The Q3 estimated-tax deadline is Tuesday. If you have been putting off working out what to send, you have the weekend, and you need less of it than you think.

Four questions, in this order. Each one can make the next unnecessary, which is why the order matters more than the arithmetic.

Tax Disclaimer: This article provides educational information based on IRS Form 1040-ES, IRC § 6654 and IRS guidance on estimated taxes, and is not tax advice. Consult a certified CPA or tax professional for individual advice.

1. Do you owe a September payment at all?

Two tests dispose of a lot of people before any calculation starts.

The first is the floor. Estimated tax applies if you expect to owe $1,000 or more for 2026 after withholding and refundable credits. Below that, there is no underpayment penalty to avoid (26 U.S. Code § 6654(e)(1)).

The second is the one almost nobody checks. If your 2025 return showed no tax liability at all, you were a US citizen or resident for that whole year, and 2025 was a full twelve months, there is no underpayment penalty for 2026 whatever you end up owing (26 U.S. Code § 6654(e)(2)). You would still pay the tax in April. You would simply owe nothing in quarters.

Pull the 2025 return and look at the total tax line before you do anything else. It is one number and it can end the exercise. If this is your first year over a 1099-K threshold, the crossover guide covers who this exemption tends to catch.

2. If you do owe, how much is actually required?

Not what you think 2026 will produce. The smaller of two protected numbers.

The prior-year test is the easy one, because you look it up instead of forecasting it: 100% of the total tax on your 2025 return, or 110% of it if your 2025 AGI was over $150,000 (IRS Form 1040-ES). Divide by four. That is a figure the IRS cannot penalise you for, however good this year turns out.

The current-year test asks for 90% of your actual 2026 tax, which is worth computing only if 2026 is running below 2025. The 1099 tax estimator gets you that number in a few minutes; the safe harbor breakdown explains which of the three tests is yours.

Pay the smaller one.

3. Are you behind on April or June?

If you are, a large payment on Tuesday will not fix it, and it is worth knowing that now rather than being surprised in April. The penalty is figured separately for each installment due date, so an earlier shortfall keeps accruing even after you catch up (Instructions for Form 2210).

Two mechanisms reach backwards. If your income genuinely arrived late in the year, Schedule AI recomputes each installment from the income you had actually received by that date. And if there are wages in your household, extra tax withheld now is treated as paid in four equal parts across the year no matter when it was withheld (26 U.S. Code § 6654(g)) — which means a W-4 change this week can still cover April.

Both are laid out in the catch-up guide. If you are going to change a W-4, do it today rather than Monday; you want as many pay periods as the rest of the year allows.

4. How, and by when exactly?

There are two different clocks and most people only know about one.

If you already have an EFTPS account, your deadline is 8 p.m. Eastern on Monday the 14th, a full day before the date on the form. If you do not, use IRS Direct Pay, which needs no enrolment and whose same-day window closes at 8 p.m. Eastern on Tuesday the 15th.

Do not start an EFTPS enrolment this week hoping to beat either. The credential arrives by post.

One reassurance worth carrying into Tuesday evening: a payment submitted on time is on time even though the money leaves your account afterwards. The confirmation number is the receipt, not the bank statement. Methods, fees and the postmark rule for anyone mailing a cheque are in the payment guide.

And then check your state, because it is not the same question

State systems run their own thresholds and their own instalment schedules. Californians have nothing due on September 15 at all — the state’s third instalment is zero percent. New York starts requiring estimated payments at a fraction of the federal floor.

Neither of those follows from anything you worked out above. The state breakdown covers four of the largest.

If the money is not there

Send part of it. The charge for underpaying is interest running at an annualized 7% on whatever remains outstanding, not a flat penalty for being short, so a partial payment on Tuesday costs strictly less than the same payment in October. There is no threshold you drop below by sending nothing, and no advantage in waiting until you can send it all.

The bottom line

Work the four questions in order and most people find the answer before question three. Check last year’s total tax line first, because a zero there ends it. If it is not zero, the prior-year safe harbor gives you a number you can look up rather than forecast, divided by four. If you are behind, change the W-4 rather than reaching for a lump sum. And whatever you send, send it before 8 p.m. Eastern on the 15th — Monday the 14th if you are paying through EFTPS.

Then put April 15, 2026; June 15, 2026; September 15, 2026; and January 15, 2027 in a calendar, because the January instalment is the one people miss after getting September right.