The Roth conversion math, without the hand-waving

Ask about Roth conversions in any retirement forum and you’ll get certainty in both directions — convert everything, convert nothing — usually within three comments of each other. The actual math is smaller and calmer than the argument. This post walks through it, and our free calculator runs it on your own numbers, in your browser, without an account.

The identity that dissolves most of the debate

Multiplication doesn’t care about order. Take $10,000 at a 25% tax rate that never changes: pay the tax now and $7,500 grows tenfold to $75,000, tax-free. Or let the full $10,000 grow tenfold to $100,000 and pay 25% on the way out — $75,000 again, to the dollar. When the rate is the same on both ends and the tax comes out of the converted amount, converting changes nothing. Anyone arguing otherwise under those assumptions is arguing with arithmetic.

The two things that actually move the answer

First: whether your rate later differs from your rate now. This sounds obvious and is where most people slip, because the two rates aren’t the same kind of number. A conversion stacks on top of everything you already earn this year, so it’s taxed at your top(marginal) rates — spanning more than one bracket if it’s large. Withdrawals in retirement fill the brackets from the bottom — standard deduction at 0%, the next slice at 10%, then 12% — so what you actually pay on withdrawals is a blended rate. Someone “in the 22% bracket” in retirement often pays 8–14% on what they withdraw. Comparing bracket to bracket quietly overstates the case for converting. The calculator computes both numbers for you from income and filing status, using the 2026 IRS brackets — neither rate is a guess you have to supply.

Second: where the conversion tax gets paid from. Pay it from money outside the account and the full converted amount grows tax-free. But those dollars weren’t free — kept, they’d have gone on earning something, and that’s the honest cost. The calculator asks you to state what they’d have earned and keeps them growing at exactly that rate on the don’t-convert side. The slower those dollars were earning, the stronger the case for using them: cash earning nothing is the cleanest fuel, while money already compounding as fast as the account adds nothing beyond paying from the conversion itself.

What a flat-rate comparison can’t see

The clean math above assumes away the things that, for real households, often decide the question: required minimum distributions forcing taxable income at 73 — 75 if you were born in 1960 or later — whether you need it or not; each traditional dollar dragging up to 85 cents of Social Security into taxation; Medicare IRMAA surcharges and ACA subsidy cliffs keyed to your income before deductions (MAGI), which a conversion raises dollar for dollar; a surviving spouse inheriting the same balances with the brackets cut in half. None of these fit in a one-page calculator honestly — they depend on your actual balances, ages, and state. A calculator that claims to handle them with three inputs is performing precision it doesn’t have.

What the calculator can and can’t settle

Not “convert” or “don’t” — that judgment depends on your whole picture, and a tool that hands you a verdict from three numbers is overreaching. But the two questions above are yours to answer honestly: is your blended tax rate on withdrawals really going to be higher than today’s marginal rate, and can you pay the tax from outside money? The calculator shows both futures side by side, in after-tax dollars, and computes the break-even tax rate where they tie — the number Vanguard’s research calls the BETR, with the same caution the page itself carries: steady-rate, single-year arithmetic makes it a directional read, not a plan. The arithmetic is the easy part — and it should be free.