Is 15 September still the Q3 date in 2026?

For calendar-year individuals, the third estimated tax installment is due 15 September 2026. Confirm if you are on a fiscal year or have an extension situation. This is process guidance, not tax advice.

Can I just pay 25% of last year’s tax?

Safe-harbor rules exist and are fact-specific (including higher-income variants). Ask your CPA which harbor you are using before you wire ‘a round number.’ A guess that misses can still be a penalty even if you ‘meant to catch up in Q4.’

What if RSUs vested in August?

Withholding on vest is not automatically enough. Send the grant/vest confirmations this week. Do not wait for the 1099 in January to discover Q3 was short.

We are donating art later this year. Does that change Q3?

Maybe for annualized income, not as a last-minute voucher hack. Appraisal and Form 8283 are a different workstream — see the 8283 guide. Do not delay the 15 September payment waiting on an appraisal.

Q3 Estimated Taxes Due 15 September 2026: What to Send Your CPA This Week vs After the 15th

September 3, 2026 · Guide · 1 min read

Today is 3 September 2026. The third estimated tax payment for most calendar-year individuals is 15 September 2026. A Dropbox dump on the 14th is how firms file a voucher with last quarter’s numbers and explain penalties in April.

Send the packet this week. Compare CPA firms in cities or near you. Charitable art gifts are a separate assignment: Form 8283 and qualified appraisals.

Send before the 15th

YTD paystubs / K-1 estimates / 1099-NEC drafts you already have.
RSU/ESPP vest confirmations since the last estimate.
Whether you are on prior-year safe harbor or annualized income — in one sentence from last quarter’s memo.
State estimate if you owe a separate voucher (rules differ; ask, do not copy federal).

Can wait until after the 15th

Perfect December projections.
A donation appraisal that has not been inspected yet.
Rewriting last year’s return because you are mad at Q2.

A CPA who asks for the vest PDFs on 4 September is doing the job. A CPA who says ‘we’ll true up at filing’ is selling you a penalty conversation. This page is not advice; it is a calendar. Book the call, then pay on time. Start at firm listings.

Frequently Asked Questions

Is 15 September still the Q3 date in 2026?: For calendar-year individuals, the third estimated tax installment is due 15 September 2026. Confirm if you are on a fiscal year or have an extension situation. This is process guidance, not tax advice.
Can I just pay 25% of last year’s tax?: Safe-harbor rules exist and are fact-specific (including higher-income variants). Ask your CPA which harbor you are using before you wire ‘a round number.’ A guess that misses can still be a penalty even if you ‘meant to catch up in Q4.’
What if RSUs vested in August?: Withholding on vest is not automatically enough. Send the grant/vest confirmations this week. Do not wait for the 1099 in January to discover Q3 was short.
We are donating art later this year. Does that change Q3?: Maybe for annualized income, not as a last-minute voucher hack. Appraisal and Form 8283 are a different workstream — see the 8283 guide. Do not delay the 15 September payment waiting on an appraisal.