IRS Interest Calculator

If you pay a tax bill after its due date, interest builds up on what you owe. This estimates that interest from the amount, the due date, the day you pay, and the yearly rate.

It is an unofficial estimate from published rules. Penalties are charged separately and are not included, and the IRS notice you receive has the figure that counts.

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Updated 2026-11-05

How it works

The rate is set every three months and published by the IRS. For individuals it is the federal short-term rate plus 3 percentage points, rounded to a whole number. Interest is compounded daily, so each day's interest is charged on the tax plus the interest already added.

The calculator counts the days from the due date to the day you pay. It then grows the amount owed by the yearly rate divided by 365 for each of those days. The interest is the growth, and the total is the amount owed plus that interest.

Worked example

If $5,000.00 is due on 2026-04-15 and paid on 2026-11-30, that is 229 days. At 7% a year, compounded daily, the interest comes to $224.46, so the total is $5,224.46.

Questions

What rate should I use?

Use the rate the IRS published for the quarter. If your debt lasted across several quarters, the rate may have changed. Run the calculator once for each stretch of time and add the results, using the interest so far as the new amount owed.

Does interest stop when I pay?

Yes. It builds up until the day the tax is paid in full. Interest is also charged on some penalties, which this does not cover.

Why does the rate change?

The rate is reset every quarter from the federal short-term rate. It has been as low as 6% and as high as 8% in recent years.

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