Q3 Estimated Taxes: What to Send by Sept. 15

The third estimated payment is the one people skip. April had the filing deadline attached to it, so it got attention. January will have the year-end panic behind it. September sits in the middle of a normal working month with nothing else on the calendar, and the payment quietly does not happen.

That is a shame, because by September you finally have real information. Eight months of income are behind you. You know whether the year is going the way you planned in March. The Sept. 15 payment is the first one you can size using facts rather than a forecast, and it is the last chance to fix a first half that came in light before the interest charge grows into something worth noticing.

What the Sept. 15 payment is, and what it is not

Estimated tax confuses people because it looks like a separate tax with its own rules. It is not. It is a prepayment against the single federal tax bill you will calculate on your 2026 Form 1040 next spring, sent in four installments because nobody is withholding for you. Employees do the same thing invisibly, one paycheck at a time.

Two consequences follow from that. If you overpay in September you get the money back as a refund, so an overshoot costs you the use of the cash and nothing else. And if your projection changes in November, the December quarter can absorb it. Nothing about the September number is permanent.

The cheapest correct number is usually sitting on last year's return

There are two ways to land in safe harbor, meaning the IRS charges you no underpayment interest even if you still owe a pile in April. You can pay 90% of what you will owe for 2026, or you can pay 100% of what you owed for 2025. If your 2025 adjusted gross income was above $150,000 ($75,000 if married filing separately), the prior-year route requires 110% instead.

The second route has an enormous practical advantage: the number is already known. Pull up your 2025 Form 1040, find total tax on line 24, divide by four, and you have an installment amount that cannot be wrong. Projecting 2026 requires you to guess at a fourth quarter that has not happened.

The prior-year route is especially useful in a year that is going well. If 2026 is running 40% ahead of 2025, safe harbor lets you pay against the smaller old number all year and settle the difference in April, interest free. That is a real cash flow benefit, as long as you actually set the difference aside instead of spending it.

Dani's year, projected forward

Dani is a freelance illustrator filing single. In March she projected $56,000 of net profit for 2026 and sent $2,800 in April and again in June. Two big retainer clients showed up in May, and she now expects to finish the year at $72,000. Here is the full-year math on that projection.

That is the honest number, and if the cash is there it is the one to send. It leaves nothing to make up in January and no interest accruing. If the mechanics behind those figures are unfamiliar, the self-employment tax explainer walks through why the 15.3% headline overstates the real cost.

The same year, run backward from her 2025 return

Now suppose Dani cannot spare $5,502 in September. The retainers pay net 45, the money is real but not in the account yet, and she needs a number that keeps the IRS off her back without draining the buffer.

The trade is straightforward. The $1,750 route protects her from underpayment interest, but she will owe roughly $5,000 with her return in April 2027, and that bill is real. Safe harbor moves the money, it does not cancel it. Anyone taking this route should be moving the difference into a separate account on the same day, which is the argument the emergency fund article makes about keeping the tax reserve away from the operating balance.

If your income arrived unevenly this summer

The four-equal-installments rule assumes income shows up in a flat line, which for seasonal work it does not. A wedding photographer earns most of the year between May and September. A tax preparer earns it between January and April. Paying a flat quarter of the annual number in April, when almost nothing has come in yet, can mean floating the IRS a loan for eight months.

One related point that trips up people with a day job: federal withholding from a W-2 is treated as paid evenly across all four installment periods, regardless of when it was actually withheld. That means a December bonus with heavy withholding can retroactively cover a spring shortfall. If you have wages alongside your freelance income, the W-2 plus side hustle walkthrough covers how the two interact.

What paying late actually costs

The underpayment charge is not a fine. It is interest, computed at the federal short-term rate plus three percentage points and compounded daily. For the quarter beginning July 1, 2026 the rate for individuals is 7%, and the IRS announced on Aug. 21 that it stays at 7% for the quarter beginning Oct. 1.

Two takeaways from those numbers. First, the charge is proportional, so a partial payment reduces it proportionally. If you can send $1,200 of a $3,000 installment, send the $1,200. There is no minimum and no penalty for paying an odd amount. Second, the total is modest enough that it should never be the reason you skip a payment out of dread. It is a nuisance charge, not a disaster, and the IRS computes it for you on Form 2210 rather than billing you a surprise.

How to send it in five minutes

Go to IRS Direct Pay, which draws from a checking or savings account with no fee and no account signup. Choose "Estimated Tax" as the reason, "1040ES" as the form and 2026 as the tax year. That last field is where the mistakes happen: selecting 2025 applies your money to a year that is already closed, and untangling it takes months.

The system verifies your identity against a prior return, so have a recent 1040 handy for the filing status and address it asks about. Save the confirmation number. If you would rather schedule payments in advance, EFTPS lets you queue all four installments up to a year out, though enrollment takes several days because it mails you a PIN. Card payments work too and carry a processor fee of roughly 2%, which is a poor trade unless you are chasing a signup bonus that beats it.

State estimated payments are separate, run on their own portals and in a few states on their own dates. If you owe state income tax on this income, handle it the same week so it does not slip.

What this sets up for January

Once the payment clears, spend 10 minutes on three things while the numbers are in front of you. Write down your year-to-date net profit, because the fourth installment on Jan. 15, 2027 will be sized off it and you will not want to reconstruct it in a holiday week. Compare that figure to the projection you used in March: if you are more than 20% off in either direction, redo the annual estimate now rather than in December. And check whether you are on pace for whatever retirement contribution you planned, since a solo 401(k) has to exist by Dec. 31 even though the funding deadline runs later.

The habit matters more than any single payment. Freelancers who treat the four dates as fixed obligations, the way rent is fixed, almost never end up with a tax problem. The ones who treat each date as a decision eventually make the wrong one in a tight month. Two installments remain in this cycle. Put Jan. 15 on the calendar today, with the amount you expect to send, and September stops being the payment people skip.