Backdoor Roth Calculator
See the true tax cost of a backdoor Roth once the pro-rata rule counts your existing pre-tax IRA money.
Want to understand the concept, not just the number? Read The Backdoor Roth Guide and more below.
How it's calculated
The backdoor Roth lets high earners fund a Roth despite the income limit. You put after-tax money into a Traditional IRA, then convert it to Roth. If the Traditional IRA holds only that new after-tax money, the conversion is tax-free. The catch is the pro-rata rule, which trips up people who already have other pre-tax IRA money.
The IRS treats all your Traditional, SEP, SIMPLE, and rollover IRAs as one pool. Take the default. You add $7,500 of after-tax money, but you already hold $42,500 of pre-tax IRA money, so your $50,000 pool is 85 percent pre-tax. Convert $7,500 and 85 percent of it, $6,375, is taxable, costing $1,530 at a 24 percent rate. Only $1,125 comes across tax-free, and $6,375 of your after-tax basis stays stuck in the IRA.
That is the trap. The backdoor is clean only when you have no other pre-tax IRA money. A common fix is to roll your pre-tax IRA into your 401(k) first, if the plan allows it, since 401(k) money is not counted in the pro-rata pool. Then the backdoor conversion is tax-free again. Sort out the pool before you convert, not after.
Assumptions
- Applies the pro-rata rule across your after-tax contribution and existing pre-tax IRA balance. Assumes no prior after-tax basis.
- Uses a single marginal rate for the taxable portion. It does not model the conversion pushing you into a higher bracket.
Last updated: 2026-08-08
These assumptions follow our general methodology.
Frequently asked questions
What is the pro-rata rule?
When you convert Traditional IRA money to Roth, the IRS treats all your Traditional, SEP, SIMPLE, and rollover IRAs as one pool. The conversion is taxed in proportion to how much of that pool is pre-tax, no matter which dollars you actually move.
Why do I owe tax on a backdoor Roth?
Because you have other pre-tax IRA money. The pro-rata rule spreads your after-tax basis across the whole pool, so most of your conversion counts as pre-tax and is taxed, even though you contributed after-tax dollars.
How do I avoid the pro-rata rule?
Empty your pre-tax IRAs before converting. The usual move is to roll them into your employer 401(k), which is not counted in the pro-rata pool. With no pre-tax IRA money left, the backdoor conversion becomes tax-free.
What happens to the basis left behind?
The after-tax basis that does not convert stays in your IRA and is tracked on Form 8606. It reduces the tax on future withdrawals or conversions, but until then it is money you already paid tax on that is stuck alongside your pre-tax funds.