Q3 Estimated Taxes Are Due Sept 15. Size the Bill in August, Not the Night Before

Most estimated-tax stress is timing, not money. You know September 15 is coming, you have a vague sense you owe something, and you keep putting off the math because the math feels like it needs the whole quarter to be over first. Here is the quiet good news: by late August it basically is. The Q3 window covers June 1 to August 31, so with a couple of weeks left you can size the payment from numbers that are almost final, set the cash aside, and let the deadline pass without a scramble.

This is a prep post, not a panic post. Below is what the Q3 payment actually covers, the two honest ways to size it, and how to get to a number in an afternoon rather than at 11pm on the 14th.

What the September 15 payment is for

US estimated taxes are paid in four uneven chunks across the year. The third installment is due September 15, 2026, and it covers the income you earned in the third payment period, June 1 through August 31. (If a due date ever falls on a weekend or legal holiday, the next business day counts as on time.)

You owe estimated tax at all if you expect to owe $1,000 or more for the year after withholding and refundable credits. For a side hustle with no withholding, that threshold is easy to cross, so most people earning real self-employment income are in scope.

Two taxes ride on that income, and it helps to see them separately:

• Self-employment (SE) tax. This is Social Security and Medicare for the self-employed: 15.3% total (12.4% Social Security + 2.9% Medicare), applied to 92.35% of your net self-employment earnings. The Social Security portion only applies up to the 2026 wage base of $184,500; the Medicare portion has no cap. One useful softener: half of your SE tax is deductible above the line when you figure income tax.
• Income tax. Your net self-employment profit also gets taxed at your ordinary income rate, stacked on any other income, after deductions. Where it lands depends on your bracket.

That is the whole shape of the bill. SE tax first, income tax on top, paid in quarterly slices.

Two honest ways to size it

There are two legitimate ways to land on a Q3 number. Pick whichever matches how much certainty you want.

Option A: estimate the actual tax on the quarter

By late August you can total your Q3 net profit (income earned June to August, minus the business expenses against it) and estimate the tax directly:

1. Take your net Q3 profit.
2. Figure SE tax: multiply by 0.9235, then by 15.3%.
3. Figure income tax on the profit at your marginal rate (remember half the SE tax is deductible).
4. Add them, and that is roughly the Q3 slice.

This is the most accurate approach because it uses real numbers from a nearly complete quarter. It also rewards good expense capture: every legitimate deduction you have logged lowers the profit the tax is calculated on.

Option B: pay the safe harbor and stop worrying

You do not have to predict your full-year tax perfectly. The IRS gives a safe harbor: you avoid an underpayment penalty if your total estimated payments hit the smaller of 90% of this year's (2026) tax or 100% of last year's (2025) tax shown on your return. If your 2025 AGI was over $150,000 ($75,000 if married filing separately for 2026), the prior-year figure is 110% instead of 100%.

The practical move: take that annual safe-harbor target, divide by four, and pay a quarter of it on September 15. Using last year's return as the basis is the calmest option because the number is already known, you are not forecasting anything. You may owe a little more (or get a little back) at filing, but you are protected from the penalty either way.

Approach	Best when	What you need
Estimate actual Q3 tax (Option A)	Your income is uneven or much higher/lower than last year	Q3 income, Q3 expenses, your bracket
Safe harbor on prior year (Option B)	Last year was a normal year and you want certainty	Your 2025 total tax (and AGI to check the 110% rule)

General guidance, not tax advice; safe-harbor rules and thresholds depend on your situation, so confirm with your accountant.

Get to a number this week

You do not need a spreadsheet from scratch. The quarterly tax calculator is built for exactly this: set it to Q3, enter your net self-employment income for the period, and it walks through the SE tax and income-tax pieces for 2026, including the half-of-SE-tax deduction, and shows both the estimated-actual figure and the safe-harbor comparison. The point is to replace a vague dread with one concrete dollar amount you can move into a separate account today.

Two things make that number honest and lower:

• Count every Q3 expense. Tax is on profit, not revenue. Software, supplies, the business share of your phone, mileage records, contractor payments: each one you have captured reduces the profit the SE and income tax sit on. If your receipts are scattered, you will overstate profit and overpay.
• Do it while the quarter is fresh. Reconstructing August expenses in December is how deductions get missed. Late August is the cheapest possible time to total the quarter, because you still remember what each charge was.

If you are newer to this and the whole estimated-tax machine still feels foreign, the side hustle starter lays out the basics, when you cross into "you should be paying estimates" territory and what to track from day one. And if you want the deeper version of the safe-harbor logic, including the June deadline that kicks off the same cycle, do you owe the IRS by June 15 covers the same rules from the Q2 angle.

Frequently asked questions

When are Q3 estimated taxes due in 2026? September 15, 2026. The payment covers income earned in the third period, June 1 to August 31. If the date falls on a weekend or legal holiday, the next business day counts as on time.

How much should I pay for Q3? Either estimate the actual tax on your Q3 net profit (SE tax of 15.3% on 92.35% of earnings, plus income tax at your rate), or pay one quarter of your annual safe-harbor target: 90% of this year's tax or 100% of last year's tax (110% if 2025 AGI was over $150,000). The quarterly tax calculator does both for you. This is general guidance, not tax advice.

What income does the Q3 payment cover? The third estimated-tax period runs June 1 through August 31. By late August it is nearly complete, which is why you can size the payment accurately before the September 15 deadline instead of guessing.

Do I really have to pay estimated taxes on a side hustle? You generally must pay estimated tax if you expect to owe $1,000 or more for the year after withholding and credits. A side hustle with no withholding crosses that quickly, so most people earning meaningful self-employment income are in scope. Confirm your situation with your accountant.

What if I skip the Q3 payment? Underpaying or skipping a quarter can trigger an underpayment penalty, calculated per period, even if you settle up at filing. Paying at least your safe-harbor amount on time is what protects you, which is why the prior-year safe harbor is the low-stress default.

Can deductions lower my Q3 estimated tax? Yes. Estimated tax is figured on net profit, not gross income, so every legitimate business expense you have captured for the quarter reduces the amount the SE and income tax are calculated on. Capturing expenses while the quarter is fresh is the easiest way to keep the number honest.

Start free, no credit card

Sizing Q3 in August turns a deadline into a checkbox. Run the quarterly tax calculator in Q3 mode, move the cash into a separate account, and September 15 stops being a date you dread.

The number is only as good as the expenses behind it, though, and that is the part scan-ai helps with. It reads each line on your receipts, maps it to the right tax line, and lets you ask "how much did I spend on software this quarter?" in plain language, so your Q3 profit (and the tax on it) reflects every deduction you actually earned. Start free with 20 receipt scans and 5 AI chats, no credit card, and walk into the deadline with your numbers already done.

This article is general information, not tax advice. Tax rules change and depend on your specific situation. Confirm anything with real stakes with a qualified accountant or tax professional.