Safe harbor (estimated tax)
Also: safe harbor, estimated tax safe harbor, 110% safe harbor, 100% safe harbor
A rule that prevents an underpayment penalty if total withholding plus estimated payments equals or exceeds 100% of the prior year's tax liability, or 110% for AGI above $150,000.
The federal estimated-tax safe harbor shields a filer from underpayment penalties when withholding plus timely estimated payments covers the lesser of 90% of the current year’s tax or 100% of the prior year’s tax. For filers with AGI above $150,000, the prior-year benchmark rises to 110%. California applies the same 110% rule with its own AGI threshold. Meeting safe harbor is simpler than accurately projecting a high-equity year, because the prior-year number is already known.
Example: a filer’s 2024 total tax was $180,000. Her 2025 AGI will exceed $150,000, so the safe harbor is 110% of $180,000, or $198,000. She confirms her 2025 withholding and estimated payments total at least $198,000 across the four quarters. Her 2025 actual tax could be any amount above that with no underpayment penalty, though the balance is still due April 15.
Common mistake: using the 100% figure when AGI exceeds $150,000. Using the wrong safe harbor amount produces small but compounding underpayment penalties, currently at 8% annualized interest.
Safe harbor matters in years with large RSU, NSO, or capital gain spikes, and in years where a Roth conversion or ISO exercise pushes actual tax well above withholding.