The IRS allows you two options: You can just prorate your contribution limit based on the number of months you actually have HDHP coverage that year, or you can contribute the full amount that would have been allowed if you'd had HDHP coverage for the whole year.

But, wait a minute… why wouldn't everyone just use the full-year contribution limit?

If you do contribute the full-year amount, then the IRS has what's called a "testing period." Basically, you have to continue to be covered by just an HDHP (remaining HSA-eligible) for the entire following year. If you don't, you'll owe income tax and an additional tax on some of the money you contributed to your HSA during the year that you only had partial-year HDHP coverage.

An example makes this easier to understand

Let's say you started a new job in July 2023, and become eligible for your employer's health plan in September. Your employer offers an HDHP and you enroll in it as a single employee.

As far as the IRS is concerned, as long as you're HSA-eligible as of December 1, they'll let you make the full-year HSA contribution, but the testing period applies. This is called the "last month" rule, because you have to be HSA-eligible as of the first day of the last month of the tax year in order to make the full-year HSA contribution.

So assuming you're still enrolled in the HDHP as of December 1 and you're participating in the health plan as an individual, you can go ahead and contribute up to $3,850 to your HSA for 2023 — and you have until April 15, 2024 to make that contribution.

Alternatively, you can just prorate your contribution for 2023: You're covered under an HDHP for five months (September to December) in 2023, so you can contribute 5/12 of the total contribution limit for 2023.

So which option should you pick?

If you go with the prorated option, you can't contribute as much to your HSA. For someone who wants to maximize their HSA contributions — either to grow them over time or to save for a large medical expenses that's expected in the near future — the lower contribution limit would be a drawback.

But the benefit to the prorated contribution limit is that it doesn't matter what sort of coverage you have the following year. If you continue to have HDHP coverage, you can continue to contribute to your HSA. But if you switch to a different plan, there's no harm done.

On the other hand, if you made the full year contribution for 2023, you'll have to maintain your HDHP coverage (or coverage under a different HDHP) for all of 2024. What happens if you don't do that? Let's say you end up leaving that job in July 2023 and switching to a different employer that only offers a non-HDHP option.

In that case, you wouldn't be HSA-eligible for all of 2024 (in other words, you "failed" the testing period), which means the IRS is going to take another look at what you contributed for 2021.

They'll take the prorated amount that you could have contributed and subtract it from the $3,850 that you actually contributed. The difference will be added to your taxable income in 2024. AND you also have to pay a 10% additional tax on that total, adding more to your tax bill.

The risk-versus-reward calculation depends in large part on when you enroll in the HDHP. If your HDHP coverage took effect in February 2022 (and you still have HDHP coverage as of December 1), you can contribute the full-year amount.

On the other hand, if your HDHP coverage starts December 1, using the last month rule lets you contribute an extra $3,162.50. But that's all money that would be subject to income tax and the additional 10% tax if you don't end up staying HSA-eligible for the whole following year.

As with any tax-related question, you'll want to consult with a tax adviser to get advice that's specific to your situation.

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