≈ $1,674 unexpected tax With a $100,000 pre-tax Traditional/SEP/SIMPLE IRA balance, a $7,500 nondeductible contribution leaves a total IRA pool of $107,500. The pro-rata rule makes only 7.0% of any conversion tax-free, so converting $7,500 produces about $6,977 of taxable income — roughly $1,674 in federal tax at a 24% marginal rate (22.3% effective on the conversion).
How the pro-rata rule taxes this conversion
A backdoor Roth is supposed to be tax-free: you contribute after-tax money, then convert it. But the IRS pro-rata rule (Form 8606) pools all your Traditional, SEP, and SIMPLE IRAs and treats every converted dollar as the same blend of after-tax and pre-tax money. With a $100,000 pre-tax balance, your $7,500 after-tax contribution is just a small slice of the pool — so most of what you convert is pulled from pre-tax dollars and taxed as ordinary income.
| Form 8606 step | Amount |
|---|---|
| Nondeductible contribution (after-tax basis) | $7,500 |
| Pre-tax IRA balance | $100,000 |
| Total IRA value (the pro-rata pool) | $107,500 |
| Non-taxable fraction (basis ÷ pool) | 7.0% |
| Amount converted | $7,500 |
| Tax-free portion of conversion | $523 |
| Taxable portion of conversion | $6,977 |
| Federal tax owed (@ 24%) | $1,674 |
The fix: roll the $100,000 pre-tax balance into your employer's 401(k) (or a solo 401(k)) before December 31. 401(k) balances are excluded from the pro-rata pool, so this leaves only your $7,500 after-tax contribution — the non-taxable fraction jumps to 100% and the whole conversion converts tax-free, saving the $1,674 above this year and more in future years if you do the backdoor Roth annually.
Why the fraction is so small: the pro-rata fraction is basis ÷ total pool. A $7,500 basis against a $107,500 pool is just 7.0%. The bigger the pre-tax balance, the closer the conversion gets to fully taxable — which is fine if you meant to do a taxable conversion, but a costly surprise if you expected the backdoor Roth to be free.
Run your exact numbers
Your real basis, conversion amount, and marginal bracket may differ. The interactive backdoor Roth pro-rata calculator computes the taxable portion, the tax owed, and the 401(k)-rollover savings for your situation — and projects the multi-year drag if you repeat the backdoor Roth annually.
Model the full conversion picture
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Open the pro-rata calculator →Other pre-tax balances
- $25,000 pre-tax IRA balance
- $50,000 pre-tax IRA balance
- $150,000 pre-tax IRA balance
- $200,000 pre-tax IRA balance
- $300,000 pre-tax IRA balance
FAQ
How much tax will I owe on a $7,500 backdoor Roth with a $100,000 pre-tax IRA?
With a $100,000 pre-tax balance and a $7,500 nondeductible contribution, the total IRA pool is $107,500, so only 7.0% of any conversion is tax-free. Converting $7,500 makes about $6,977 taxable as ordinary income — roughly $1,674 in federal tax at a 24% marginal rate. State tax, if any, is on top.
Why is most of my backdoor Roth taxable?
The pro-rata rule pools all your Traditional, SEP, and SIMPLE IRAs together. Your $7,500 after-tax contribution is a small slice of a $107,500 pool, so only 7.0% of each converted dollar is treated as the after-tax money. The other 93.0% is pulled from pre-tax dollars and taxed.
How do I make this conversion tax-free?
Roll the $100,000 pre-tax balance into an employer 401(k) (or solo 401(k)) that accepts roll-ins before December 31. That removes it from the pro-rata pool, leaving only your $7,500 after-tax contribution, so the full conversion converts tax-free and you save about $1,674 this year.
Does my 401(k) balance change this calculation?
No. 401(k), 403(b), and most solo 401(k) balances are excluded from the pro-rata rule. Only IRA money counts — which is exactly why moving the pre-tax IRA into a 401(k) fixes the problem.
Worked example assumes a $7,500 nondeductible contribution (the 2026 IRA limit for those under 50), a $7,500 conversion, no prior basis, and a 24% marginal rate. The pro-rata test uses your December 31 IRA balances. Educational only, not tax advice — consult a CPA before converting.