Q3 Estimated Tax Payments: Deadlines, Rules, and How to Avoid Penalties (2026 Guide)
Your third-quarter 2026 estimated tax payment is due Tuesday, September 15, 2026. It covers income you earned from June 1 through August 31, and the date applies to your federal payment and your Illinois or Missouri state payment alike. The date is the easy part. Figuring out whether you owe, and how much, is where business owners in O’Fallon, Breese, and Chesterfield get tripped up, and 2026 carries a wrinkle that makes last year’s shortcut less reliable than usual.
Find Out If You Owe
Not sure whether Illinois, Missouri, or both want a payment this quarter?
Do you owe a Q3 payment at all?
The tax system runs on pay-as-you-go. Employees never think about it because their employer withholds tax from every paycheck. If you run a business or take income as an independent contractor, nobody is withholding on your behalf, so you send the tax in yourself four times a year.
Whether you owe those payments comes down to a dollar threshold, and this is where Illinois and Missouri part ways. Federally, and for Illinois, you generally owe estimated payments once you expect your tax for the year to top $1,000 after subtracting withholding and credits. The IRS estimated tax rules and Illinois Form IL-1040-ES both draw the line at that same $1,000 figure.
Missouri sets the bar at $100, filed on Form MO-1040ES. A Chesterfield business owner can owe Missouri estimated payments on a level of side income that would never cross the federal or Illinois line. If you moved from the Illinois side to the Missouri side, or picked up Missouri-source income this year, that $100 threshold is the one that catches people off guard. The deadline is shared, but the trigger that puts you on the hook is not.
Illinois S corps and partnerships owe one more thing
If your Illinois business is an S corporation or a partnership, there is a second obligation with nothing to do with your personal return. Illinois charges a personal property replacement tax of 1.5% on the entity’s net income, paid by the business itself before any profit reaches your individual Form IL-1040. Missouri has no equivalent entity-level tax for pass-throughs.
So an Illinois S corp owner is tracking two separate things: the 1.5% the company owes on its net income, and the personal estimated payments the owner makes on the income that flows through. If your business sits on the Illinois side, build the replacement tax into your quarterly planning rather than discovering it at filing.
How much to send so you avoid a penalty
Once you know you owe, the amount that keeps you out of penalty territory follows a safe-harbor rule. Pay the smaller of 90% of what you will owe for 2026, or 100% of what you owed for 2025. If your adjusted gross income last year was over $150,000, that prior-year figure climbs to 110%. Hit one of those marks through withholding and estimated payments combined, and an underpayment penalty is off the table even when your final bill comes in higher.
Most people lean on the prior-year number because it is fixed and known. Divide last year’s tax by four, send that each quarter, and you are covered. For 2026, that shortcut is weaker than it used to be.
Why 2026 makes last year’s number a weaker guide
The One Big Beautiful Bill Act, signed in July 2025, changed several rules that feed straight into your 2026 liability. Two matter most for business owners: the Section 199A qualified business income deduction was made permanent and adjusted, and 100% bonus depreciation came back for qualifying purchases. Both move the gap between your gross income and your taxable income, which is the gap your estimated payments are supposed to track.
Put a number on it. Say you bought a $60,000 work truck for the business this year. Under restored 100% bonus depreciation, you can deduct the entire $60,000 in 2026 rather than spreading it across several years. That single deduction can pull your taxable income well below last year’s, which means a safe-harbor payment sized to your 2025 tax would have you sending far more than 2026 calls for, with the excess tied up until you file. Run the situation the other way and the risk flips: a strong revenue year with no offsetting purchases can push your real liability past last year’s number, and the comfortable prior-year safe harbor quietly leaves you underpaid.
Last year’s figure is a shaky guide this year. We’ll run a real projection so your September payment fits 2026.
So this is the year to run a real 2026 projection instead of copying the old figure, and it is where a proactive tax planning approach earns its keep. The point is to walk into September with a number built for this year, which is the difference between having options and getting handed a surprise in April.
What a missed September 15 costs
The penalty for underpaying works like interest, charged on the amount you were short and running from the due date until you pay. Illinois calculates it at the federal short-term rate plus three points, and Missouri adds interest to a delinquent installment on the same clock. Timing is the lever: an installment you catch up a week late costs little, while one you let ride until January accrues the whole way.
If cash flow is the reason a payment is at risk, treat that as a signal to act early. A missed estimate is often the first visible sign of a deeper cash flow problem, and the payment is far easier to manage when you plan the quarter than when the voucher is already past due.
Earning on both sides of the river? You’re running two systems
Here is the part national tax guides skip. If you live in Illinois and earn in Missouri, or the reverse, you are running two estimated-tax systems at once: two thresholds ($1,000 versus $100), two forms (IL-1040-ES versus MO-1040ES), two portals, and two sets of safe-harbor math. September 15 is the one date they share.
The mistake we see most is treating the federal figure as if it covers the states, then learning in April that Missouri wanted payments all along. Before September 15, run the Illinois and Missouri numbers on their own before assuming one federal figure covers both. If you are not sure which state wants what, that is a short conversation that heads off an April surprise. Book a free call and we will map your quarters across both states, so the only thing left to remember is the date.
FAQ
When is the Q3 2026 estimated tax payment due? The third-quarter 2026 estimated tax payment is due Tuesday, September 15, 2026. It covers income earned from June 1 through August 31 and applies to both federal payments and Illinois or Missouri state payments.
Who has to make estimated tax payments in Illinois? You generally owe Illinois estimated payments if you expect your Illinois tax to exceed $1,000 for the year after subtracting withholding and credits. Payments are made on Form IL-1040-ES.
What is Missouri’s estimated tax threshold? Missouri requires estimated payments once you expect to owe $100 or more in state income tax after withholding and credits, filed on Form MO-1040ES. That is lower than the federal and Illinois $1,000 threshold.
How do I avoid an underpayment penalty? Pay the smaller of 90% of your current-year tax or 100% of your prior-year tax through withholding and estimated payments combined. If your prior-year adjusted gross income was over $150,000, use 110% of the prior-year figure.
What happens if I miss the September 15 deadline? The penalty works like interest on the amount you underpaid, running from the due date until you pay. Making the payment as soon as possible after the deadline limits how much accrues.
Do I owe estimated taxes in both Illinois and Missouri if I work across the state line? Possibly. Illinois and Missouri run separate systems with different thresholds, forms, and portals, so income sourced in each state is evaluated on that state’s rules. Run both calculations rather than assuming your federal payment covers the states.
We’ll map your quarters across Illinois and Missouri so the only thing left to remember is the date.
