What a conversion costs in federal tax — and the Medicare IRMAA cliff it might trip.
2026 IRS federal brackets & standard deduction; CMS 2026 IRMAA tiers, single $109K / MFJ $218K (cms.gov, checked 2026-09-10). Assumes the full amount is pre-tax (no Form 8606 basis). Social Security is taxed under IRS Publication 915; the senior deduction for 65 and older applies for 2025–2028 (irs.gov). IRMAA uses a 2-year MAGI lookback, so this year's conversion affects Medicare premiums two years later. Estimates only — not tax advice.