A taxpayer files four months late owing $20,000 and assumes the worst case is "a few hundred bucks plus interest." The real bill: $4,000 in penalties before interest. Penalty math is rate-driven, stacked across categories, and almost always larger than candidates guess.
Congress built the taxpayer-penalty regime to enforce two duties: file on time, pay what you owe. Penalties stack from light (late information return) to severe (criminal evasion), with each tier added because earlier tiers proved insufficient. Knowing why each rate was set makes the structure memorable.
HIGH-FREQUENCY: The §6651(a)(1) penalty is 5% of the unpaid tax for each month or partial month the return is late, up to a maximum of 25%. A partial month counts as a full month, one day past the due date triggers the first 5%.
The penalty exists because tax administration depends on returns arriving on time. Without a return, the IRS cannot match third-party data or close out a tax year. The 5% rate per month is steep on purpose.
Common mistakes
- Adding 5% and 0.5% to get 5.5% per month. When FtF and FtP overlap, FtF is reduced by FtP for shared months. Combined rate is 5%, not 5.5%. Trap: a four-month-late scenario showing $X x 5.5% x 4.
- Computing penalties on gross tax instead of net unpaid tax. Both §6651 penalties apply only to the net unpaid balance. A taxpayer who owes $30,000 in tax but had $25,000 withheld accrues penalties on $5,000, not $30,000.
- Mixing substantial authority with reasonable basis. Substantial authority (≈40%) avoids §6662 with no disclosure. Reasonable basis (≈20%) requires Form 8275 disclosure. "Reasonable basis is sufficient on its own" is the trap answer.
Bottom line
- Failure to file (§6651(a)(1)): 5% of net unpaid tax per month, max 25%
- Failure to pay (§6651(a)(2)): 0.5% of net unpaid tax per month, max 25%, running from April 15 regardless of any extension
- When both run, FtF is reduced by FtP for shared months, so the combined rate is 5% per month, never 5.5%; combined cap with FtP is 25%
- §6651 60-day minimum: the smaller of $510 or 100% of unpaid tax for returns over 60 days late
Exam shortcut
When a question stacks §6651(a)(1) and §6651(a)(2), the combined rate during the overlap is always 5% per month, never 5.5%. Pick the answer that respects the absorption rule. Beyond five months, FtF is capped at 25% and only FtP keeps accruing at 0.5%. For §6662 vs §6663, remember they are mutually exclusive. Any choice combining 20% and 75% on the same underpayment is wrong.
The full lesson (about 2,378 words, 16 min read) adds 2 worked examples, all 6 common mistakes, a self-check, free in the app.
Learning objectives
- I.C3
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