What is the pro-rata rule on a backdoor Roth?

Question

Accepted Answer

When you convert any traditional IRA money to Roth, the IRS treats ALL your traditional/SEP/SIMPLE IRA balances as one pot. The tax-free fraction of the conversion equals your after-tax basis divided by the total pot — you cannot choose to convert only the after-tax dollars. A large pre-tax rollover IRA is the most common reason a 'tax-free' backdoor Roth turns out mostly taxable.

Backdoor Roth Pro-Rata Calculator