How the IRS actually calculates penalties
If you miss a tax deadline, the IRS doesn't just charge one penalty — it can stack three separate charges. They compound on each other and grow every month you wait.
1. Failure-to-file penalty (IRC § 6651(a)(1))
5% of unpaid tax per month or part of a month, up to a maximum of 25%. Caps at 5 months. If your return is more than 60 days late, the minimum penalty is the smaller of $510 (2025 inflation-adjusted) or 100% of the tax owed.
2. Failure-to-pay penalty (IRC § 6651(a)(2))
0.5% of unpaid tax per month or part of a month, up to a 25% cap. Increases to 1% per month if the IRS issues a final notice of intent to levy. Decreases to 0.25%/month if you're on an approved installment agreement.
3. Interest (IRC § 6621)
The IRS interest rate is the federal short-term rate + 3 percentage points, updated quarterly. For most of 2024 through 2025 it held at 8% per year. Critical detail: it compounds daily, not monthly or yearly. Our estimator uses simple annual to keep math digestible — the real number will be a touch higher.
The two penalties interact. In any month where both failure-to-file and failure-to-pay apply, the failure-to-file penalty is reduced by the failure-to-pay amount (so the combined cap is 5%/month, not 5.5%). The penalties also cap at different totals — FTF at 25% after 5 months, FTP at 25% after 50 months.
What about missed quarterly estimated payments?
Quarterly estimated tax underpayment is calculated differently — it's under IRC § 6654, not § 6651. There's no "failure-to-file" or "failure-to-pay" penalty per se. Instead, the IRS charges an interest-style penalty from the missed due date until the earlier of:
- The date you actually pay the underpayment, or
- April 15 of the following year (when the underpayment is "resolved" by your annual return).
The rate is the same federal short-term + 3%. Safe harbor exceptions mean you avoid the penalty entirely if you paid 100% of last year's tax (110% if AGI > $150k) or 90% of this year's tax in withholding + estimates.
Can I get penalties abated?
Sometimes, yes. The IRS offers first-time penalty abatement (FTA) for filers with a clean compliance history — typically removing FTF + FTP for one tax year if you've been on time the previous three. You request it by calling the IRS or writing a short letter. Reasonable-cause abatement is also available if you can document why you missed (illness, disaster, etc.) — much harder to qualify for.
State penalties — how to think about them
Most states roughly mirror the federal structure: a failure-to-file penalty (often 5%/month, capped at 25%), a failure-to-pay penalty (around 0.5-1%/month), and interest at 5-10% APR. A few states have lower thresholds or different mechanics. No income tax states (AK, FL, NV, NH, SD, TN, TX, WA, WY) won't charge anything on the income side — you might still owe other state-level taxes (sales, franchise, etc.).
Our estimator uses a conservative middle estimate (≈ 6% APR plus a small monthly penalty) for states with income tax. For an exact number, check your state's Department of Revenue penalty schedule.