The § 408(d)(2) pro-rata rule decides how much of a traditional-to-Roth conversion is taxable — basis comes out tax-free, everything else is ordinary income. No FICA, no self-employment tax.
Line 8 — what moved from traditional to Roth this year.
Last year's line 14 carryforward plus this year's nondeductible contributions.
Line 6 — ALL traditional/SEP/SIMPLE IRAs combined (§ 408(d)(2) aggregates them). Zero for a clean backdoor Roth.
Taxable amount of the conversion (2025)
$—
Enter the conversion amount to run the 8606 math.
The pro-rata trap
You can't convert "just the basis." § 408(d)(2) aggregates every traditional IRA you own, so a pre-tax balance sitting anywhere makes part of the conversion taxable — the backdoor Roth only works cleanly when the year-end traditional balance is zero. Save the conversion and the Forecaster stacks the taxable slice into your brackets as ordinary income (no FICA, no SE tax).