Key takeaways
- The IRS third-quarter calendar: individuals pay the third installment of 2026 estimated tax on Sept. 15, 2026 using Form 1040-ES. Tuesday, not a weekend, not a federal holiday. No automatic extra day.
- The period is June 1–Aug. 31, not “the third calendar quarter” in the payroll sense. January–March was April 15. April–May was June 15. September–December is Jan. 15, 2027 (or skip that voucher if you file the 2026 return by Feb. 1, 2027 and pay in full — 1040-ES instructions).
- You generally must pay estimates if you expect to owe $1,000+ after withholding and refundable credits, and withholding will be under the smaller of 90% of 2026 tax or 100% of 2025 tax (110% if 2025 AGI was over $150,000; $75,000 if married filing separately for 2026). Source: 2026 Form 1040-ES.
Estimated tax September 15 is the search because people treat “quarterlies” like rent on the 15th of every third month and then miss that the IRS skipped July. There is no July 15 individual 1040-ES. If you freelance, take a K-1, sell a rental, or cash a big 1099 in June–August and you only withhold from a W-2, this voucher is the one that usually bites.
What Sept. 15 is
Official line from the IRS small-business third-quarter calendar: “Individuals: Pay the third installment of your 2026 estimated tax — Use Form 1040-ES.” Same date: calendar-year S corps and partnerships that took a six-month extension file 1120-S / 1065. Corporations deposit their third estimated installment (Pub. 542 / EFTPS). Do not mix those piles. This piece is the individual 1040-ES.
You can pay the whole year on April 15. Most people split four ways. Paying late on one installment can still draw a penalty even if April 2027 shows a refund. That is §6654 / Form 2210 territory, not “the IRS will understand.”
Who has to pay
1040-ES general rule — both must be true:
- You expect to owe at least $1,000 for 2026 after subtracting withholding and refundable credits.
- Withholding and those credits will be less than the smaller of 90% of 2026 tax or 100% (or 110%) of 2025 tax, with 2025 covering a full 12 months.
W-2 employees with no side income usually never see this form. A new W-4 with extra withholding can replace a voucher if the extra dollars actually hit the 941. Gig, farm (special rules), fishing (special rules), partnership, S-corp K-1, taxable Social Security plus IRA withdrawals, and a one-time capital gain are the usual 1040-ES population.
Student-loan RAP payments do not count as estimated tax. Different agency, different form: RAP student loans.
The 90 / 100 / 110 harbor
Penalty avoidance is the lesser of:
- 90% of the tax that will show on the 2026 return, or
- 100% of the tax on the 2025 return — unless 2025 AGI was more than $150,000 ($75,000 MFS for 2026), then use 110% of 2025 tax.
Farmers and fishers: the 110% swap does not apply the same way; see Pub. 505. Do not copy a blog’s $150,000 line onto a Schedule F year without reading that chapter.
Worked sketch, not your return: 2025 tax $40,000, 2025 AGI $120,000. Prior-year harbor is $40,000. Four equal installments $10,000, counting withholding that already happened. If 2025 AGI was $180,000, prior-year harbor is $44,000. If 2026 will be a down year, 90% of 2026 may be cheaper than 110% of 2025. Run both. The worksheet is in the 1040-ES PDF, not in a TikTok calculator.
HSA contributions lower AGI if you have an HDHP. That can move a 110% cliff. It is still a health account first: HSA contribution limits for 2026.
How to pay without a fee trap
Free path: IRS.gov/Payments — Direct Pay from a U.S. checking or savings account, or an IRS Online Account. Direct Pay’s own page: personal estimated tax is in scope; $10 million cap is a business/other limit, not your 1040-ES.
EFTPS: Treasury still runs it. IRS now says new individual enrollments are closed; existing individual users can still pay “for now” and are being pushed to Online Account / Direct Pay, with a later-2026 cutover flagged on EFTPS.gov. Do not start a five-day EFTPS enrollment on Sept. 14. The third-quarter calendar still warns EFTPS users: schedule by 8 p.m. ET at least one calendar day before the due date.
Credit-card processors (PAY1040, ACI, etc.) charge a fee. That fee is not estimated tax. Mailing a check with a 1040-ES voucher still works; the IRS has to receive it by the 15th, not “postmarked whenever.”
Cash sitting for the voucher can live in a HYSA until Direct Pay pulls it. That rate is not a tax credit: I bonds vs high-yield savings.
Uneven income and Form 2210
Equal quarters assume income showed up evenly. A August asset sale with nothing in Q1 is how people overpay April and still get tagged later, or underpay June and owe a penalty that 2210 Schedule AI (annualized income installment method) would have shrunk. That schedule is extra math. It is the legal way to match the required installment to when the money actually arrived.
Withholding is treated as paid equally through the year unless you prove otherwise. Extra W-2 withholding in December can still cover an underpaid September voucher for penalty purposes. That is a feature. It is also how people raid a December paycheck to fix a June miss.
What this date is not
Not the 2026 Form 1040 due date (April 2027). Not the Social Security COLA announcement (that uses Q3 CPI-W, a different formula): Social Security COLA 2027. Not NVIDIA’s Aug. 26 print. Not a state estimated-tax holiday — many states also want money on the 15th; check your DOR.
Putting the IRS pull on a credit card to “keep the HYSA intact” is how utilization jumps on the statement the bureaus see: credit utilization ratio.
Pay the voucher. Keep the confirmation number. If you cannot pay, still file later; estimates and the annual return are different clocks. Penalty interest accrues from the installment date. That is in Pub. 505, not a rumor.
Education, not tax advice. Thresholds, Direct Pay, and EFTPS rules change. Use IRS.gov, Form 1040-ES, and Publication 505. This is not a recommendation of how much to send or which harbor to pick.
