Why we built it
Most free penalty calculators get the math wrong. Some apply a single flat interest rate to periods that actually spanned half a dozen rate changes; others skip the interest that accrues on penalties themselves, or stop updating when the IRS announces new rates. The result is estimates that can be off by hundreds of dollars, in either direction, on exactly the day someone is deciding what to do about a scary IRS notice.
What's My Penalty? is an independent, free project built to a simple standard: the number you see should match what the IRS itself would compute.
How we keep it accurate
- Verified rates, every quarter. The quarterly §6621 interest table is checked against IRS announcements each quarter (currently verified through Q3 2026), and inflation-adjusted penalty amounts are updated annually from IRS sources.
- The IRS's own method. Interest compounds daily with quarter-boundary rounding, the same Rev. Proc. 95-17 approach IRS transcripts use. Penalties follow IRC §6651, §6654, §6655, §6662, §6698, and §6699, including the fiddly details: the 4.5% concurrent-month reduction, 60-day minimums, per-owner amounts, safe harbors, and the evenly-spread-withholding rule.
- Validated output. The engine is locked down by an automated test suite, including scenarios verified to the penny against independent known-good computations.
- Honest limits. Every result states its assumptions (single payment, calendar-year filer, no prior notices), and results for future dates are flagged when they rely on rates the IRS hasn't announced yet. The IRS's own figure always controls.
Privacy by architecture
Every calculation runs in your browser. The amounts and dates you type are never sent to us. There's no server that could store them. No accounts, no signup. The details are in the privacy policy.
What's here
- The calculator: late-filing, late-payment, estimated-tax, and accuracy penalties with daily-compounded interest, for individuals, C corporations, S corporations, and partnerships
- Guides to the rules the penalties come from
- A FAQ and glossary for quick answers
Contact
Questions, corrections, or suggestions: contact@whatsmypenalty.com. Corrections are taken seriously. If you can show a number is wrong, it gets fixed.