The question this actually settles
Pre-tax and Roth contributions differ in exactly one respect: when the government takes its share. A traditional IRA or pre-tax 401(k) deferral reduces your taxable income now and is taxed as ordinary income when you withdraw. A Roth contribution gets no deduction and is never taxed again, provided the distribution is qualified. Everything else — the investments, the growth, the compounding — is identical.
That makes the decision a comparison of two tax rates, not a comparison of two investment products. If your marginal rate in retirement matches your marginal rate today, and you invest the deduction the traditional route hands you, the two options finish in exactly the same place. That is not a rule of thumb; it falls out of the algebra, and the calculator reproduces it as a break-even figure.
The reason most online comparisons get this wrong is the deduction. Putting $7,000 into a Roth costs you $7,000 of money you have already been taxed on. Putting $7,000 into a traditional account at a 24% marginal rate costs you only $5,320 after the deduction, leaving $1,680 in your pocket. Compare the two accounts without doing something with that $1,680 and you have compared unequal outlays, which flatters whichever account you forgot to fund.
This calculator invests the $1,680 in a taxable side fund each year, so both options cost you exactly $7,000 out of pocket. The traditional side of the ledger then has two pieces — the taxed retirement balance plus the after-tax side fund — and the comparison is honest. If you would in practice spend the refund rather than invest it, untick the side-fund box; the calculator tells you plainly what that assumption does to the answer.
Why the break-even rate is a ratio, not a guess
Write F for the balance the contribution stream builds in either account before any tax — the same number both times, since both hold the same investments. The Roth is worth F at retirement, full stop. The traditional account is worth F(1 − t_ret) plus the side fund S.
Set them equal and solve. F(1 − t*) + S = F gives F − F·t* + S = F, so F·t* = S, so t* = S / F. The break-even retirement tax rate is simply the side fund expressed as a fraction of the account balance. Nothing else enters it — not the return, not the horizon, except through their effect on those two quantities.
Now check the special case. If the side fund pays no tax of its own, it is built from deposits of C·t_now compounded exactly like the main account's deposits of C, so S = t_now · F and the break-even collapses to t* = t_now. This is the symmetry that makes the whole comparison tractable: with no drag on the side fund, the two accounts are equal precisely when the two tax rates are equal. Give the side fund a positive tax rate and, in every case where that fund actually shows a gain, S shrinks below t_now·F, so t* falls below your current rate. (At a zero return there is no gain, nothing is taxed, and the break-even sits exactly on t_now whatever rate you enter.) Taxing the side fund therefore tilts the comparison toward the Roth, and the size of the tilt is the drag on that fund.
The direction to remember is this: the traditional total falls as the retirement rate rises, while the Roth total is flat. So a retirement rate below the break-even leaves the traditional route ahead, and a rate above it leaves the Roth ahead. The table and chart in the results show that crossing explicitly for your own numbers.
Worked example: $7,000 a year for 25 years, 24% now, 32% later
Assume $7,000 a year for 25 years at 7%, a 24% marginal rate today, a 32% marginal rate in retirement, and a side fund held untouched and taxed at 15% on its gain at the end.
- Annuity factor. 1.0725 = 5.427433, so (5.427433 − 1) ÷ 0.07 = 63.249037.
- Balance in either account. 7,000 × 63.249037 = $442,743.26.
- Roth spendable value. $442,743.26 — a qualified distribution is untaxed.
- Traditional after tax. 442,743.26 × (1 − 0.32) = $301,065.42.
- Side-fund deposits. 7,000 × 24% = $1,680 a year, so 25 × 1,680 = $42,000 of basis.
- Side fund before its own tax. 1,680 × 63.249037 = $106,258.38.
- Tax on the side fund. The gain is 106,258.38 − 42,000 = $64,258.38, taxed at 15% = $9,638.76, leaving $96,619.62.
- Traditional total. 301,065.42 + 96,619.62 = $397,685.04.
- Roth advantage. 442,743.26 − 397,685.04 = $45,058.22.
- Break-even. 96,619.62 ÷ 442,743.26 = 21.82%. Because the expected retirement rate of 32% is well above 21.82%, the Roth wins here — and it would still win at any retirement rate above 21.82%, even though that is below the 24% rate you pay today.
That last point is the one worth sitting with. The 15% tax on the side fund costs the traditional route about $9,600, which is why the break-even lands two percentage points below the current marginal rate rather than exactly on it.
How to pick your two tax rates
Your current marginal rate is the easy one: it is the federal band your last dollar of income falls into, plus your state rate if your state taxes income and does not exempt retirement contributions. Use the marginal rate, never the effective rate — the deduction comes off the top of your income, so it is worth the top rate.
The retirement rate is the hard one, and it is a forecast rather than a fact. Three considerations dominate. First, most retirees have lower gross income than they did while working, which argues for a lower rate. Second, required minimum distributions from pre-tax accounts start in your seventies and can push a large balance into a higher band whether you want the money or not; a $2 million pre-tax balance throws off roughly $75,000 in its first required distribution year under the Uniform Lifetime Table in IRS Publication 590-B, which on top of Social Security can leave a retiree in a higher band than they ever paid while employed. Third, statutory rates themselves change — the current federal schedule is a policy choice, not a constant, and forecasting it twenty-five years out is guesswork.
Because of that uncertainty, treat the break-even figure as a decision boundary rather than a prediction. If your break-even comes out at 22% and you think your retirement rate will be somewhere between 12% and 32%, the calculator has not chosen for you — it has told you the coin lands near the middle of your range, which is the classic argument for splitting contributions between the two account types and hedging.
Two structural factors sit outside the arithmetic and both favour the Roth. A Roth IRA has no required minimum distributions during the original owner's lifetime, so it can be left untouched and passed on. And at the contribution limit, a Roth contribution is economically larger than a traditional one: $7,000 of after-tax money is worth more than $7,000 of pre-tax money, and the limit is the same for both. A saver who genuinely maxes out every year is therefore sheltering more real value in the Roth. Model that case with the Roth IRA growth calculator.
After-tax retirement money at each possible retirement rate
| Marginal rate in retirement | Traditional + side fund | Roth | Roth advantage |
|---|---|---|---|
| 0% | $539,362.88 | $442,743.26 | −$96,619.62 |
| 10% | $495,088.55 | $442,743.26 | −$52,345.29 |
| 15% | $472,951.39 | $442,743.26 | −$30,208.13 |
| 20% | $450,814.23 | $442,743.26 | −$8,070.97 |
| 21.8249% (break-even) | $442,743.26 | $442,743.26 | $0 |
| 24% | $433,104.50 | $442,743.26 | $9,638.76 |
| 32% | $397,685.04 | $442,743.26 | $45,058.22 |
| 37% | $375,547.87 | $442,743.26 | $67,195.39 |
The Roth column never moves, because a qualified distribution carries no tax. Every dollar of difference comes from the traditional column, and the crossing point is the break-even rate the calculator reports.
Deductibility is not automatic
A traditional IRA contribution is only deductible in full if neither you nor your spouse is covered by a workplace retirement plan, or if your modified adjusted gross income falls below a threshold the IRS resets annually. Above the range the contribution is still permitted but non-deductible, which destroys the entire premise of this comparison — there is no tax saving to invest on the side, so a non-deductible traditional contribution is worse than a Roth contribution in nearly every case. Roth IRA contributions have their own separate MAGI phase-out, above which savers commonly use a conversion instead. Pre-tax and Roth 401(k) deferrals have no income limits at all, which is why the same comparison often points to a different answer inside a workplace plan than it does for an IRA. Check the current-year thresholds in IRS Publication 590-A before you rely on either deduction.
Assumptions this model makes, and where they bite
- One flat rate in retirement. Real withdrawals fill the standard deduction and the lower bands before reaching your top rate, so the effective tax on a traditional balance is usually below the marginal rate you enter. That biases the comparison toward the Roth. If you expect modest withdrawals, enter a blended rate rather than your top band.
- Contributions arrive at year end. This is the conservative convention. Funding in January instead adds roughly one extra year of growth to every contribution, in both options equally, so it moves both totals but barely touches the break-even.
- The side fund is a single asset with a single tax rate. Real taxable portfolios throw off dividends taxed yearly and gains taxed on sale. The two side-fund settings bracket that reality; run both and see whether the conclusion changes.
- No state-tax arbitrage. Moving from a high-tax state while working to a no-tax state in retirement is one of the strongest arguments for pre-tax contributions, and it is not modelled separately — fold it into your two rates.
- No required minimum distributions. Pre-tax balances must begin distributing in your seventies whether you need the money or not; Roth IRAs need not during your lifetime. That difference is real and this model ignores it.
- Contribution limits are treated as equal. They are equal in nominal dollars, which means the Roth limit is larger in economic terms. If you consistently max out, that advantage is worth more than most of the arithmetic above.
How this fits with the rest of the decision
Order of operations matters more than account type. Capture the full employer match before anything else, because that return dwarfs any tax effect here — the 401(k) growth calculator shows what the match alone contributes over a career. Then choose the wrapper using the break-even rate above, then think about where the money is invested.
Splitting contributions between pre-tax and Roth is not a failure to decide; it is a defensible hedge against tax-rate uncertainty, and it gives you two pools with different tax characteristics to draw on in retirement, which is what makes withdrawal sequencing possible. Many savers use pre-tax deferrals in peak earning years and Roth contributions in early-career or low-income years, which follows the arithmetic exactly.
Two adjacent decisions use different tools. A Roth conversion — moving existing pre-tax money into a Roth and paying tax now — turns on the same rate comparison but adds the question of where the tax payment comes from and how much of a single year's income you are willing to push into a higher band. And if you are considering pulling money out of a retirement account early rather than deciding where to put it, price that first with the early withdrawal penalty calculator; the combined income tax and 10% additional tax usually exceeds anything the Roth-versus-traditional choice is worth.
For comparing a retirement contribution against a completely different use of the money — repaying a loan, buying an asset, funding a business — put both on a discounted basis with the net present value calculator or find the hurdle rate with the internal rate of return calculator.