If you are self-employed, earn gig income, receive investment or rental income, or have too little tax withheld from wages, September is a useful time to check whether your 2026 tax payments still match your income.
The federal and California schedules are not identical. That difference matters before you send a payment based on habit or assume that nothing is due.
The federal September 15 deadline
For calendar-year individuals, the third 2026 federal estimated-tax payment is due September 15, 2026. The payment period generally covers income earned from June 1 through August 31.
The IRS can assess an underpayment penalty when enough tax is not paid by the applicable payment-period deadline—even when the taxpayer later receives a refund. The correct amount depends on the full return, prior-year information, withholding, payments already made, and whether income was earned unevenly during the year.
Review the current 2026 Form 1040-ES and IRS Publication 505 before calculating or changing a federal payment.
California normally uses a different installment pattern
California's standard individual estimated-tax schedule is generally 30% for the first installment, 40% for the second, 0% for the third, and 30% for the fourth. Under that standard schedule, no third installment is normally required for September.
That does not mean every California taxpayer can ignore their state projection until January. A payment calculation may change when income increases, income is earned unevenly, prior estimates were too low, withholding changes, or special rules apply. California also provides a September payment voucher for situations in which a taxpayer is making a payment.
Use the current 2026 California Form 540-ES instructions instead of copying the federal installment amount automatically.
Information to review before paying
Gather current information rather than relying only on last year's return:
- Year-to-date business income and deductible expenses
- W-2 wages and federal and California withholding
- Estimated payments already made and prior-year overpayments applied
- Investment, rental, retirement, or other income
- Changes in filing status, dependents, credits, or major deductions
- Expected income and expenses for the rest of 2026
- Your 2025 federal and California returns
If your income is seasonal or changed sharply, ask whether recalculating or using an annualized-income method is appropriate.
Withholding may be another option
Someone who has both W-2 wages and self-employment or gig income may be able to increase withholding from wages instead of relying entirely on separate estimated payments. The IRS recommends reviewing the withholding calculation and submitting a new Form W-4 when an adjustment is appropriate.
Withholding and estimated payments interact, so review both before changing either one.
Pay through an official channel
Federal payment options are available through the IRS payments page. California individuals can review options through FTB Web Pay.
Confirm the tax year, payment type, amount, and confirmation details before submitting. Save the payment confirmation with your tax records.
The best September review answers two separate questions: what should be paid federally, and whether the California projection still works under California's own schedule.
If your income changed, your records are incomplete, or you are unsure how prior payments affect the calculation, INTELLIX Tax LLC can help organize the information and discuss the next practical step for your situation.