A taxpayer may contribute the full amount to an HSA using the last-month rule if the taxpayer was an eligible individual on the first day of the last month of the tax year (December 1 for most taxpayers).
If the taxpayer does not qualify to contribute the full amount for the year, the contribution is determined by using the sum of the monthly contribution limits rule.
Full Contribution for Last-Month Rule
The last-month rule allows eligible individuals to make a full contribution for the year even if they were not an eligible individual for the entire year. They can make the full contribution for the year if:
Chris, age 53, became an eligible individual on December 1, 2011. He has family HDHP coverage on that date. He fully expects to meet the testing period by remaining eligible through December 31, 2012. Under the last-month rule, he can contribute up to $6,150 for the 2011 tax year.