The safe harbor rule is the reason you do not have to predict your exact tax bill to avoid a penalty. The IRS gives you a target based on a number you already know, last year's tax, and says: hit this, and we will not charge you an underpayment penalty regardless of what you owe when you file in April.
That target is either 100% or 110% of last year's tax. The 110% version catches people off guard, so let us go through it carefully.
The two safe harbor tests
You only need to pass one of these, not both. Aim for whichever is lower.
| Test | What you pay in | Best when |
|---|---|---|
| Current-year test | 90% of this year's actual total tax | Income is flat or down from last year |
| Prior-year test | 100% of last year's total tax (110% if last year's AGI was over $150,000) | Income is rising, since last year's number is fixed |
There is also a small-taxpayer rule: if you owe less than $1,000 after withholding and credits, there is no penalty at all. And the payments must actually be made by each quarterly deadline; the safe harbor is a total target, but each installment still has its own due date.
Who pays 110% instead of 100%
The higher threshold applies when your prior-year adjusted gross income exceeded $150,000, or $75,000 if you are married filing separately. AGI is the number on line 11 of your Form 1040, not your salary and not your taxable income. This trips people up because AGI includes things like capital gains and business income, so you can cross $150,000 in a year your W-2 wages alone never would have.
Note that it is the prior year's AGI that matters, not this year's. If your AGI was $140,000 in 2025, you get the 100% test for 2026 even if you earn $400,000 this year. The whole point of the prior-year test is that it is based on a finished, filed return.
A worked example
You filed your 2025 return. Form 1040 line 24, "total tax," shows $22,000. That is your liability after credits but before withholding, the number that drives safe harbor.
Two freelancers, same tax, different AGI
Maya and Dev owed identical tax last year, but Dev's higher AGI means he must pay $550 more per quarter to be safe. The difference is invisible on the payment voucher; it only matters for whether the penalty applies.
Now the payoff. Suppose Maya's 2026 income doubles and her actual 2026 tax comes out to $41,000. She paid $22,000 during the year. She still owes $19,000 in April, but she owes no underpayment penalty, because she cleared the prior-year safe harbor. The safe harbor protects you from the penalty, not from the balance itself. That is the deal, and for a freelancer with a breakout year it is an excellent one.
When to use the 90% current-year test instead
The prior-year test is not always the cheapest option. If this year is shaping up well below last year, the prior-year target ties up cash you do not need to send. Say your 2025 tax was $22,000 but your 2026 tax will only be about $12,000. The prior-year test demands $22,000 (or $24,200); the current-year test demands only $10,800, which is 90% of $12,000. In a down year, estimate this year's tax and aim for 90% of it.
The catch: the current-year test requires predicting a year that has not finished. In a rising year, that is risky. The prior-year number is known the moment last year's return is filed, which is why most freelancers default to it. My rule: rising or uncertain income, use prior-year. Falling income with good visibility, use the 90% test.
How to find your number
- Pull last year's Form 1040. Find line 24, "total tax." That is your liability, not your refund or your balance due at filing.
- Check line 11, AGI. Over $150,000 ($75,000 if married filing separately)? Multiply line 24 by 1.10. Otherwise multiply by 1.00.
- Divide by 4. That is each quarterly payment.
- Compare against 90% of this year's projected tax if you have a down year. Pay whichever is lower.
Frequently asked questions
Where exactly is "total tax" on my return?
Form 1040, line 24. It is your tax liability after nonrefundable credits, before any withholding or estimated payments are subtracted. It is not the refund or the amount you owed when you filed; those are lines 34 and 37.
Does the 110% rule apply to state estimated taxes?
No. This is a federal rule. States have their own underpayment rules and their own safe harbors, and they vary widely. Check your state's revenue department before assuming the same math applies.
What if I owed zero tax last year?
Then 100% of zero is zero, and you are in safe harbor as long as you make the required timely payments, effectively any positive amount each quarter. This is common for people who had no tax liability in their first year of freelancing.
Does safe harbor mean I will not owe anything in April?
Not necessarily. Safe harbor protects you from the underpayment penalty. If your actual tax exceeds what you paid in, you still owe the balance when you file. A great year can still mean a large check in April, just without a penalty attached.