September 15 Checkpoint for Founders
Hit safe harbor, lock in deductions, and sidestep 7% IRS interest before year-end.
September 15 isn’t just another due date — it’s the moment you either get ahead of penalties or let them compound. For 2025, this checkpoint brings three big levers for U.S. founders: (1) dialing in Q3 estimated taxes to meet safe harbor, (2) shipping extended S-corp or partnership returns with clean K-1/K-3 packages, and (3) front-loading deductions unlocked by this year’s law changes. Below is the playbook, with numbers that matter now.
1. Estimated taxes: safe harbor math you can actually use
For calendar-year individuals, the third estimated tax payment is due Monday, September 15, 2025. The IRS safe harbor remains: pay 90% of your 2025 tax or 100% of your 2024 tax (110% if your 2024 AGI > $150,000) to avoid underpayment penalties.
Penalty rate reality check: the IRS underpayment interest rate is 7% (Q3 2025), compounded daily. Every underpaid “quarter” is priced separately, so catching up in Q4 doesn’t erase Q1–Q3 shortfalls.
Annualize if your income is lumpy. When income is back-loaded (common for founders), the Annualized Income Installment Method (Form 2210 Schedule AI) reweights what each quarter “should” have been; it often cuts or eliminates penalties versus the straight 25/25/25/25 method. Use Pub. 505 Worksheet 2-7 to compute required installments and file Form 2210 with Schedule AI.
Worked example (solo founder with uneven income):
YTD through 8/31/25: $420,000 pass-through income, $120,000 W-2 wages; withholdings YTD $18,000.
Projected full-year tax: $165,000. Prior-year (2024) total tax: $140,000; 2024 AGI: $380,000.
Safe harbor target = 110% × 2024 tax = $154,000 (because AGI > $150k). You’ve paid $18,000 via withholding; to be a line-of-sight safe harbor by 9/15, you’d want to have paid ≈ a cumulative of $154,000. If Q1+Q2 estimates were $0, send $136,000 on 9/15 to cross safe harbor and stop penalty exposure for the rest of 2025. If cash-tight, compute Schedule AI; if 80% of revenue came after June 1, your required Q1/Q2 under AI will be far lower than 25% each. (
Small but costly miss: founders who try to “catch up in January” forget that January 15, 2026, is for Q4, not for prior-quarter shortfalls; Q3 underpayments still accrue 7% until fixed.
2. Extensions due: what September 15 returns must get right
Calendar-year partnerships (Form 1065) and S corporations (Form 1120-S) on extension are due September 15, 2025. Get K-1s out and, if relevant, satisfy the K-2/K-3 “one-month date” rule: if any partner/shareholder requested K-3 info by August 15, 2025, you must file K-2/K-3 with the return and furnish K-3.