The third-quarter estimated tax payment is due on 15 September 2026, covering income earned between 1 June and 31 August.
Most people who owe it spend the week beforehand doing the same futile exercise: trying to forecast a full year of unpredictable income in order to work out what to send. There is a rule that makes that unnecessary, and it is the most useful thing in the tax code for anyone with variable earnings.
The safe harbour
You avoid the underpayment penalty by satisfying any one of these. Not all three — any one.
| Threshold | What it requires |
|---|---|
| Under $1,000 | You will owe less than $1,000 after withholding and credits |
| 90% rule | Pay 90% of this year’s tax — requires a forecast |
| Prior-year rule | Pay 100% of last year’s tax — requires no forecast at all |
If your prior-year adjusted gross income exceeded $150,000 (or $75,000 married filing separately), that last row becomes 110% of last year’s tax.
Why the prior-year rule is the one that matters. Last year’s tax is a number you already have — it is printed on the return you filed. Divide it by four, pay that each quarter, and the underpayment penalty cannot reach you even if you triple your income this year. You will still owe the extra tax in April; you simply will not be penalised for not having predicted it. For anyone whose income arrives in unpredictable lumps — contractors, freelancers, anyone with a good quarter and a bad one — this converts an impossible forecasting problem into arithmetic you can do in ten seconds.
What the payment actually covers
The quarters are not equal calendar quarters, which surprises people. The third-quarter payment covers June, July and August — three months, due two weeks after the period closes.
That compression is why September catches people out. The money earned in August is due before most freelancers have chased the invoices for it.
How the penalty works
It is not a flat fine. It is calculated on how much was underpaid and for how long, at an interest rate the IRS sets.
Two consequences follow. First, a small shortfall paid soon after the date is a small penalty rather than a disaster. Second, it accrues per period — missing 15 September starts the clock in September, not in April. Paying everything in one lump in January does not cure the earlier quarters.
If cash is short this month
- Pay something. The penalty scales with the shortfall, so a partial payment reduces it proportionately.
- Pay as soon as you can rather than waiting for the next quarter. The clock runs on days.
- If you have a job as well as freelance income, increasing withholding through your employer is treated as paid evenly across the year — which can retrospectively cover earlier quarters in a way a direct payment cannot.
That third point is genuinely useful and almost never mentioned: withholding and estimated payments are not treated the same way by the calculation.
The third-quarter estimated tax payment for 2026 is due on 15 September 2026, covering income earned between 1 June and 31 August 2026. Safe harbour thresholds described — owing less than $1,000 after withholding and credits, paying at least 90% of the current year’s tax, or paying 100% of the prior year’s tax, rising to 110% where prior-year adjusted gross income exceeded $150,000 or $75,000 for married filing separately — are set out in IRS guidance including Form 1040-ES and Form 2210 instructions, and were summarised by Kiplinger, NerdWallet, H&R Block and TaxAct. The underpayment penalty is calculated by reference to the amount and duration of underpayment at an interest rate determined by the IRS. This article is general information about published rules and is not tax advice; individual circumstances vary, state requirements differ, and anyone unsure of their position should consult a qualified tax professional or the IRS directly.
Frequently Asked Questions (FAQ)
When is the next estimated tax payment due?
15 September 2026. It is the third-quarter payment and covers income earned from 1 June through 31 August 2026.
Who has to make estimated payments?
People with income that has not had tax withheld from it — typically the self-employed, freelancers, business owners, landlords and some investors. If your income arrives with tax already deducted, this generally does not apply to you.
What is the safe harbour rule?
A set of thresholds that protect you from the underpayment penalty regardless of what you actually end up owing. Meeting any one of them is enough: owing under $1,000 after withholding and credits; paying at least 90% of this year’s tax; or paying 100% of last year’s tax.
What changes if I earned more than $150,000 last year?
The prior-year threshold rises from 100% to 110%. If your previous-year adjusted gross income exceeded $150,000 — or $75,000 if married filing separately — you must pay the lower of 90% of this year’s tax or 110% of last year’s.
What is the penalty if I miss it?
An underpayment penalty calculated on how much was underpaid and for how long, at an interest rate set by the IRS. It is charged per period, so a late third-quarter payment accrues from 15 September rather than from next April.
Can I just pay it all in January instead?
Not without a penalty. Estimated tax is due in instalments across the year, so paying the full amount late still leaves the earlier periods underpaid.
