- Mortgage interest deduction
- A federal itemized deduction for interest paid on a loan used to buy, build or improve your home. Interest is deductible on up to $750,000 of mortgage debt, or $375,000 if married filing separately (IRS limit applied by this calculator, 2026). This estimate applies the $750,000 limit to every filer.
- Itemized deductions
- Specific expenses, such as mortgage interest, state and local taxes and charitable gifts, listed individually on your federal return instead of taking the standard deduction. Mortgage interest reduces tax only if you itemize. This calculator assumes you do and that all the interest counts, so its figure is the maximum possible saving.
- Standard deduction
- A flat amount, set each year by the IRS and based on filing status, that you may subtract from income instead of itemizing. You benefit from the mortgage interest deduction only by the amount your total itemized deductions exceed it. For many households with modest loans, the real saving is smaller than shown here, or zero.
- Marginal tax rate
- The rate applied to your last dollar of taxable income, here meant as your federal and state rates combined. Each deductible dollar of mortgage interest saves tax at this rate. At a 24% marginal rate, $18,000 of deductible interest lowers your tax by about $4,320, assuming you itemize.
- Tax savings
- Deductible interest multiplied by your marginal tax rate: the reduction in income tax from deducting mortgage interest. It lowers the after-tax cost of the loan but never exceeds the interest itself; a dollar of interest paid to save a fraction of a dollar in tax is still a net cost.
- First-year interest
- The total interest paid during the first twelve mortgage payments. Because the balance is highest at the start, this is normally the largest annual interest figure and the largest annual deduction. Both decline every year as principal is repaid, so the average annual saving over the loan is lower.
- Amortization
- The schedule by which equal monthly payments gradually pay off the loan, with early payments going mostly to interest and later ones mostly to principal. It is why the interest deduction is front-loaded: the tax benefit is largest in the first years and shrinks steadily toward the end of the term.
- Total interest paid
- The sum of all interest over the full loan term if every scheduled payment is made. Compare it with total tax savings: the difference is the after-tax interest cost. Selling, refinancing or prepaying the loan would reduce both figures, since the calculator assumes the loan runs its full term.
