Two-Year Lookback

The rule that Medicare uses your federal tax return from two years ago to set your current premium, because that is the most recent IRS data available. Your 2026 Part B and Part D IRMAA reflects 2024 MAGI. The rule has two practical consequences. First, large income spikes from Roth conversions, capital gains, or one-time events affect Medicare premiums two years later, often catching retirees by surprise. Second, the time to do IRMAA-aware planning is two years before each premium year, not the year itself.