Quarterly estimated taxes in 2026: dates, amounts and the safe harbor rule

If you own a business and nobody withholds tax from your profit, the IRS expects you to pay during the year in four installments. Illinois expects the same. Miss them, or send too little, and both charge an underpayment penalty that works like interest on the shortfall.

The fix is usually simple arithmetic. Here is what we set up for most of our owner clients.

The 2026 due dates

Payment	Covers income earned	Due
First	January 1 to March 31	April 15, 2026
Second	April 1 to May 31	June 15, 2026
Third	June 1 to August 31	September 15, 2026
Fourth	September 1 to December 31	January 15, 2027

The periods are uneven on purpose. The second covers two months and the fourth covers four. Illinois uses the same four dates for individuals on Form IL-1040-ES.

Who has to pay

You need federal estimated payments if you expect to owe at least $1,000 for 2026 after subtracting withholding and credits. Illinois uses a $1,000 threshold too, after its own credits and withholding. Most sole proprietors, partners and S corporation owners with a profitable business cross both lines.

How much to send: the safe harbor

You avoid the federal penalty if your payments, made on time, add up to at least one of these:

• 90% of your 2026 tax, which you only know after the year ends, or
• 100% of your 2025 tax, the total on last year’s Form 1040, or
• 110% of your 2025 tax if your 2025 adjusted gross income was over $150,000 ($75,000 if married filing separately).

The second and third options are the useful ones, because they use a number you already have. If profit jumps this year, you still owe the difference in April 2027, but no penalty.

A worked example

Marisol runs a two-chair salon in downtown Naperville as a sole proprietor. Her 2025 return shows total tax of $18,400 and adjusted gross income of $96,000, so the 100% rule applies.

• Federal: $18,400 divided by four is $4,600 per payment.
• Illinois: her 2025 Illinois tax was $4,610, so about $1,153 per payment on Form IL-1040-ES.

She schedules both in January for all four dates and stops thinking about it. If her bookings fall off in the fall, we lower the last two payments at the September planning meeting.

Paying, and keeping proof

Pay the IRS through IRS Direct Pay or EFTPS and choose “Estimated tax” and the tax year 2026. Pay Illinois through MyTax Illinois. Both give a confirmation number: save it with the year’s tax file. A check by mail works, but it leaves you depending on the post office near a deadline.

Three situations that change the plan

You own an S corporation and take a salary. You can cover some or all of your estimated tax by raising the federal and Illinois withholding on your own paycheck. Withholding counts as paid evenly through the year even if it happens in December, which makes it a good catch-up tool.

Your income is seasonal. A landscaper who earns most of the year’s profit from May to October can use the annualized income method on Form 2210, Schedule AI, to pay smaller amounts early in the year. It takes more paperwork at filing time, so we use it only when the savings are worth it.

You missed a payment. Pay as soon as you can. The penalty is figured day by day on each late installment, so a payment two weeks late costs far less than one caught at filing time.

What we do for planning clients

At each quarterly meeting we look at the books through the last month, compare profit with last year, and send you the federal and Illinois amounts for the next payment in writing. Most owners want one number for each date; that is what you get.