There are certain dates on the calendar that small business owners know by heart.

Payroll day. Rent day. Maybe the day your biggest client usually pays their invoice.

September 15 probably is not one of them.

But if you own a business, are self-employed, receive partnership or S corporation income, or earn income that does not have enough tax withheld, it probably deserves a spot on your calendar.

September 15, 2026 is the due date for the third federal estimated tax payment of the year. Colorado’s third individual estimated income tax payment is also due September 15.  

And before you simply send the same amount you paid last time, this is a good moment to ask a much better question:

Does that number still make sense?

When is the next estimated tax payment due?

For most calendar-year taxpayers, the third 2026 federal estimated tax payment is due September 15, 2026. The fourth payment is generally due January 15, 2027.

Estimated taxes are how the IRS collects tax during the year on income that does not already have enough withholding attached to it. That can include self-employment income, business income, interest, dividends, rental income, capital gains, and other taxable income.

This matters because our tax system is essentially pay-as-you-go. Waiting until you file your return next spring to pay everything can create an underpayment penalty, even in some situations where you ultimately receive a refund.

Who actually needs to make estimated tax payments?

For individuals, including sole proprietors, partners, and S corporation shareholders, estimated payments are generally required when you expect to owe at least $1,000 in tax after subtracting withholding and refundable credits.

That is why estimated taxes are not just a “self-employed person” issue.

You might have employees, run payroll, receive a W-2 from your own company, and still need estimated payments because your withholding does not cover the tax created by the rest of your income.

This is especially common for growing business owners. The business makes more money, distributions increase, investment income changes, or something unexpected happens during the year. Meanwhile, the estimated payment amount is still based on numbers calculated months ago.

The business changed. The tax estimate did not.

How much are you supposed to pay?

This is where people tend to get nervous, but the basic federal framework is fairly straightforward.

Generally, taxpayers can avoid an estimated tax underpayment penalty by paying enough during the year to cover the smaller of 90% of the current year’s tax or 100% of the previous year’s tax. If the prior year’s adjusted gross income exceeded $150,000, or $75,000 if married filing separately, the prior-year threshold generally increases from 100% to 110%.

You may hear this referred to as the “safe harbor.”

But there is an important distinction: paying enough to avoid a penalty does not necessarily mean you will owe nothing when you file your return.

Those are two different goals.

One approach focuses on avoiding an under-payment penalty. The other focuses on estimating your actual tax liability closely enough that you do not get hit with a painful tax bill next spring.

At Savvion HQ, we think business owners deserve to know the difference.

Why September is a great time to recalculate

Your first estimated payment was due back in April. Think about how much may have changed since then.

Maybe sales are running ahead of plan. Maybe you hired two people. Maybe margins tightened. Maybe you increased your own compensation. Maybe you sold an investment. Maybe the business had a slow first quarter followed by a fantastic summer.

Any of those things can change your tax picture.

The IRS recommends recalculating estimated taxes when your income estimate changes. Current-year estimates should take expected income, deductions, credits, and applicable tax law into account rather than blindly relying on last year’s numbers.

In other words, September is not just another payment deadline. It is a checkpoint.

What should a business owner review before September 15?

You do not need to turn yourself into a tax accountant for the weekend. But you do want a clear picture of what has happened so far this year.

A useful September tax checkup looks something like this:

1.      Review your year-to-date profit and loss statement and make sure the bookkeeping is reasonably current. Then compare actual performance with what you expected at the beginning of the year.

2.      Project the rest of 2026. If revenue, expenses, payroll, owner compensation, or distributions have changed significantly, update the forecast.

3.      Look at what has already been paid through estimated payments and withholding.

4.      Recalculate the expected federal and Colorado tax obligation using your updated numbers.

5.      Decide whether the September payment should stay the same, increase, or decrease.

6.      Make sure the cash is available before the payment deadline instead of discovering the tax obligation three days before it is due.

That last part matters more than people realize.

Tax planning and cash flow planning should not live in separate rooms.

Knowing that you owe $20,000 is useful. Knowing three months ahead of time that you will owe $20,000 is much more useful.

What Colorado business owners should know

Colorado also uses estimated income tax payments for income that is not sufficiently covered by withholding. The Colorado Department of Revenue lists estimated payment dates of April 15, June 15, September 15, and January 15 for calendar-year individuals. Payments can be made through Revenue Online, and the state provides Form DR 0104EP for calculating individual estimated income tax payments.

For many Colorado business owners, that means there are two numbers to think about before September 15, not one.

Federal. And Colorado.

It sounds obvious when you say it out loud, but state estimates are surprisingly easy to overlook when someone is focused on the larger federal payment.

What if your business income is unpredictable?

Not every business earns money evenly throughout the year.

A landscaper in Colorado Springs probably does not have the same revenue pattern as a consulting firm. A retailer may earn a huge percentage of its annual profit during the holidays. A construction company may have one unusually profitable project land in a single quarter.

The tax rules recognize that.

If your income arrives unevenly throughout the year, the IRS allows certain taxpayers to use an annualized income installment method that may reduce or avoid an underpayment penalty that would otherwise result from uneven income.

This is one of those areas where having somebody actually look at the story behind the numbers can matter.

Four equal payments are simple. Business rarely is.

Can you increase withholding instead?

Sometimes.

If you also receive wages, additional withholding may be an alternative to making estimated tax payments. A taxpayer can submit a new FormW-4 requesting additional withholding from wages.

For some business owners who are already receiving payroll from their company, reviewing withholding can be part of the overall tax strategy.

That does not mean increasing withholding is always the right answer. It means there may be more than one way to solve the problem.

The goal is not simply to send money to the IRS.

The goal is to understand what you owe, why you owe it, when it needs to be paid, and how it fits into the financial health of your business.

That is a much better way to run a company.

Frequently asked questions

When is the September 2026 estimated tax payment due?

The third federal estimated tax payment for calendar-year taxpayers is due September 15, 2026. Colorado’s third individual estimated income tax payment is also due September 15.

Do all small business owners have to make quarterly estimated tax payments?

No. Individuals such as sole proprietors, partners, and S corporation shareholders generally need estimated payments when they expect to owe at least $1,000 after subtracting withholding and refundable credits. Your specific situation depends on your income, withholding, prior-year tax, and other factors.

What happens if I do not pay enough estimated tax?

You may owe an underpayment penalty if you do not pay enough tax during the year or make required payments late. The IRS generally provides safe-harbor rules based on current-year or prior-year tax, although special rules apply in certain circumstances.

Should I just pay the same amount I paid last quarter?

Not necessarily. If your income, deductions, credits, withholding, or business performance changed, your estimated tax calculation may need to change too. The IRS recommends recalculating when your income estimate changes.

A September tax checkup can save a lot of January stress

Nobody opens a business because they are excited about estimated tax payments.

But this is exactly the kind of small financial checkpoint that can prevent a much bigger headache later.

If your business looks different today than it did when your estimates were originally calculated, do not assume the old number is still the right number.

Take a look now.

Review the books. Look at your year-to-date profit. Project where the year is headed. Check what you have already paid. Then make sure your tax plan matches the business you actually have today.‍

If you are staring at your numbers thinking, “I have no idea whether this is right,” that is exactly the kind of  thing Savvion is here for.  

You should not have to piece together tax advice, bookkeeping  data, payroll information, and cash flow decisions on your own just because  you are the one in charge.

Sources

• IRS Publication 505 (2026), Tax Withholding and Estimated Tax

• IRS Estimated Tax FAQ

• IRS Topic No. 306, Penalty for Underpayment of Estimated Tax

• Colorado Department of Revenue, Individual Income Tax Estimated Payments

• Colorado Department of Revenue, Business Income Tax Estimated Payments

General information only. Tax rules and individual circumstances vary. Readers should consult a qualified tax professional regarding their specific situation.

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