model designed for comparison, not a full retirement plan. Key limitations include the following:
- Contribution timing: assumes end-of-year deposits; real payroll contributions occur throughout the year.
- Tax complexity: uses marginal rates and does not model progressive brackets, deductions, credits, Social Security taxation, IRMAA or Medicare surcharges, or state taxes.
- Taxable account drag: models capital gains tax only at retirement; it does not model annual dividend taxes or interim realized gains.
- Plan rules: does not model contribution limits, catch-up contributions, vesting schedules, or match formulas tied to pay periods.
- Returns: assumes constant returns; real markets are volatile and sequence-of-returns risk can matter.
Treat the output as a decision aid rather than a guarantee. If you are making a high-stakes contribution choice, confirm plan details, tax assumptions, and account rules with plan documents or a qualified tax or financial professional.
Planning notes: what this comparison captures and what it does not
The Roth-versus-traditional decision is often summarized as pay tax now or pay tax later, but the real comparison depends on at least three moving parts: your current marginal tax rate, your retirement marginal tax rate, and whether you invest the tax savings created by traditional contributions. This calculator intentionally keeps the model focused so that you can run scenarios quickly without burying the key decision under dozens of secondary assumptions.
It treats employer contributions consistently as traditional dollars in both strategies and gives you a scenario table around your assumed retirement tax rate. That table matters because many real decisions are sensitive to tax-rate uncertainty. If you want to extend the analysis further, common next steps include adding state taxes, modeling different account fees, considering required minimum distributions, and thinking about how future withdrawals interact with Social Security and Medicare rules. Those are valuable refinements, but the break-even framework here gives you a strong first pass that is specific, numerical, and easier to reason about than a rule of thumb.
