How can I calculate the cost increase to my taxes for paying off my house early?

[DoninGA]

Paying off the mortgage on your personal residence has no effect on your taxes in most cases.  The mortgage interest you pay is entered on Schedule A as an itemized deduction.

The total of all your itemized deductions on Schedule A must be greater than the standard deduction for your filing status to have any tax benefit.  So if you are not itemizing deductions, your mortgage interest paid has on effect on your tax return.

Standard deductions for 2024

Single - $14,600 add $1,950 if age 65 or older
Married Filing Separately - $14,600 add $1,550 if age 65 or older
Married Filing Jointly - $29,200 add $1,550 for each spouse age 65 or older
Head of Household - $21,900 add $1,950 if age 65 or older

[rjs]

The only way that paying off the mortgage on your home could increase your tax is if you itemize deductions on your tax return. After paying off the mortgage you would no longer have a deduction for mortgage interest, so your tax might increase. (But you also are no longer paying the interest. The cost of paying the interest is much higher than the amount of tax that you save from the deduction, so it's a net saving overall, even if you pay more tax.)

The amount that your tax would increase depends the amount of mortgage interest, your other itemized deductions, your tax bracket, and other factors. The easiest way to calculate the difference would be to prepare what-if tax returns with and without the mortgage interest deduction and see how much difference it makes.

As DoninGA said, if you do not itemize deductions, paying off your mortgage will not make any difference in your tax.