How the Failure-to-Pay Penalty Is Calculated Under IRC 6651(a)(2)

The failure-to-pay penalty is the slow one. It does not jump at you the way the failure-to-file penalty does. It just keeps ticking, month after month, until it reaches 25 percent of the tax you showed on your return. Most people never check it. They should, because it is one of the easiest numbers on an IRS notice to verify by hand.

What the statute says

IRC 6651(a)(2) adds 0.5 percent of the amount shown as tax on the return for the first month the tax goes unpaid, plus another 0.5 percent for each additional month or fraction of a month, up to 25 percent in the aggregate. The clock runs from the date prescribed for payment, determined with regard to any extension of time for payment. Reasonable cause is a defense.

Note what extends the clock and what does not. An extension of time to file under Form 4868 does not extend the time to pay. The payment due date for a 2025 individual return is April 15, 2026, extension or not. The extension guide walks through that trap in detail.

The base changes every month

This is the part people miss. The failure-to-file penalty is computed once, on what you owed at the due date. The failure-to-pay penalty is recomputed every month. Under IRC 6651(b)(2), the amount shown as tax is reduced, for each month, by any part of the tax paid on or before the beginning of that month and by credits claimed on the return.

IRM 20.1.2.3.8.1 describes the method plainly: for each penalty month, multiply the unpaid tax at the start of that month by the penalty rate, then add up all the months. When nothing changes for several months, months x unpaid tax x rate gets you there faster.

How penalty months are counted

IRM 20.1.2.3.8.4.2 says the first penalty month begins the day after the payment due date. Each month ends on the same day of the month as the due date. For a balance due April 15, 2026, month one runs April 16 through May 15, month two runs May 16 through June 15, and so on. A payment made in the middle of a month does not help that month. It helps the next one.

Worked example: one balance, no payments

Say you file your 2025 return on time and show $10,000 due. You pay nothing until December 1, 2026. December 1 falls in the month that runs November 16 through December 15. That is month eight.

Worked example: partial payments

Same $10,000. This time you pay $2,000 on June 30, 2026, $3,000 on September 10, 2026, and the last $5,000 on January 20, 2027. Watch how each payment only counts starting with the next month that begins after it is made.

The June 30 payment landed in month three, so month three was still charged on $10,000. If that same $2,000 had gone in on June 15 instead, month three would have been charged on $8,000. Fifteen days, ten dollars. Small, but it compounds across a bigger balance and a longer timeline.

Where your payment goes matters

The penalty is charged on unpaid tax. So the question is whether your payment reduced tax or went somewhere else. For penalty computation purposes, IRM 20.1.2.3.8.2 applies payments to tax first, then to other assessed penalties, then to the failure-to-pay penalty, and finally to interest. That ordering keeps the failure-to-pay base as low as possible. The broader rules for voluntary and involuntary payments are in how the IRS applies payments.

Reaching the 25 percent cap

At 0.5 percent per month, a balance that sits untouched reaches the 25 percent cap after 50 months. That is four years and two months. On $10,000, the penalty tops out at $2,500. Two things move that date:

• A notice of intent to levy. Under IRC 6651(d), the rate rises to 1 percent per month for months beginning more than 10 days after the notice. The cap arrives much sooner. See the 1 percent rate guide.
• An installment agreement. Under IRC 6651(h), an individual who filed on time pays 0.25 percent per month while an installment agreement is in effect. The cap arrives much later. See the 0.25 percent rate guide.

The cap is applied per assessment. IRM 20.1.2.3.8.1 notes that tax shown on the return gets its own 25 percent limit, and additional tax billed later in a separate notice gets its own computation. One can be capped while the other is still running.

When failure to file is also in the picture

If the return was also late, both penalties run at once for the first few months. IRC 6651(c)(1) reduces the failure-to-file penalty by the failure-to-pay amount for each overlapping month, so you are not charged 5.5 percent a month. The details are in the overlap guide.

One more wrinkle. If the IRS prepares a substitute return for you under IRC 6020(b), IRC 6651(g)(2) treats that substitute as your filed return for failure-to-pay purposes. So the failure-to-pay penalty runs on the tax the IRS computed, from the original payment due date.

Interest is separate, and it compounds

Interest under IRC 6601 runs on the unpaid tax from the due date, compounded daily under IRC 6622. Interest on the failure-to-pay penalty itself does not start until the penalty is assessed and not paid within 21 days of notice and demand (10 business days if $100,000 or more), under IRC 6601(e)(2)(A). The penalty interest guide shows that timing with numbers.

How to check the number on your notice

1. Find the tax shown on the return and every payment and credit with its effective date on your account transcript.
2. List each penalty month starting the day after the payment due date.
3. For each month, take the unpaid tax at the start of the month and multiply by the rate in effect for that month (0.5%, 1%, or 0.25%).
4. Add the months, and stop when the total reaches 25% of the tax shown.

If your total is lower than the IRS total, look for a payment the IRS posted to a different year or applied after its actual date. That happens more than you would think.

Worked example: a substitute for return

Say you never filed a 2022 return, which was due April 18, 2023. The IRS prepares a substitute return under IRC 6020(b) and assesses $12,000 of tax. Because IRC 6651(g)(2) treats that substitute as your filed return for failure-to-pay purposes, the 0.5 percent monthly penalty under 6651(a)(2) runs from the original payment due date, not from the assessment.

Penalty months begin on the 19th of each month. By October 18, 2026, 42 months have run. At 0.5 percent a month, that is 21 percent of $12,000, or $2,520, assuming no levy notice raised the rate. Eight more months at the standard rate and it reaches the $3,000 cap. Meanwhile the failure-to-file penalty, which the substitute return does not stop, has been sitting at its own cap since 2023. Filing your own return, even years late, can reduce the tax if the substitute overstated it, and every penalty computed on that tax shrinks with it.

The bottom line

The failure-to-pay penalty is half a percent a month on whatever tax is still unpaid when each month begins. It rewards early payments, even small ones, and it punishes ignoring the levy notice. Penalty relief exists for this one too; see the firm's penalty abatement page. Before you ask for relief, know the number. Have someone run it if the notice does not add up.