To enter mortgage interest:

1. From within your TaxAct® return (Online or Desktop) click Federal. On smaller devices, click in the upper left-hand corner, then select Federal.
2. Click Itemized or Standard Deductions to expand the section, then click Interest expenses
3. Click Yes to enter information from Form 1098

Per IRS Publication 17 Your Federal Income Tax (For Individuals), page 169:

Amount Deductible

Note. Interest on home equity loans and lines of credit are deductible only if the borrowed funds are used to buy, build, or substantially improve the taxpayer's home that secures the loan. As under prior law, the loan must be secured by the taxpayer's main home or second home (qualified residence), not exceed the cost of the home, and meet other requirements

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.

Fully deductible interest. 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.)

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 (but see binding contract exception below), to buy, build, or substantially improve your home (called home acquisition debt), but only if throughout 2018 these mortgages plus any grandfathered debt totaled $1 million or less ($500,000 or less if married filing separately).
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 2018 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 of Pub. 936 for more detailed definitions of grandfathered debt and home acquisition debt.

You can use Figure 24-A (on page 170 of IRS Publication 17) to check whether your home mortgage interest is fully deductible.

Note. You must be legally liable for the mortgage in order to deduct the mortgage interest.