Backdoor Roth Pro-Rata Calculator: How Much Tax Your Pre-Tax IRA Triggers
A backdoor Roth is only tax-free if you hold no pre-tax money in any Traditional, SEP, or SIMPLE IRA. If you do, the IRS pro-rata rule (Form 8606) taxes a slice of every conversion — and that slice can be most of it. This calculator shows the taxable portion, the tax you'll owe, and how much you'd save by rolling the pre-tax balance into a 401(k) before you convert. 2026 figures, computed entirely in your browser.
Why the backdoor Roth surprises high earners
The backdoor Roth is the standard workaround for people who earn too much to contribute to a Roth IRA directly (above roughly $168,000 single / $252,000 married filing jointly in 2026). You put after-tax money into a Traditional IRA, then convert it to Roth. In a clean setup the conversion is tax-free, because you already paid tax on the contribution.
The trap is the pro-rata rule. The IRS does not let you convert "just" the after-tax dollars. It pools every Traditional, SEP, and SIMPLE IRA you own and treats each converted dollar as the same blend of after-tax and pre-tax money. So if you have an old rollover IRA from a former 401(k) sitting alongside your new nondeductible contribution, most of your "tax-free" backdoor Roth is suddenly taxable ordinary income.
The pro-rata formula (IRS Form 8606)
Two numbers drive everything:
- Total after-tax basis — this year's nondeductible contribution plus any prior basis you haven't converted.
- Total IRA value — the December 31 balance of all your Traditional + SEP + SIMPLE IRAs, including the new contribution. 401(k)s are excluded.
The non-taxable fraction is basis ÷ total IRA value. Whatever you convert, that fraction comes out tax-free and the rest is taxed as ordinary income:
The calculator above does this for your numbers, then stacks the taxable portion on your other income through the exact 2026 federal brackets (Rev. Proc. 2025-32) — so if the conversion straddles a bracket, you see the true blended tax, not a flat rate. The key insight: the bigger your pre-tax balance, the smaller the tax-free fraction — and a $7,500 contribution on top of a six-figure pre-tax balance is taxed almost in full.
The fix: roll the pre-tax IRA into a 401(k) first
Because employer plans are excluded from the pro-rata pool, the clean fix is to move your pre-tax IRA money out of the IRA system entirely — into your current employer's 401(k), or a solo 401(k) if you're self-employed, provided the plan accepts incoming rollovers. Do this before December 31 of the conversion year, because the pro-rata test uses your year-end IRA balance.
Once the pre-tax balance is zero, your only IRA money is the after-tax contribution, the non-taxable fraction is 100%, and the backdoor Roth is tax-free again. The "what-if" line in the calculator shows exactly what that rollover saves you this year, and the multi-year projection shows the compounding payoff if you do the backdoor Roth annually.
Worked examples (2026, $7,500 contribution)
Each example assumes a $7,500 nondeductible contribution (the 2026 IRA limit under 50) and a $7,500 conversion at a 24% marginal rate, with no prior basis:
| Pre-tax IRA balance | Tax-free fraction | Taxable portion | Tax owed (24%) |
|---|---|---|---|
| $0 (clean backdoor) | 100% | $0 | $0 |
| $25,000 | 23.1% | $5,769 | $1,385 |
| $50,000 | 13.0% | $6,522 | $1,565 |
| $100,000 | 7.0% | $6,977 | $1,674 |
| $150,000 | 4.8% | $7,143 | $1,714 |
| $200,000 | 3.6% | $7,229 | $1,735 |
| $300,000 | 2.4% | $7,317 | $1,756 |
Notice how fast the tax-free fraction collapses. Past about $100,000 of pre-tax money, your backdoor Roth is effectively a fully taxable conversion — which is fine if you intended a conversion, but a costly surprise if you expected it to be free.
Frequently Asked Questions
What is the pro-rata rule for a backdoor Roth?
When you convert from a Traditional IRA to a Roth, the IRS treats every dollar as a proportional mix of your after-tax basis and pre-tax balance across all your Traditional, SEP, and SIMPLE IRAs. You can't cherry-pick only the after-tax money, so any pre-tax balance makes part of your backdoor Roth taxable.
Do 401(k) balances count toward the pro-rata rule?
No. 401(k), 403(b), and most solo 401(k) balances are excluded. Only Traditional, SEP, and SIMPLE IRAs count. That exclusion is exactly why rolling a pre-tax IRA into a 401(k) before converting removes the pro-rata tax.
How do I avoid the pro-rata tax on a backdoor Roth?
Roll your entire pre-tax Traditional/SEP/SIMPLE IRA balance into an employer 401(k) that accepts roll-ins before December 31 of the conversion year. That zeroes the pre-tax pool, leaving only your after-tax contribution, so the full conversion is tax-free.
Is the backdoor Roth conversion reversible?
No. Since the 2017 Tax Cuts and Jobs Act, Roth conversions are irrevocable — you can't recharacterize a conversion back to a Traditional IRA. Clear any pre-tax balance before you convert, because you can't undo a conversion that triggered an unexpected pro-rata bill.
Plan the whole conversion picture, not just one year
A backdoor Roth is one piece. QuantCalc models conversions against tax brackets, the ACA cliff, IRMAA surcharges, and RMDs across 10,000 market paths — so you can see the full multi-year tax arc.
PRO unlocks: the full multi-year backdoor projection, Roth conversion optimizer, IRMAA & ACA modeling, tax-torpedo visualization, 10,000 simulations, and PDF reports.