How to Use This Tool
Enter last year's tax and this year's expected tax. The useful output is the smallest amount you can pay in during the year without a penalty.
The penalty is not for owing money
The underpayment penalty is charged for paying too little as you earn. The US tax system is pay-as-you-go, and the penalty enforces the timing rather than the total.
So you can end the year owing a very large balance and owe nothing extra for it, provided you met one of the safe harbours:
- 90% of this year's total tax. Requires knowing this year's tax, which is the problem with it.
- 100% of last year's total tax. A known number, available in January.
- 110% of last year's, if last year's AGI was over $150,000.
Meeting any one is enough. There is also a small-balance let-off: if you end up owing less than $1,000 after withholding and credits, no penalty applies regardless.
Why the prior-year rule is the useful one
If your income rose sharply, the prior-year safe harbour is based on the smaller old number. Last year $28,000 of tax, this year $52,000: paying in $30,800 — 110% of last year — protects you completely, and the remaining $21,200 is simply due in April with no penalty attached.
That is a real cash-flow benefit and it is entirely legitimate: the rule exists precisely because you cannot know a rising year's tax in advance. The catch is remembering that the $21,200 is still owed. A safe harbour prevents a penalty; it does not reduce the tax.
The reverse case matters too. If your income fell, the prior-year figure may be far more than you need to pay, and the 90%-of-this-year test is the cheaper one.
Timing, and the withholding trick
Estimated tax is quarterly, and the quarters are not equal calendar quarters. The payments are generally due in mid-April, mid-June, mid-September and mid-January of the following year, so the second "quarter" is two months long.
Crucially, an estimated payment counts when you make it. Paying everything in January does not cure three missed quarters; the penalty already accrued on each one.
Withholding is treated differently. Tax withheld from wages is deemed paid evenly across the year no matter when it was actually taken. So increasing withholding on a December payslip, or on a year-end bonus, can repair an underpayment from March that an estimated payment cannot touch. If you have any wage income at all, this is usually the cleanest fix late in the year.
Details that change the answer
- The penalty is interest, not a fine. It is computed at a rate the IRS sets each quarter, on the shortfall for the days it was short. A small, brief underpayment costs very little.
- Uneven income can be handled with the annualised income method, which matches payments to when the income arrived. It requires more paperwork and helps a great deal if your year is lumpy.
- The $150,000 threshold for the 110% rule is $75,000 if married filing separately.
- State estimated tax is separate, with its own rules and its own safe harbours.
Figures are for tax year 2026. This is arithmetic for planning, not tax advice.
