The Q3 Estimated Tax Deadline Is September 15. Here Is What You Need.

Third-quarter estimated taxes are due Tuesday, September 15, 2026. The quarter covers income you earned from June 1 through August 31 — which, despite the name, is three months rather than the four-month stretch the calendar might suggest.

If you are self-employed, contracting, driving, renting property, or running anything that does not withhold tax for you, this is one of four dates that decide whether you owe a penalty next April. Here is what to send and how to work out the number without a full afternoon of arithmetic.

Do you actually owe this quarter?

The general rule: you need to make estimated payments if you expect to owe at least $1,000 in tax for the year after subtracting withholding and refundable credits.

Two situations catch people out:

You have a W-2 job as well. Withholding from that job counts toward your total. If it is large enough to cover the tax on your side income too, you may owe nothing quarterly. Many people in this position over-pay estimates they never needed to make.

You had a big Q3. Income is not assumed to be spread evenly across the year. A contractor whose summer is triple their winter genuinely owes more for this quarter, and paying one-quarter of an annual estimate will leave them short.

The safe harbor is the number to know

You do not have to predict this year’s tax perfectly. The IRS provides a safe harbor: pay enough and you avoid the underpayment penalty regardless of how the year actually turns out.

The prior-year route is far easier, because last year’s number is already known. Take the total tax from your prior-year return, apply 100% or 110%, divide by four, and that is your quarterly payment. You are not guessing at anything.

The 90%-of-current-year route pays less if you are having a worse year than last year. It costs more effort, since you have to estimate the current year with reasonable accuracy.

Working out the number for a real quarter

If you are taking the current-year route, the shape of the calculation is this:

1. Total your Q3 income — everything received June 1 through August 31.
2. Subtract your Q3 deductible expenses. This is the step that decides how much you send, and the one most people under-do.
3. Apply self-employment tax of 15.3% to the net, covering both halves of Social Security and Medicare.
4. Apply income tax at your marginal rate on the net.
5. Subtract any withholding from a W-2 job or other source.

Step two is where the money is. Every legitimate expense you failed to capture is an amount you pay tax on unnecessarily, at a combined rate that for many self-employed people lands north of 30%. A summer of uncaptured fuel and materials receipts is not a rounding error.

If your receipts are already in one place, this is a lookup rather than a project. Ask for your June-through-August totals by category, subtract, and you have step two done. If they are in a glovebox, a truck console, and three email accounts, budget an evening — and then fix the process before December, because the Q4 and annual versions of this are worse.

How to pay

Fastest is IRS Direct Pay at irs.gov — bank transfer, no account required, no fee, immediate confirmation. Select “Estimated Tax” and the correct tax year, which is the field people most often get wrong. A payment posted to the wrong year creates a credit sitting in one year and a penalty in the other.

Also available: EFTPS (requires enrollment ahead of time, so not useful if you are reading this in September), card payment through an approved processor (works, but carries a processing fee of roughly 2%), or a paper 1040-ES voucher by mail, postmarked by the deadline.

Do not forget state. Most states with an income tax run their own estimated payment schedule, often on the same date. Paying federal and forgetting state is one of the more common ways to get a surprise letter.

What happens if you miss it

Less catastrophic than people fear, but not free. The underpayment penalty is effectively interest on the amount you were short, running from the deadline until you pay. It accrues per quarter, which produces the outcome that surprises people: you can be owed a refund for the year overall and still be penalized for a quarter you underpaid.

The practical implication is that a late payment still beats a skipped one. If you cannot pay the full amount by the 15th, pay what you can on the 15th. The penalty scales with both the shortfall and the days it stays outstanding, so partial and prompt is meaningfully cheaper than complete and late.

Setting up so December is easier

Three habits separate the people for whom this is a ten-minute task from those for whom it is a lost weekend.

Capture receipts when you get them. Not weekly, not monthly. The receipt in your hand has about a minute before it becomes a receipt in your pocket, and the ones in your pocket do not make it into the deduction column.

Move a percentage as income arrives. A separate account, 25–30% of every payment received, moved the day it lands. Then the quarterly payment is a transfer rather than a cash-flow event.

Put all four dates in the calendar now. January 15, April 15, June 15, September 15. They do not move much year to year, and the reminder costs nothing.

The Q4 payment is due January 15, and the annual return lands after that. Whatever you fix between now and then gets used twice.