IRS late-filing & late-payment penalty calculator

Two clocks run at once and they do not simply add up. The §6651 mechanics: 5% a month to file, 0.5% a month to pay, both capped at 25%. Interest sits on top and is not modelled here.

You enter	the unpaid tax, how many months the return was late, and how long the tax stayed unpaid.
It applies	§6651: 5% a month to file, 0.5% a month to pay, the 25% caps, and the combined-month rule that stops them simply adding up.
You get	the penalty before interest, with each component shown separately so you can see which clock is costing you.

A worked example

$10,000 unpaid and three months late lands at $1,500 in penalties before interest — 4.5% + 0.5% for the three months both clocks ran. Change the inputs below to your shape.

Unpaid tax at the due date (USD)Months the return is late a part month counts as a whole oneMonths the tax stayed unpaidReturn is more than 60 days late (triggers the minimum penalty)

$1,500.00

Penalties before interest

Failure-to-file (5%/month, 25% cap): $1,350.00
Failure-to-pay (0.5%/month, 25% cap): $150.00
Penalties before interest: $1,500.00

In months where both penalties run, failure-to-file drops to 4.5% so the combined month is 5%, not 5.5%.

An estimate, not a quote. The IRS produces the figure that actually applies to you — check there before you rely on a number. Mechanics only: interest (reset quarterly, compounding daily) is on top, and relief regimes are not modelled.

What this deliberately leaves out

Reasonable cause, first-time abatement, and the installment-agreement and levy-notice rate changes all move the real number, and each is argued rather than computed — your CPA or enrolled agent does that. The $525 minimum is the 2026 figure and is adjusted for inflation annually. Figures verified 2026-08-14.

Would you rather we handled it?Late filings compound, and the relief argument is easier to make with clean books behind it.

Talk to an accountant See what we do

What to do with this number: Take the number to whoever files: penalty relief (reasonable cause, first-time abatement) is argued by your CPA or enrolled agent, and the IRS computes the official figure. This page tells you the size of the problem, not the final bill.

The figures behind this tool, with their sources and verification dates: the companion guide. When the statute or schedule changes, the guide and the tool move together.