TAX PLANNING · SEPTEMBER 1, 2026

September 15 Is Not a Tax Deadline Problem. It’s a Cash-Planning Problem.

September 15 is the third estimated-tax payment deadline for many owners. But treating it as a calendar task misses the more useful question: does the payment still reflect the business you actually have?

Individuals—including sole proprietors, partners, and S-corporation shareholders—generally must make estimated payments if they expect to owe $1,000 or more when filing. Corporations generally face the requirement at $500 or more. IRS: Estimated taxes

The deadline is therefore a forecast checkpoint. If sales, payroll, distributions, margins, or owner withholding changed since June, repeating the prior payment without revisiting the model is not efficiency. It is guesswork.

What is due on September 15, 2026?

For estimated-tax purposes, the year is divided into four payment periods. The IRS third-quarter calendar lists Sept. 15, 2026, among the relevant third-quarter due dates; an owner’s particular filing and payment obligations depend on entity type and facts. IRS: Third quarter tax calendar

For owners with pass-through income, the payment is personal even when the cash was generated in the business. That is why tax planning and cash planning have to meet before the deadline.

Why should an estimated payment start with a forecast?

The IRS says estimated tax is based on expected adjusted gross income, taxable income, taxes, deductions, and credits. It also says to adjust for changes in your situation and tax-law changes. IRS: Estimated taxes

That means the payment should be informed by what has changed since the last estimate:

• year-to-date profit compared with plan;
• payroll and owner compensation actually paid;
• distributions taken or planned;
• material deductions, credits, and capital purchases; and
• expected Q4 revenue and margin.

A payment can be technically timely and still be financially wrong for the year.

What should an owner reconcile before September 15?

Start with the books. Not the bank balance alone, and not last year’s return alone.

A practical pre-payment review answers four questions:

1. What did we earn through August? Use closed books, then isolate one-time items rather than treating them as recurring profit.
2. What has already been paid toward tax? Include estimates, withholding, and any payments made through the business or individual account.
3. What cash is actually available after operating commitments? Taxes are an obligation, but the payment plan still has to coexist with payroll, debt service, and working capital.
4. What does the rest of the year plausibly look like? A growing business, a seasonal business, and a business with uneven income should not all use the same shortcut.

The output should be a decision: pay as projected, increase the payment, revise a future payment, or investigate whether the model is incomplete.

Can withholding be part of the solution for an S-corp owner?

Potentially. The IRS notes that a taxpayer receiving salaries and wages can avoid estimated payments by requesting additional withholding through a new Form W-4. Whether that is appropriate for an owner depends on payroll, compensation, and the broader tax plan. IRS: Estimated taxes

The point is not that every owner should solve the issue through withholding. The point is that estimated payments are not the only lever. A good plan considers the available levers before the year closes.

What is the Monday-morning move?

Run a September tax-and-cash checkpoint: close the books through August, compare year-to-date profit with the last tax projection, list all tax paid to date, and model Q4 before releasing the payment.

If the answer changes, make the decision while there is still another quarter to act—not in March, when the only remaining option is explaining the gap.

Sources

1. Internal Revenue Service, Estimated taxes, accessed Aug. 31, 2026.
2. Internal Revenue Service, Third quarter tax calendar, accessed Aug. 31, 2026.
3. Internal Revenue Service, 2026 Form 1040-ES, Estimated Tax for Individuals, accessed Aug. 31, 2026.

This article is educational and general in nature. Tax obligations and payment amounts depend on facts specific to the taxpayer. Consult qualified tax advisors about your situation.

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