Quarterly Estimated Taxes: Safe Harbors, Due Dates and the 2026 Penalty Rate

September 15, 2026 is two deadlines, not one

Two separate obligations land on the same date, and missing either one carries its own penalty. **Q3 estimated tax for 2026.** The third installment of your 2026 estimated tax is due, on Form 1040-ES. This is a payment against the year you are currently living in. **Extended 2025 partnership and S-corp returns.** If a calendar-year partnership or S corporation filed Form 7004 back in March for an automatic six-month extension, Form 1065 or Form 1120-S is due now, along with each partner's or shareholder's Schedule K-1. Late filing here is charged per partner or shareholder per month, not as a percentage of tax, so it accrues quickly on a multi-owner entity even when no tax is owed. The two interact in a way that catches people out. A K-1 arriving on September 15 is what finalizes your **2025** income, and your 2025 tax is the number your 2026 prior-year safe harbor is measured against. So an owner waiting on a late K-1 may not know their own safe-harbor target until the same day the Q3 payment against it falls due. Where that happens, pay against a conservative estimate of 2025 tax rather than skipping the installment, because interest runs from the due date and not from the date the K-1 turned up. The extended individual return (Form 1040 or 1040-SR) for 2025 is due a month later, on October 15, 2026.

Who has to pay quarterly, and who is exempt outright

Two conditions must both be met. You expect to owe at least $1,000 in tax for 2026 after subtracting withholding and refundable credits, and you expect withholding plus refundable credits to be less than the smaller of your two safe harbors. Common profiles: sole proprietors, single-member LLCs, partners, S-corp shareholders, gig and platform workers, landlords, active traders, retirees taking large required minimum distributions, and W-2 employees with substantial side income or a one-off capital gain. If you are a household employer, include household employment taxes when estimating your 2026 tax, provided you will already have income tax withheld somewhere or would have to make estimated payments anyway. **The first-profitable-year exemption.** Under §6654(e)(2) no penalty applies at all, whatever you paid, if your 2025 tax year was a full 12 months, you had no tax liability for 2025, and you were a US citizen or resident throughout it. Someone whose business made nothing in 2025 and is having a strong 2026 owes no estimated tax penalty for 2026 at all. They will owe the tax at filing, but the §6654 charge cannot apply. This is the single most commonly missed exemption among people in their first profitable year.

The three safe harbors

You only have to clear one. The penalty is measured against whichever required annual payment is smaller. **90% of current-year tax.** Total payments of at least 90% of the tax shown on your 2026 return. Useful when income is falling, because it stops you prepaying against an inflated prior year. **100% of prior-year tax.** Total payments of at least 100% of the tax shown on your 2025 return, which must cover all 12 months. The planning favorite, because the target is a fixed number the moment your 2025 return is filed. Your 2026 income can then triple without creating a penalty. **110% of prior-year tax.** The same test, raised, if your 2025 AGI was more than $150,000, or more than $75,000 where your 2026 filing status is married filing separately. Note the threshold is tested on the prior year's AGI, not the current year's. The deliberate consequence of the prior-year harbor is that a very good 2026 costs you nothing in penalty terms as long as you paid against 2025. You will owe the balance at filing, but no penalty accrues on it before then.

The 2026 penalty is 7% interest, not a flat fine

The §6654 charge is not a fixed percentage of the shortfall. It is interest, set under §6621 as the federal short-term rate plus 3 percentage points, reset every quarter and compounded daily. It runs from each missed installment date until the earlier of the date you pay or the filing deadline. For 2026 the federal short-term rate is 4%, so the underpayment rate is **7% per year** for both the quarter beginning July 1 and the quarter beginning October 1, 2026. Because it is a daily-compounding interest charge rather than a penalty on the whole year, a shortfall paid a few weeks late costs a few weeks of interest. This is also why paying something late still beats paying nothing: interest stops accruing on the part you have paid. There is no reasonable-cause waiver for §6654 in the way there is for late filing. The relief that does exist is narrow, covering casualty, disaster or other unusual circumstances, retirement after reaching 62, or disability.

Withholding is credited a quarter to each due date, which is how you fix a late underpayment

This is the most useful and least known rule in the whole regime, and it is the reason a missed Q1 or Q2 payment is often still repairable in the autumn. Under §6654(g), tax withheld from wages is deemed paid in four equal parts, one on each installment due date, regardless of when it was actually withheld. Withhold an extra $8,000 from a December paycheck and the statute treats $2,000 of it as having been paid on April 15, $2,000 on June 15, $2,000 on September 15 and $2,000 on January 15. So a taxpayer with any W-2 income, including an S-corp owner on their own payroll, can file a new Form W-4 late in the year, push withholding up, and retroactively cure an underpayment that estimated payments alone could not fix. An estimated payment made in December only counts in December. Withholding made in December counts across the whole year. The rule cuts both ways and is a default, not a mandate. If your withholding was actually front-loaded and the even-quarters assumption hurts you, you may establish the dates the amounts were genuinely withheld instead, by checking box D in Part II of Form 2210 and attaching the form to your return.

Annualized income installment method (Form 2210, Schedule AI)

If income arrives unevenly, a Q4 commission, a single capital gain, a project that only started in July, the default assumption that you owed a quarter of the year's tax by April 15 over-penalizes you. The annualized income installment method under §6654(d)(2) lets you allocate income to the period in which it was actually earned, so each installment is recalculated against year-to-date income. It is materially more compliance work and requires Schedule AI, but it frequently removes an early-quarter penalty entirely for seasonal and lumpy earners.

Farmers and fishermen

If at least two-thirds of your gross income for either 2025 or 2026 comes from farming or fishing, a separate regime applies. The 90% current-year test drops to 66 2/3%, and the 110% high-income rule does not apply to you at all regardless of AGI. The payment schedule differs too: the obligation can be met with a single installment by January 15, 2027, and filing and paying in full by March 1, 2027 removes the estimated tax penalty for the year.

How to pay

IRS Direct Pay for free ACH transfer from a bank account, your IRS Online Account, EFTPS for free ACH and the most reliable option for recurring business payments, debit or credit card through an approved processor where fees apply, or a paper voucher mailed with Form 1040-ES. Schedule payments in advance rather than relying on remembering the date. EFTPS allows scheduling up to a year ahead, which suits a fixed prior-year safe harbor target well, because the four amounts are known and identical.

State estimated tax runs separately

Most states with an income tax operate their own estimated tax regime with their own safe harbors, and clearing the federal test does not clear the state one. Due dates commonly mirror the federal April, June and September dates, but fourth-quarter dates and safe-harbor percentages vary, and a handful of states use a different schedule altogether. Some states conform to the federal safe harbor and impose no separate penalty where it is met. Check the rules for each state you have a filing obligation in, which for multi-state owners may be several.

Statute references

• Failure by individual to pay estimated income tax — IRC §6654
• Amount of required installments and the safe harbors — IRC §6654(d)(1)
• Annualized income installment method — IRC §6654(d)(2)
• Withheld tax deemed paid in equal parts on each due date — IRC §6654(g)
• Determination of the interest rate, short-term rate plus 3 points — IRC §6621