Estimated Taxes: What’s Due September 15

Source: California Franchise Tax Board

If you have income that nobody withholds tax from, September 15 is worth putting on your calendar. It’s the federal deadline for the third estimated tax payment of 2026, covering what you earned from June through August.

Who this actually applies to

If your income is a regular paycheck, you can mostly stop reading. Your employer withholds tax every pay period, so you’re square with the IRS by default.

It matters when money arrives without withholding — 1099 or consulting work, a side business, rental income, a large investment gain, or RSUs that vested without enough tax taken out. In those cases the IRS can charge an underpayment penalty even if you pay every dollar you owe next April. The penalty is for paying late, not for paying short.

California doesn’t want anything in September

This is the part that catches people. California front-loads its estimated payments: 30% in April, 40% in June, 0% in September, and 30% in January.

So if you’re a California resident, September 15 is a federal-only date. The FTB isn’t expecting a payment until January 15.

How much to send

There are two ways to land on a number. You can project what you’ll owe for the year and pay a quarter of it, or you can use the safe harbor — pay 100% of what your 2025 tax came to, spread across the four payments. If your 2025 adjusted gross income was over $150,000, that threshold is 110% instead. Hit it and the IRS won’t penalize you, whatever this year turns out to be.

Safe harbor tends to be the easier path when your income is uneven, which it usually is for the people this applies to.

What to do before the 15th

Pull last year’s return and find your total tax — that’s the number safe harbor keys off. Then add up any income this year that arrived without withholding. If those two things don’t tell you clearly whether you owe something, that’s a better conversation to have before the 15th than after.

This is educational only — not tax advice, and not a recommendation for your situation. Estimated tax rules turn on details specific to you, so confirm anything here with your tax preparer before acting on it.