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HSA / Medicare excess contribution calculator

Still working past 65 on an employer HDHP? Enrolling in Medicare Part A can be backdated up to 6 months -- see how much of your HSA contributions that backdating would turn into an excess contribution, and the 6% excise tax if it isn't withdrawn in time.

✓ TestedMatches Medicare.gov's 6-month retroactive Part A rule and IRS Publication 969's excess-contribution rule
Likely excess HSA contribution–

Based on Medicare.gov's rule for signing up for premium-free Part A after your initial enrollment window: coverage "starts 6 months back from when you sign up or when you apply for benefits from Social Security," but "can't start earlier than the month you turned 65." IRS Publication 969: "Beginning with the first month you are enrolled in Medicare, your contribution limit is zero," and that rule "applies to periods of retroactive Medicare coverage" -- so contributions made during a backdated window become excess contributions. The 6% excise tax (Form 5329) applies for every year an excess contribution stays in the account; withdrawing the excess (and any earnings on it) by your tax filing deadline, including extensions, avoids it. Applying for Medicare or Social Security exactly in your 65th-birthday month, or any time within your initial enrollment window, creates no retroactive window at all. More on the mechanics and a worked example in the Medicare Advantage vs. Original Medicare guide; for the rest of the partial-year HSA rules see the HSA contribution limit calculator.

How it works

Medicare.gov's rule for signing up for premium-free Part A after your initial enrollment window backdates coverage up to 6 months, but never earlier than the month you turned 65. IRS Publication 969 is direct that your HSA contribution limit is zero from the first month you're enrolled in Medicare, including retroactively, so contributions made in a backdated window become excess contributions with a 6% excise tax (Form 5329) for every year they aren't withdrawn. Enrolling 14 months after turning 65 while contributing $500/month backdates the full 6 months: a $3,000 excess and a $180 excise tax if it stays in the account; enrolling right at 65 creates no retroactive window at all. The practical fix: stop HSA contributions at least 6 months before you apply for Medicare or Social Security past 65. More in the Medicare Advantage vs. Original Medicare guide.

Worked examples