The 5-Day Countdown To September 15: What To File Now
Updated: August 2026 · Industry Advisors · Education, filed, 2006.
Five business days from the September 15 Q3 estimated-tax deadline is when late payments start compounding into a real bill. If today is anywhere between September 10 and 14, this is the checklist that fits.
Day −5 (Sept 10) · Pull Last Year's Form 1040
Open last year's return. The line you need is line 24, Total Tax. This is the base for the prior-year safe harbor. If AGI on last year's return was above $150,000, multiply by 110%; otherwise multiply by 100%. Divide by four — that's the quarterly amount. This is the same math covered in the September 15 estimated-tax deadline post; the version below is the compressed 5-day workflow.
Day −4 (Sept 11) · Compute The Safe-Harbor Number
The alternate route: 90% of this year's projected tax, divided by four. Use it only if this year is materially lower than last. If you're not sure, use the prior-year route — it's safer, provable, and cannot be second-guessed by an examiner. The safe-harbor calculator post walks through worked examples.
Day −3 (Sept 12) · Confirm Bank Funds
ACH from IRS Direct Pay clears in 1–2 business days. Make sure the balance is there by the 12th. If you're paying by card, budget for the 1.75–1.99% processor fee.
Day −1 (Sept 14) · Pay Federal And State
Federal at IRS Direct Pay (Form 1040-ES, tax year 2026). State at your state's estimated-tax portal on the same day. Screenshots of both confirmations belong in your bookkeeping folder. If you have moved states in the past year, foreign qualification questions are covered in the foreign qualification post.
Day 0 (Sept 15) · Confirm And Archive
Verify both confirmations show up in your bank statement. Log the amounts in bookkeeping under Estimated Tax Payments. Move on — nothing else is due until January 15.
Quick Check
Three questions on the safe-harbor rule. Tap each to reveal the answer.
Q1. Last year's Form 1040 line 24 read $18,000. AGI was $135,000. What is the safe-harbor annual amount?
Answer: $18,000. AGI is under $150K, so 100% applies. Quarterly = $4,500.
Q2. Same facts but AGI was $190,000. What is the annual safe-harbor?
Answer: $19,800. AGI over $150K triggers the 110% multiplier. Quarterly = $4,950.
Q3. You paid nothing in Q1 and Q2. On Sept 15 you pay the full $13,500 cumulative Q1–Q3. Are you penalty-free for those quarters?
Answer: Partially. You stop accruing penalty on Sept 15, but Q1 and Q2 amounts were late — a small underpayment penalty applies to the days those amounts sat unpaid. Better than nothing.
Where This Connects
Estimated tax is one of twelve recurring items on the Q4 compliance checklist. If Q3 goes clean, Q4 planning starts in October alongside the S-Corp election window decision and any year-end housekeeping that has slipped.
Sources And Further Reading
The Everyday Owner's Blueprint includes the 12-month tax and compliance calendar with every quarterly deadline pre-mapped.
Industry Advisors · 30 Wall Street, New York, NY 10005. Education, not legal or tax advice.
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