Estimated Tax Safe Harbor: The 100 and 110 Percent Rules
How to avoid the underpayment penalty regardless of what you owe — the prior-year and current-year safe harbors and who gets 110 percent
Reviewed by the Fairlight Accounting cross-border tax team — U.S. & Canadian Tax Desks
The estimated tax safe harbor protects a taxpayer from the underpayment penalty regardless of what is ultimately owed. Pay, through withholding and estimates in equal installments, either 90 percent of this year's tax or 100 percent of last year's (110 percent if last year's adjusted gross income exceeded US$150,000), and no penalty applies. The balance is still due in April.
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Who must make estimated payments?
Anyone who expects to owe US$1,000 or more when the return is filed, after subtracting withholding and credits — sole proprietors, partners, S corporation shareholders, landlords, investors, and retirees without enough withholding. The requirement is enforced by a penalty (the Form 2210 guide) computed quarter by quarter, so paying the full amount in January does not cure an underpayment in April. No penalty applies if the tax on the return minus withholding (estimated payments are not counted) is under US$1,000, or if the taxpayer had zero tax liability last year (a full twelve-month year, as a U.S. citizen or resident).
The two safe harbors
| Safe harbor | Requirement | Who uses it |
|---|---|---|
| Prior-year (100 percent) | Withholding plus estimates equal at least 100 percent of last year's total tax, paid in four equal installments | Taxpayers whose income is rising — pays a known amount, penalty-proof, with the growth's tax due in April |
| Prior-year (110 percent) | Same, at 110 percent of last year's tax, for taxpayers whose prior-year adjusted gross income exceeded US$150,000 (US$75,000 married filing separately) | Higher earners — the 110 percent applies to the whole computation, not just the excess |
| Current-year (90 percent) | Withholding plus estimates equal at least 90 percent of this year's tax, in four equal installments | Taxpayers whose income is falling — pays less through the year than the prior-year harbor would require |
| Annualized income installment method | Each installment covers the tax on income actually earned through that quarter, annualized (Form 2210 Schedule AI) | Seasonal and lumpy incomes — a small installment in a slow quarter is not an underpayment |
Farmers and fishermen (two-thirds of gross income from farming or fishing in the current or prior year) have their own rule: one installment by January 15 of the lesser of 66⅔ percent of the current year's tax or 100 percent of last year's (the 110 percent rule does not apply to them), or filing by March 1 with full payment.
Why the 110 percent rule catches people
The test is prior-year adjusted gross income, and it applies to the whole prior-year tax figure. A taxpayer whose AGI was US$160,000 last year with US$32,000 of tax must pay US$35,200 through the year to be safe under the prior-year harbor — not US$32,000. The most common penalty among higher earners is exactly this: paying "last year's tax" in installments when 110 percent of it was the requirement. The test looks backward one year only; a taxpayer whose AGI drops below US$150,000 returns to 100 percent the following year.
Equal installments, and withholding's special status
The prior-year and current-year harbors require the amount to be paid in four equal installments by the quarterly due dates — a large payment in September doesn't fix a small one in April, because the penalty is computed per quarter. Withholding is different: tax withheld from wages, pensions, or an S corporation owner's salary is treated as paid evenly through the year regardless of when it was actually withheld. A December bonus with heavy withholding, or a W-4 change in November, therefore cures the whole year — the mechanism every S corporation owner uses, and a W-2 employee with side income can use, to avoid quarterly payments entirely.
What the safe harbor does not do
It does not reduce the tax. A taxpayer who pays 100 percent of a US$20,000 prior-year tax through the year and owes US$45,000 this year has no penalty and a US$25,000 balance due April 15 — the reason the trade guides on this site pair the safe harbor with a reserve: a fixed percentage of every receipt set aside so the April balance is a transfer, not a loan. It also does not apply to the state; most states have their own safe harbors with different thresholds and percentages, and a few have none.
Worked example
A consultant's prior-year return: AGI US$165,000, total tax US$38,000. This year's projected tax: US$58,000. Prior-year safe harbor at 110 percent: US$41,800, in four installments of US$10,450 by April 15, June 15, September 15, and January 15. Paying that, she owes US$16,200 on April 15 with no penalty. Alternatively, the current-year harbor: 90 percent of US$58,000 = US$52,200, in four installments of US$13,050 — more through the year, a smaller April balance, and the risk that if actual tax comes in above projection she falls under 90 percent and owes a penalty. She chooses the prior-year harbor and reserves 30 percent of every receipt; the reserve covers the April balance. Her colleague paid US$38,000 in four installments — 100 percent, not 110 — and owed a penalty on a US$950 shortfall in each quarter — US$3,800 in all, roughly US$170 to US$180 of penalty at the 2026 underpayment rates.
Frequently asked questions
What is the estimated tax safe harbor?
Paying at least 90 percent of the current year's tax, or 100 percent of last year's (110 percent if last year's AGI exceeded US$150,000), in equal quarterly installments through withholding and estimates — which eliminates the underpayment penalty regardless of the balance due.
Who must use the 110 percent rule?
Taxpayers whose prior-year adjusted gross income exceeded US$150,000 (US$75,000 if married filing separately). The 110 percent applies to the entire prior-year tax.
Does the safe harbor eliminate the balance due?
No. It eliminates the penalty. Any tax above the amount paid through the year is due with the return on April 15.
What if I had no tax last year?
No estimated tax penalty applies for this year if you had zero tax liability last year, the prior year was a full twelve months, and you were a U.S. citizen or resident — regardless of what you owe this year.
Official sources
The IRS states: “Generally, most taxpayers will avoid this penalty if they owe less than $1,000 in tax after subtracting their withholdings and credits, or if they paid at least 90% of the tax for the current year, or 100% of the tax shown on the return for the prior year, whichever is smaller.” — Internal Revenue Service, Estimated taxes, https://www.irs.gov/businesses/small-businesses-self-employed/estimated-taxes
The IRS states: “Use Form 2210 to see if you owe a penalty for underpaying your estimated tax and, if you do, to figure the amount of the penalty.” — Internal Revenue Service, About Form 2210, Underpayment of Estimated Tax by Individuals, Estates, and Trusts, https://www.irs.gov/forms-pubs/about-form-2210
Next step
Fairlight Accounting is a cross-border accounting and tax practice with a U.S. Tax Desk and a Canadian Tax Desk. Our U.S. Tax Desk handles estimated-tax planning under the prior-year, current-year, and annualized methods, withholding strategies for S corporation owners, reserve rules, and state safe-harbor coordination. See pricing or book a call.
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