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 estimated payments on everything else, and it charges for the shortfall even when the April return ends in a refund.[1]

No bill arrives ahead of the date, and no form announces it, so this post covers the whole system: who owes the payments, when, how much counts as 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 it lands differently by structure:

Sole proprietors and partners: pay personally with Form 1040-ES once they expect the year's balance to reach $1,000, because the business's profit is taxed on the owner's return, as How to Pay Yourself explains.[1]
S-corp shareholders: face the same $1,000 test on the personal return, though a salary run through payroll withholds tax along the way and shrinks what the estimated payments must cover.[1]
C-corporations: pay their own estimated tax once the corporation expects to owe $500 or more, on their own schedule: the 15th day of the 4th, 6th, 9th, and 12th months of the tax year, which for a calendar-year corporation puts the last payment on December 15, not in January.[2]
Farmers and fishermen: with at least two thirds of gross income from farming or fishing owe a single payment on January 15, 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

Two months feed the June payment while three feed September's and four feed January's, so a summer-heavy business meets its biggest period at exactly this deadline. A mailed payment counts by its postmark, and a 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, and a return that owes less than $1,000 after withholding escapes the penalty regardless.[3]

Last year's tax is a known number on the 2025 return, which makes the reliable plan concrete: pay one fourth of it at each date, and no penalty applies no matter what 2026 turns out to earn. Owners whose 2025 adjusted gross income topped $150,000 ($75,000 married filing separately) substitute 110 percent of last year's tax.[3]

What underpaying costs

The underpayment penalty is interest by another name: 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] It is figured period by period, so a missed September payment keeps accruing even for a filer whose spring return shows a refund.[3]

The arithmetic happens on Form 2210 at filing time, and most tax software fills it in without being asked, which is why the penalty tends to surprise people 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] 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 cure a spring shortfall after the fact, which no September estimated payment can do.

Match the payments to the income

A business that earns most of its money in one season can annualize instead, computing each period's required payment from the income actually earned by that date on Schedule AI of Form 2210.[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 federal adjusted gross income, tops $75,000 in both last year and this one ($150,000 filing jointly).[6] Owners under that line never owe Arizona a quarterly payment at all, even while the federal ones run.

Above the line, the dates match the federal ones, and the target is 90 percent of this year's Arizona tax or 100 percent of last year's, with last year usable only if an Arizona return was filed for it. Payments go in on Arizona Form 140ES or electronically at AZTaxes.gov.[6]

The other September 15

The same date is the extended filing deadline for calendar-year S-corp and partnership returns, six months past the March 16 original.[7] A Schedule K-1 arriving from one of those returns this month can change what the third payment should cover, so anyone expecting one should set the payment after the K-1 is in hand.

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 moves; a free consultation before the fifteenth settles those while there is still time to act on the answers.