Q3 is the third quarter of the calendar year and covers July, August, and September. All three months fall inside it in full: 31 days in July, 31 in August, and 30 in September, for 92 days total. That makes Q3 tied with Q4 as the longest quarter of the year and slightly longer than Q1 or Q2.
Most individuals and small businesses use this July-through-September window because the IRS defaults to a calendar tax year for personal income reporting. By the time Q3 begins, you have half a year of actual results, which is enough to project where you’ll land in December and still leave room to adjust.
When Q3 Doesn’t Mean July Through September
Not every organization’s Q3 lines up with the calendar. A fiscal year can start on the first day of any month other than January, which shifts every quarter with it. Once a business picks a fiscal year, changing it requires IRS approval through Form 1128.1Internal Revenue Service. Tax Years
The most prominent shifted schedule is the U.S. federal government’s. Its fiscal year runs October 1 through September 30, which makes federal Q3 the April-through-June window.2USAGov. Federal Budget Process If you work with federal contracts or grants, the government’s Q3 arrives while calendar-year businesses are still in their Q2.
Many retailers use a separate system called the 4-5-4 calendar, maintained by the National Retail Federation, which divides each quarter into four-week, five-week, and four-week periods and starts the year in early February.3National Retail Federation. 4-5-4 Calendar Retail Q3 under that calendar runs roughly August through October.
The September 15 Estimated Tax Deadline
The third quarterly estimated tax payment for individuals is due September 15, which lands before Q3 itself ends. In 2026, September 15 is a Tuesday, so no weekend or holiday adjustment applies.4Internal Revenue Service. Form 1040-ES (2026) When the date does fall on a Saturday, Sunday, or D.C. legal holiday, the IRS moves it to the next business day.5Internal Revenue Service. Publication 509 (2026), Tax Calendars
You calculate and pay using Form 1040-ES. It covers income that isn’t subject to regular withholding: self-employment earnings, freelance income, rent, dividends, and capital gains.4Internal Revenue Service. Form 1040-ES (2026) Individuals who expect to owe $1,000 or more after withholding and credits generally need to make these payments. Corporations have a separate rule with a $500 threshold.6Internal Revenue Service. Estimated Taxes
Each installment should equal 25 percent of your required annual payment, spread across four due dates: April 15, June 15, September 15, and January 15 of the following year.7Office of the Law Revision Counsel. 26 USC 6654 – Failure by Individual to Pay Estimated Income Tax Because September 15 arrives before the quarter closes, part of the payment is a projection. Keeping clean records through July and August makes that estimate much more accurate.
How to Avoid the Underpayment Penalty
Missing the deadline triggers a penalty that works like interest on the shortfall, calculated using the IRS underpayment rate (7 percent through at least late 2025; the rate for Q3 2026 had not been announced at the time of writing).8Internal Revenue Service. Quarterly Interest Rates It runs from the missed date until you pay.
You avoid the penalty entirely if you meet any one of these safe harbors:9Internal Revenue Service. Underpayment of Estimated Tax by Individuals Penalty
- Your total tax due after withholding and credits is less than $1,000.
- You paid at least 90 percent of the tax shown on your current-year return.
- You paid at least 100 percent of the tax shown on your prior-year return.
The prior-year method has a catch for higher earners. If your adjusted gross income for the preceding year was over $150,000 ($75,000 if married filing separately), the threshold rises from 100 percent to 110 percent of the prior year’s tax.7Office of the Law Revision Counsel. 26 USC 6654 – Failure by Individual to Pay Estimated Income Tax Freelancers and small business owners who had one strong year followed by a leaner one often get tripped up here. When income drops, the 90 percent current-year method usually produces a lower required payment.
Deadlines That Land Right After Q3 Closes
Form 941 for Employers
Employers who withhold income tax, Social Security, and Medicare from paychecks report those amounts on Form 941 each quarter. For Q3 (July through September), Form 941 is due October 31. If you deposited all payroll taxes in full and on time during the quarter, you get an automatic extension to November 10.10Internal Revenue Service. Instructions for Form 941
Federal unemployment tax follows a separate rule. If your accumulated FUTA liability reaches $500 or more by the end of Q3, deposit it by October 31. Below $500, you carry it into the next quarter.11Internal Revenue Service. Employment Tax Due Dates
Filing Form 941 late triggers a failure-to-file penalty of 5 percent of the unpaid tax for each month the return is overdue, capped at 25 percent. A separate failure-to-pay penalty of 0.5 percent per month runs alongside it, with the filing penalty reduced by the payment penalty when both apply.12Internal Revenue Service. Failure to File Penalty
Form 10-Q for Public Companies
Public companies file Form 10-Q with the SEC after each of the first three fiscal quarters (Q4 is covered by the annual 10-K instead). For a calendar-year company, the Q3 10-Q is due 40 days after September 30 for large accelerated and accelerated filers (November 9) and 45 days after for all other public companies (November 14).13SEC.gov. Form 10-Q Filing Form 12b-25 buys an additional five calendar days when a company can’t meet the window.14eCFR. 17 CFR 240.12b-25 – Notification of Inability to Timely File
State Estimated Payments
Most states with an income tax require quarterly estimated payments on a schedule similar to the federal one, and September 15 is a common state deadline as well. Rates and safe harbor thresholds vary, and state interest rates for underpayment tend to run somewhat higher than the federal rate. A missed state payment creates a penalty separate from any federal one, so check your state department of revenue for the exact rules that apply to you.