Estimated tax safe harbor rules: the 90%, 100% and 110% tests
Two tests, and you only have to pass one. Which one is cheaper depends entirely on whether this year is better or worse than last.
Last reviewed August 8, 2026. Informational only — not tax advice.
The two tests
The IRS charges an underpayment penalty when you have not paid enough tax during the year. But it also gives you two ways to be certain you are safe before you know what the year will bring. Passing either is enough.
| Test | What you must pay during the year | Best when |
|---|---|---|
| Current-year test | 90% of the tax shown on this year’s return | Income is flat or falling, or you can estimate this year accurately |
| Prior-year test | 100% of last year’s total tax (110% if prior-year AGI was over $150,000, or over $75,000 if married filing separately) | Income is rising, or this year is hard to predict |
Why the prior-year test is the safer bet
The current-year test has a circularity problem: to know whether you paid 90% of this year's tax, you need to know this year's tax — which you do not, until the year ends. If you underestimate and land at 85%, the penalty applies retroactively to every quarter.
The prior-year test has no such problem. Take the total tax line from last year's Form 1040, multiply by 1.0 or 1.1 depending on your prior-year AGI, divide by four, and pay that. Whatever happens this year — a windfall, a huge capital gain, a business that triples — you cannot be penalised. You will still owe the balance in April, but without penalty or interest for underpaying during the year.
The 110% threshold in detail
The higher 110% requirement kicks in on prior-year adjusted gross income, not on current-year income and not on taxable income:
| Filing status | Prior-year AGI threshold | Required percentage above it |
|---|---|---|
| Single | $150,000 | 110% |
| Married filing jointly | $150,000 | 110% |
| Head of household | $150,000 | 110% |
| Married filing separately | $75,000 | 110% |
These thresholds are fixed in statute and are not indexed for inflation, so more filers cross them every year.
The safe harbor applies quarter by quarter
A common and expensive misreading: the safe harbor is not a year-end target you can hit with one December payment. The required amount is divided across the four due dates, and each quarter is tested separately. Paying the whole year's safe harbor amount in December still leaves you penalised for the first three quarters.
The exception is withholding, which the IRS spreads evenly across the year no matter when it happened. That is why a late-year withholding increase can rescue an underpaid year in a way a late-year estimated payment cannot — explained further in the Schedule AI guide.
When no penalty applies at all
- The $1,000 de minimis rule. If your remaining balance after withholding and refundable credits is under $1,000, there is no penalty.
- No prior-year liability. If you owed no tax for the previous year, you were a US citizen or resident for the whole of it, and it covered twelve months, no penalty applies this year.
- Waivers. The IRS can waive the penalty for casualty, disaster, or other unusual circumstances, and for filers who retired after 62 or became disabled during the year.
Choosing between them
Run both and pay the smaller. If this year is clearly worse than last, the 90% current-year test will require less — but leave a margin, because falling short of 90% costs you the protection entirely. If this year is better than last, or simply unpredictable, the prior-year test is almost always the cheaper and the safer choice.
The calculator evaluates both tests against your figures for each quarter and shows which one you are currently passing.
Frequently asked questions
- What is the estimated tax safe harbor?
- A safe harbor is a payment level that protects you from the underpayment penalty no matter how much tax you end up owing. You are safe if your withholding and estimated payments for the year total at least 90% of this year’s tax, or at least 100% of last year’s total tax — 110% if your prior-year adjusted gross income was over $150,000 ($75,000 if married filing separately).
- Is it 100% or 110% of last year’s tax?
- It is 110% if your adjusted gross income on last year’s return was more than $150,000, or more than $75,000 if you file married filing separately. Otherwise it is 100%. The threshold is based on prior-year AGI, not current-year income.
- What counts toward the safe harbor?
- Both estimated tax payments and tax withheld from wages, pensions, retirement distributions and similar sources. Refundable credits also reduce the tax figure the safe harbor is measured against.
- Do I owe a penalty if I underpay by a small amount?
- No. If the total tax you still owe after withholding and credits is under $1,000, no underpayment penalty applies, even if you missed both safe harbors. You also owe no penalty if you had no tax liability at all in the prior year and that year covered twelve months.
- Does hitting the safe harbor mean I owe nothing in April?
- No. The safe harbor only protects you from the penalty. If you paid 110% of last year’s tax but this year’s tax is far higher, the shortfall is still due by the April filing deadline — you simply are not charged a penalty for having paid it late.