The September 15 Last-Minute Estimated-Tax Payment Guide

Updated: August 2026 · Industry Advisors · Education, not legal or tax advice.

It's September 14 or 15. Q3 estimated tax is due today or tomorrow. This is the 15-minute execution guide — no theory, just where to click, what to enter, and how to confirm.

Step 1 · Compute (3 Minutes)

1. Open last year's Form 1040.
2. Line 24 = your Total Tax. Call it X.
3. If prior-year AGI > $150K: multiply X × 1.10. Otherwise X × 1.00.
4. Divide by 4 = each quarterly payment.
5. Subtract what you already paid in Q1 (April) and Q2 (June). Remainder is what to pay today.

Full explanation of the math is in the safe-harbor one-number post. The September 15 deadline overview covers who owes.

Step 2 · Pay Federal (5 Minutes)

1. Go to irs.gov/payments.
2. Click Make a Payment → Bank Account (Direct Pay).
3. Reason: Estimated Tax. Apply payment to: 1040ES. Tax year: 2026.
4. Verify identity with a prior return (drop-down asks which year's return to match against).
5. Enter routing + account. Enter payment amount. Submit.
6. Screenshot the confirmation number.

If you're paying by card, use one of the IRS-authorized processors (fees 1.75–1.99%).

Step 3 · Pay State (5 Minutes)

Log in to your state's estimated-tax portal, select Q3 (or September 15 payment), enter the amount, submit. Screenshot the confirmation. If you moved states this year, check with a licensed preparer — you may owe part-year to both.

Step 4 · Archive (2 Minutes)

Create a folder "2026 Q3 Estimated Tax" and drop in:

• Federal confirmation screenshot
• State confirmation screenshot
• The math you used (a scan of the calculation on paper or a 3-line email to yourself)

Update bookkeeping: post the federal payment to Estimated Tax Payments — Federal, the state payment to Estimated Tax Payments — State. Category matters because these are prepayments, not deductions.

If You Cannot Pay The Full Amount Today

Pay whatever you can. The underpayment penalty is calculated per day the shortfall exists. A partial payment reduces the accrual immediately. Better than skipping.

If you're facing a cash shortage that will not resolve by year-end, consider whether:

• Revising your safe-harbor route to 90% of current-year estimated tax (may be lower if income dropped)
• Talking to a CPA about a Q4 catch-up rather than a Q3 catch-up

Common Last-Minute Mistakes

• Paying on Form 1040 balance due instead of Estimated Tax. IRS Direct Pay defaults to "Extension" on the wrong day of year — check the reason selector.
• Wrong tax year. Payments applied to 2025 do not help your 2026 safe harbor.
• ACH bounced. Insufficient funds in the linked account triggers a $75 IRS fee plus penalty accrual. Verify balance before submitting.
• Missing the state. Federal payment does not satisfy state obligations. Both must land the same day.

Where This Connects

Q3 is one of four quarterly checkpoints on the Q4 compliance calendar. If Q3 goes clean, the next scheduled action is S-Corp election review in November. If you had a big income spike this year, adjust the January 15 Q4 payment accordingly — the year-end housekeeping checklist covers the recalibration.

Sources And Further Reading

• IRS · Estimated Tax FAQs
• EFTPS · Electronic Federal Tax Payment System

The Everyday Owner's Blueprint includes the 12-month tax and compliance calendar with every quarterly deadline pre-mapped.

Industry Advisors · 30 Wall Street, New York, NY 10005. Education, not legal or tax advice.