September 2026 · Taxes, Small Business

Quarterly Estimated Taxes: Who Pays, How Much, and When

The third payment of 2026 is due September 15, and the safe harbor turns the amount from a guess into last year's number divided by four

September 15 is the third estimated tax payment deadline of 2026. Federal income tax is pay-as-you-go: the IRS expects tax to arrive as the money is earned, through withholding on wages, or through estimated payments on everything else. Fall short during the year, and there is a penalty, even when the April return ends in a refund.[1]

No bill arrives ahead of the date, and no form announces it. This post covers the whole system: who owes the payments, when they are due, how much is enough, and what Arizona asks for on top.

Who has to pay

The test is what the return will owe after withholding and credits, and the answer depends on how the business is set up:

Sole proprietors and partners: pay personally, with Form 1040-ES, once they expect to owe $1,000 or more for the year. The profit is taxed on the owner's personal return, as How to Pay Yourself explains, so the payments are personal too.[1]
S-corp shareholders: face the same $1,000 test on the personal return. A salary run through payroll withholds tax along the way, so the estimated payments only need to cover what the withholding does not.[1]
C-corporations: pay their own estimated tax once the corporation expects to owe $500 or more. The installments follow the corporation's own calendar, the 15th day of the 4th, 6th, 9th, and 12th months of its tax year, so a calendar-year corporation makes its last payment on December 15, not in January.[2]
Farmers and fishermen: owe just one payment, due January 15, if at least two-thirds of their gross income comes from farming or fishing, and even that one disappears for anyone who files and pays in full by March 1.[3]
First-year owners: owe nothing during the year if last year's total tax was zero, the prior tax year covered a full twelve months, and they were a U.S. citizen or resident the whole year.[1]

The four dates are not really quarters

The year splits into four payment periods, and only the first is an actual quarter:[3]

Payment due	Income it covers
April 15	January 1 through March 31
June 15	April 1 through May 31
September 15	June 1 through August 31
January 15 of the next year	September 1 through December 31

The periods are uneven: the June payment covers two months of income, the September payment covers three, and January's covers four. That is why a summer-heavy business meets its biggest payment right at this deadline. A mailed payment counts by its postmark, and a due date that lands on a weekend or holiday moves to the next business day.[1]

How much is enough: the safe harbor

Nobody knows the year's exact tax in September, and the rules never demand it. The safe harbor blocks any underpayment penalty when payments plus withholding reach either 90 percent of this year's tax, or 100 percent of last year's, whichever is smaller.[3] A return that ends up owing less than $1,000 after withholding escapes the penalty regardless.

Last year's tax is a known number, sitting on the 2025 return, and that makes the reliable plan simple: pay one-fourth of it at each date, and no penalty applies, no matter what 2026 turns out to earn. There is one adjustment: owners whose 2025 adjusted gross income topped $150,000, or $75,000 married filing separately, use 110 percent of last year's tax instead.[3]

What underpaying costs

The underpayment penalty is really just interest: the IRS applies its quarterly underpayment rate, 7 percent a year for the current quarter, to each period's shortfall for as long as it goes unpaid.[4] The math runs period by period, so a missed September payment keeps accruing, even when the spring return shows a refund.[3]

The arithmetic happens on Form 2210 at filing time, and most tax software fills it in automatically. That is why the penalty tends to surprise people: it shows up a year after the underpayment happened.

Two repairs for a year that is already behind

Raise withholding before December 31

Withheld tax gets special treatment: one-fourth of the year's total withholding counts as paid on each due date, no matter when it actually came out of the check.[5] So an owner on an S-corp payroll, or anyone with a W-2 job on the side, can raise withholding for the last months of the year with a new Form W-4, and repair a spring shortfall after the fact. An estimated payment cannot do that, because estimated payments only count on the day they are made.

Match the payments to the income

A business that earns most of its money in one season can annualize instead: Schedule AI of Form 2210 figures each period's required payment from the income actually earned by that date, rather than assuming it arrived evenly.[5] For a pool company or a landscaping crew, whose summer dwarfs its winter, that schedule exists for exactly this shape of year.

The Arizona payments

Arizona runs its own estimated system, with a higher bar: payments are required only when Arizona gross income, which for a full-year resident is simply federal adjusted gross income, tops $75,000 in both last year and this one, or $150,000 filing jointly.[6] Under that line, no Arizona quarterly payment is owed at all, even while the federal ones run.

Above it, the dates match the federal ones, and the target works the same way: 90 percent of this year's Arizona tax, or 100 percent of last year's, though last year's number only counts if an Arizona return was actually filed for it. Payments go in on Arizona Form 140ES, or electronically at AZTaxes.gov.[6]

The other September 15

September 15 is also the extended filing deadline for calendar-year S-corp and partnership returns, six months past the March 16 original.[7] A Schedule K-1 from one of those returns can change what the third payment should cover, so anyone expecting one should wait for the K-1, then set the payment.

The bottom line

Pay one-fourth of last year's tax at each date, and September 15 becomes routine. The harder questions are whether annualizing beats the safe harbor, how much a withholding change can still repair, and what an incoming K-1 shifts; a free consultation before the fifteenth settles those questions while there is still time to act on the answers.