Q3 Estimated Tax Prep 2026 (Sept 15 Deadline Worksheet)

Q3 is the quiet quarter. Q1 lands on Tax Day with the prior-year return. Q2 lands in mid-June. Q4 lands in January with year-end paperwork on the desk. Q3 sits in the middle of summer, next deadline three months away, the previous one ten weeks behind.

It is also the last estimate that can meaningfully change the year. A Q3 payment in September leaves one more chance to true things up in January. If the year has gone differently than projected, whether higher income, lower income, or a one-time event, Q3 is the recalibration point.

The September 15, 2026 deadline

Per Form 1040-ES and the IRS estimated tax FAQ, the third quarter payment period covers June 1 through August 31, with a payment due date of September 15. In 2026 that date falls on a Tuesday, so no weekend or holiday shift applies. Payments must be initiated by end of day.

The IRS treats “on time” as the date the payment is sent, not received. A check postmarked September 15 is on time. An IRS Direct Pay or EFTPS payment scheduled for September 15 is on time.

Notice the Q3 period is three months long (June, July, August). Q2 was only two. The periods are not calendar quarters.

Who has to file a Q3 payment

The threshold rule: estimated payments are required when expected tax owed (after withholding and refundable credits) is at least $1,000. The typical population:

• Self-employed sole proprietors, single-member LLCs, freelancers
• S-corp owners taking distributions
• Rental property owners with positive net income
• W-2 employees with side income, large capital gains, or RSU vests not fully withheld
• Retirees with pension or distribution income beyond what is withheld

W-2-only filers with adequate withholding generally do not need to file estimates. The W-4 already spreads withholding across paychecks. The exceptions: a mid-year raise, a bonus with flat 22 percent withholding that under-covers a higher bracket, or an equity event.

What the worksheet needs

A Q3 worksheet runs on year-to-date numbers as of the end of August. The inputs:

The Quarterly tab in the Annual Tax Planner (Premium). Q3 covers June through August with a September 15 due date. The estimated income, estimated tax and amount paid columns are filled in by hand, and the totals row sums them across the year.

If a Q1 or Q2 confirmation number is missing, the IRS account at irs.gov/account shows posted payments. EFTPS keeps a payment history. IRS Direct Pay sends confirmation emails by default. Any of those sources confirms what has been paid before the math runs.

A blank cell is still useful information - if you cannot find a Q1 confirmation in any of those places, the Q1 payment may not have been made, and that changes the Q3 calculation.

The safe harbor math

The underpayment penalty rule avoids penalty when withholding plus estimated payments equal at least the smaller of:

• 90 percent of the current year’s tax, or
• 100 percent of the prior year’s tax (110 percent if prior AGI exceeded $150,000)

Two paths to the same finish line. Some filers find the prior-year number easier because it is a fixed dollar amount they can divide by four. Others find the current-year projection more accurate when income is materially different. Both are valid, and writing the two numbers down side by side is what makes the choice visible.

Prior-year safe harbor

Safe harbor target per quarter = Prior year total tax / 4
                               = Prior year total tax * 1.10 / 4   (if prior AGI > $150,000)

By Q3, three quarters of the year have passed. The cumulative target is three quarters of the annual safe harbor amount. Compare cumulative withholding plus Q1 plus Q2 estimates against that target. The gap (if positive) is the Q3 minimum to stay at safe harbor.

Current-year safe harbor

Projected annual tax = (YTD net income annualized) * effective rate
Cumulative target through Q3 = Projected annual tax * 0.90 * 0.75
Q3 minimum = Cumulative target - YTD payments

The current-year path is more accurate when the year has clearly diverged from last year. It also requires a defensible projection of the effective rate, which is non-trivial for variable income. The Quarterly Estimated Tax Spreadsheet walkthrough covers the rate tables.

A worked example

Marcus is a freelance video editor, filing single. His 2025 federal tax liability (line 24 of last year’s 1040) was $18,000 on an AGI of $96,000.

Prior-year path.

• Safe harbor annual target: $18,000 (prior AGI under $150,000, so no 110 percent uplift)
• Quarterly safe harbor: $18,000 / 4 = $4,500
• Cumulative through Q3 (three quarters): $13,500
• Q1 paid: $4,500. Q2 paid: $4,500. Total paid: $9,000.
• Q3 minimum to stay at safe harbor: $13,500 - $9,000 = $4,500

If Marcus pays $4,500 on September 15, his prior-year safe harbor is intact and no penalty applies regardless of how 2026 lands.

Current-year path.

• YTD gross (Jan to Aug): $96,000. Annualized: $144,000. Net of expenses: $112,000.
• Effective combined federal plus SE rate (rough): 27 percent. Projected federal tax: $30,200.
• 90 percent of projected: $27,180. Cumulative through Q3 (three quarters): $20,385.
• Already paid: $9,000. Q3 minimum on current-year path: $11,385.

Two different numbers. Marcus’s year is running ahead of last year by a wide margin. Paying $4,500 keeps him at prior-year safe harbor (no penalty) but leaves a sizable balance due at filing. Paying $11,385 closes more of the gap, with a bigger cash outflow now.

Both numbers come out of the same year-to-date figures. Which one to pay is the conversation.

Filing the Q3 payment

Three filing methods, all to the same Treasury account:

1. IRS Direct Pay. Free, no enrollment, takes a checking account routing and account number. Confirmation emailed immediately.
2. EFTPS. Free, requires enrollment (PIN mailed in 7-10 days). Allows scheduling all four estimates in advance and stores payment history.
3. Mail with Form 1040-ES voucher. The 2026 Form 1040-ES PDF includes a payment voucher for each quarter. Mail with a check. Postmark by September 15 counts as on time.

Debit and credit card payments work but route through a third-party processor with a fee: a flat charge of roughly $2.10 for a debit card, or about 1.75 to 1.85 percent for a credit card. Most rewards programs return less than that on tax payments.

Whichever method, save the confirmation number somewhere it can be found at filing time. The Annual Tax Planner logs the amount paid against each quarter on its Quarterly tab, and the Quarterly Tax Payment Tracker logs the amount paid, the date paid and what is left for that quarter. Neither has a cell for the confirmation string itself, so that stays with your own payment records.

If Q1 or Q2 was missed or underpaid

The penalty is calculated per quarter. A missed Q1 has been accruing the underpayment rate since April 15. A missed Q2 has been accruing since June 15. Per the IRS quarterly interest rate schedule, the 2026 underpayment rate is 7 percent in Q1 and 6 percent in Q2 (federal short-term rate plus 3 percent), so the dollar damage on a moderate underpayment is modest.

Catching up at Q3 stops the meter on future quarters but does not erase the prior periods. The mechanic: interest accrues on the unpaid amount for the days unpaid. A Q1 shortfall of $3,000 caught up on September 15 sat unpaid for roughly 153 days; at a blended rate near 6.5 percent annual, that is around $80.

The penalty is computed at filing on Form 2210. The IRS sometimes computes it on the filer’s behalf and bills it; some filers compute their own to avoid surprises.

State estimated taxes

Most states with income tax require quarterly estimates that mirror the federal schedule, but rules and forms differ. Some line up with the federal dates; others diverge. California’s third-quarter estimate uses a different payment percentage from the federal flat 25 percent.

Each state department of revenue publishes its own schedule and safe harbor rule. The state site is the source of truth for the current year’s voucher and due dates. For filers in no-income-tax states (Florida, Texas, Tennessee, Washington, and several others), this section is moot.

After September 15

Two things worth doing in the same session as the payment:

1. Log the payment. The Annual Tax Planner takes the amount paid on the Q3 row of its Quarterly tab; the Quarterly Tax Payment Tracker takes the amount and the date paid. The method and the confirmation string from IRS Direct Pay or EFTPS go with your own records.
2. Update the year-end projection with the September YTD numbers. The Q4 estimate due January 15, 2027 will use these. Q4 is also when prior-year safe harbor decisions get locked in - if December income is unusually high or low, that changes the picture.

The mid-year tax checkup walkthrough covers the broader review that pairs with the Q3 prep. Some filers run them together: the checkup surfaces what changed, the Q3 worksheet turns it into a payment. For freelancers and sole proprietors, the 25 self-employed deductions for 2026 is the reference for what counts as a deductible expense when running the YTD net income line.

When the worksheet hands off to a CPA

Three patterns from the Q3 worksheet that often produce a CPA conversation:

• Large gap between current-year and prior-year safe harbor. A 2x or 3x difference usually means the year has materially shifted. A CPA can quantify the cash flow trade-off.
• YTD payments running below the cumulative target. Whether to top up via a larger Q3 estimate or to increase W-2 withholding (which counts as paid evenly across the year and can patch prior-quarter shortfalls).
• One-time events. Selling a property, exercising stock options, a large bonus, a Roth conversion. Each shifts federal and state estimate logic in ways that are hard to project mid-year without the transaction docs.

A worksheet organizes the numbers. It does not weigh strategic timing or year-end harvesting. Those belong in the conversation the spreadsheet prepares.

Which tax template fits

If the whole tax year is the question, the Annual Tax Planner is the wider workspace. If quarterly payments are the whole question, the lighter tracker covers that on its own.

• Annual Tax Planner - Income logged by type with an editable rate on each, deductions by category, a credits field and a net tax liability on the dashboard, plus a documents checklist and a four-row quarterly log of what has been paid.
• Quarterly Tax Payment Tracker - One annual estimate split four ways, with due dates calculated from the tax year and columns for amount paid, date paid and what is left each quarter.

Both open in Excel and Google Sheets. The data stays on your device.

Related

• Mid-Year Tax Checkup: 8 Things Worth Reviewing in June - The broader 60-minute review that pairs with the Q3 worksheet.
• Quarterly Estimated Tax Spreadsheet: Safe Harbor Rule Explained - The full walkthrough on the 2026 due dates and both safe harbor paths.
• 25 Self-Employed Tax Deductions for 2026 - The deductible-expense reference for the YTD net income line.