Generally, home mortgage interest is any interest you pay on a loan secured by your home (main home or a second home). The loan may be a mortgage to buy your home, a second mortgage, a line of credit, or a home equity loan.

You can deduct home mortgage interest if all the following conditions are met.

You file Form 1040 and itemize deductions on Schedule A (Form 1040).
The mortgage is a secured debt on a qualified home in which you have an ownership interest.

Fully deductible interest. In most cases, you can deduct all of your home mortgage interest. How much you can deduct depends on the date of the mortgage, the amount of the mortgage, and how you use the mortgage proceeds. If all of your mortgages fit into one or more of the following three categories at all times during the year, you can deduct all of the interest on those mortgages. If one or more of your mortgages does not fit into any of these categories, your deduction will be limited.

The three categories are as follows.

Mortgages you took out on or before October 13, 1987 (called grandfathered debt).
Mortgages you took out after October 13, 1987, to buy, build, or improve your home (called home acquisition debt), but only if throughout 2013 these mortgages plus any grandfathered debt totaled $1 million or less ($500,000 or less if married filing separately).
Mortgages you took out after October 13, 1987, other than to buy, build, or improve your home (called home equity debt), but only if throughout 2013 these mortgages totaled $100,000 or less ($50,000 or less if married filing separately) and totaled no more than the fair market value of your home reduced by (1) and (2).

The dollar limits for the second and third categories apply to the combined mortgages on your main home and second home.