What portion of high deducible health plan (HDHP) premiums can I subtract if some of the expense is included in my federal Itemized Deductions?

To calculate the amount you are allowed to subtract:

1. Divide your limited medical expenses from your federal Schedule A, Line 4 by your total medical expenses from your federal Schedule A, Line 1.
2. Multiply this ratio by your total HDHP premiums paid.
3. Subtract that amount from your total HDHP premiums paid.

Example: The taxpayer has $1,000 in high deductible health insurance premiums. They also have $7,000 of other medical expenses which means they have total medical expenses of $8,000. After the limitation, the taxpayer is only allowed to deduct $2,000 of medical expenses. The $1,000 deduction must be reduced by $250 (($2,000 ÷ $8,000) x $1,000), which means the taxpayer is allowed to deduct $750.