Per IRS Publication 936 Home Mortgage Interest Deduction, page 2:

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 any one mortgage fits into more than one category, add the debt that fits in each category to your other debt in the same category.) If one or more of your mortgages doesn’t fit into any of these categories, use Part II of this publication to figure the amount of interest you can deduct.

The three categories are as follows.

1. Mortgages you took out on or before October 13, 1987 (called grandfathered debt).
2. Mortgages you (or your spouse if married filing a joint return) took out after October 13, 1987, and prior to December 16, 2017 (see binding contract exception below), to buy, build, or substantially improve your home (called home acquisition debt), but only if throughout 2019 these mortgages plus any grandfathered debt totaled $1 million or less ($500,000 or less if married filing separately). Exception. A taxpayer who enters into a written binding contract before December 15, 2017, to close on the purchase of a principal residence before January 1, 2018, and who purchases such residence before April 1, 2018, is considered to have incurred the home acquisition debt prior to December 16, 2017.
3. Mortgages you (or your spouse if married filing a joint return) took out after December 15, 2017, to buy, build, or substantially improve your home (called home acquisition debt), but only if throughout 2019 these mortgages plus any grandfathered debt totaled $750,000 or less ($375,000 or less if married filing separately).

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

See Part II for more detailed definitions of grandfathered debt and home acquisition debt.

To enter your qualifying interest into the TaxAct® program:

1. From within your TaxAct return (Online or Desktop), click Federal. On smaller devices, click the menu icon in the upper left-hand corner, then select Federal
2. Click Itemized or Standard Deductions to expand the category and then click Interest expenses
3. Click Yes on the first screen and proceed with the questions to enter your information