September 15 Estimated Taxes: A Cash Plan for 2026

September 15, 2026 is the regular due date for the third federal estimated income tax installment for calendar-year individuals. For an owner whose cash is already committed to payroll, suppliers, and ongoing jobs, that payment deserves a place in this week's cash plan.

The immediate task is to confirm the amount due, identify the money available to fund it, and check what the payment leaves for upcoming commitments. A tax estimate and a bank balance answer different questions; review them together before moving money.

What the September Deadline Covers

The IRS calendar associates the third payment period with June 1 through August 31. That does not mean the installment is simply the tax on those three months' profit. The calculation depends on the applicable method, earlier payments, withholding, and other tax information. See 2026 Form 1040-ES for the individual payment instructions.

This article concerns an owner's individual federal estimates, including owners of pass-through businesses. Corporate estimated payments and payroll deposits follow their own rules. Florida does not impose a personal income tax, but businesses can have Florida corporate income tax obligations and other taxes to plan for.

September 15 is the regular deadline; applicable IRS disaster relief or other special rules can change a taxpayer's obligations. Confirm any exception against the IRS disaster-relief notices, rather than assuming it applies.

Confirm the Payment Before Planning the Withdrawal

Gather the current estimate, year-to-date books, prior estimated-payment confirmations, and current withholding information. Ask whether the amount still fits the year and the payment method being used. A strong summer or a weak August is a reason to review the calculation, not to change the voucher based on the checking balance alone.

Our quarterly estimated-tax guide covers the general planning framework. Here, the output you need is a confirmed September payment amount and a clear plan for funding it.

For a pass-through owner, company cash and personal tax obligations also need to be distinguished. Identify the personal funds already reserved and any properly recorded owner payment needed from the business. Do not assume the owner's individual tax payment is a deductible company operating expense.

Build a Cash View Through the Next Operating Cycle

Use a specific date range that includes the tax payment and the next payroll and supplier-payment cycle. Then list:

• Cash currently available, separated by business, personal, and tax-reserve account.
• Payments already issued or pending that have not cleared.
• Additional payroll, vendor bills, debt payments, and other commitments due during the period.
• Customer receipts expected during that period, with a realistic collection date.
• The confirmed tax payment and the funds intended to cover it.

Start with a reconciled cash figure and label what it already includes. Do not subtract an outstanding check twice: if it has already reduced the book cash balance, deducting it again understates available cash. Likewise, a transfer between accounts is not new money when both accounts are already included in the same forecast.

Keep uncertain receipts separate from funds already available. A submitted invoice or retained amount cannot fund an outgoing payment until it is collected. For the wider process, see cash-flow forecasting for contractors and progress billing and collections.

A Simple Funding Example

Suppose the owner and bookkeeper identify $24,000 of business cash after accounting for outstanding payments already reflected in the books. Another $15,000 is needed for commitments in the forecast period and has not yet reduced that figure. That leaves $9,000 before any owner payment or additional operating cushion.

If the owner's confirmed individual estimate is $7,000 and there is no separate personal tax reserve, funding the full amount from the business would leave only $2,000 of that projected headroom. An expected $8,000 customer receipt should appear separately until its timing is dependable.

These amounts are an illustration of cash timing, not a tax calculation or a recommendation to distribute all remaining cash. The point is to expose the shortfall or narrow cushion before the payment date.

What to Do if the Cash Is Short

Contact your tax preparer promptly to confirm the required amount and review the payment history. Follow up on genuinely due receivables and review optional spending against existing commitments. Do not treat money reserved for payroll taxes or other obligations as spare cash.

An underpayment can produce a penalty even if the year's final tax is later paid in full. IRS Publication 505 explains that the penalty is determined separately for each payment period. Paying later does not automatically erase an earlier shortfall, and changing the estimate requires a supportable calculation.

Use the IRS payment options to make the appropriate payment, check the tax year and payment type, and retain the confirmation. Reconcile the withdrawal when it clears; a saved plan to pay is not evidence that a payment was completed.

Start Planning for January After September

The next regular individual estimated-tax installment date is January 15, 2027, as shown in the IRS 2026 payment calendar. After the September payment, update the forecast and the tax reserve as income, withholding, and obligations change.

KDM Accounting Services helps owners connect current books, receivables, upcoming commitments, and estimated-tax planning. Contact KDM Accounting Services to review the numbers before the next payment puts pressure on the checking account.