If your HDHP coverage ends mid-year, your HSA contribution limit will be prorated based on the number of months you were HSA-eligible. To calculate this, divide the annual contribution limit by 12 and multiply by the number of months you maintained HDHP coverage.
The IRS provides a “Limitation Chart and Worksheet” on Form 8889 to help you determine the exact amount you can contribute. It’s a simple calculation and can be completed quickly.
For guidance, review IRS Form 8889 or consult a tax professional.
Source: "Publication 969 (2018), Health Savings Accounts and Other Tax-Favored Health Plans.”
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