Tax Extension Calculator

Filing an extension pushes back the date your return is due, but the payment deadline does not move with it. Estimating what you are likely to owe before that original date passes is the key to staying clear of penalties and interest, and that is exactly the figure this tool helps you project.

You enter the total tax you expect to owe for the year and what you have already paid through paycheck withholding or estimated tax installments; the difference is the balance due by the original deadline. The calculator then shows what that balance grows to if it is still unpaid on October 15: the failure-to-pay penalty of 0.5% a month for six months (3%) plus six months of interest at 7% a year (3.5%), the IRS underpayment rate for the second half of 2026. If at least 90% of your tax is paid by the original due date and the rest comes with the return, the IRS treats you as having reasonable cause for the late-payment penalty during the extension; the calculator does not apply that exception.

The trap worth avoiding is treating an extension as permission to delay payment as well; paying your best estimate by the deadline keeps penalties at bay. When the exact number is uncertain, leaning toward a higher estimate is the safer side to err on, since paying slightly too much costs far less than coming up short.