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IRC §6651 late-file and late-pay rates
Failure to file is generally 5% per month (max 25%); failure to pay is generally 0.5% per month (max 25%).
Under IRC §6651(a)(1), the failure-to-file penalty is generally 5% of the unpaid tax for each month or fraction of a month the return is late, up to 25%.
Under IRC §6651(a)(2), the failure-to-pay penalty is generally 0.5% of the unpaid tax for each month or fraction of a month, up to 25%.
When both apply for the same months, the failure-to-file amount is reduced by the failure-to-pay amount for those months. Noticemath implements that overlap rule in deterministic code.
Special cases (fraud rates, minimum dollar amounts, extensions, reasonable cause) are outside the G1 input set and may change results.