These penalties aren't one-time fees. Once assessed, they're added to your premium for as long as you're enrolled, which for most people means the rest of their life.
Part B: for every full 12-month period you were eligible for Part B but didn't enroll, without qualifying employer coverage, add 10% of the standard premium, permanently. Part D: for every full month you went without creditable drug coverage after your initial enrollment window closed, add 1% of the national base beneficiary premium, rounded to the nearest ten cents, permanently.
If you or your spouse had qualifying coverage through current employment, generally a large enough employer plan, you can usually delay enrollment penalty-free and sign up during a Special Enrollment Period once that coverage ends. This calculator assumes you've already confirmed you don't qualify for that exception. If you're not sure, that's the first thing to check before treating this number as final.
A $20/month penalty sounds small. Over 20 years that's roughly $4,800, and Part B premiums also rise most years, so the real total is usually higher than a flat multiplication suggests. Treat the number above as a floor, not a ceiling.
This doesn't model IRMAA income-related surcharges, which are separate from and stack on top of any late enrollment penalty. It also assumes the standard premium and base beneficiary premium stay flat across your selected years, when in practice they've increased most years historically. Confirm your specific situation with Medicare or a licensed advisor before making enrollment decisions based on this estimate.