What Philadelphia Business Owners Need to Do Before September 15
If you’re self-employed, run an S-corp, or earn income that doesn’t have taxes withheld automatically, mark your calendar: September 15, 2026 is your third-quarter estimated tax deadline. Miss it, and the IRS doesn’t just send a reminder — it charges a penalty, even if you’re planning to pay everything in full when you file next April.
Here’s who this applies to, how to calculate what you owe, and how to avoid the most common (and most expensive) mistakes.
Who Actually Has to Pay Estimated Taxes?
The IRS expects estimated payments from anyone who won’t have enough tax withheld throughout the year — which includes most:
- Freelancers and independent contractors (1099 income)
- Sole proprietors and single-member LLCs
- S-corp owners taking distributions on top of salary
- Partners in a partnership or multi-member LLC
- Real estate investors with significant rental income
- Anyone with substantial investment, dividend, or capital gains income
As a general rule, if you expect to owe $1,000 or more in tax for the year after withholding and credits, the IRS wants quarterly payments — not one lump sum in April.
The 2026 Estimated Tax Deadlines
Estimated taxes are paid in four installments throughout the year:
- Q1 2026: April 15, 2026
- Q2 2026: June 16, 2026
- Q3 2026: September 15, 2026
- Q4 2026: January 15, 2027
Q3 is the one people forget most often — it falls right in the middle of back-to-school season and Q4 planning, with no obvious “tax season” cue to remind you.
How to Calculate What You Owe
1. Start with Last Year’s Tax Liability
The simplest safe-harbor method, outlined on IRS Form 1040-ES: pay 100% of what you owed last year (110% if your prior-year adjusted gross income was over $150,000), divided into four equal payments. This protects you from underpayment penalties even if this year’s income is higher than expected.
2. Or Calculate This Year’s Actual Liability
If your income has changed significantly — a big new client, a slow quarter, a business sale — a straight percentage of last year’s taxes may leave you overpaying or underpaying. A mid-year projection based on actual year-to-date numbers is more accurate, especially if profits are trending up.
3. Don’t Forget Self-Employment Tax
Estimated payments aren’t just income tax — for sole proprietors and single-member LLC owners, they also cover the 15.3% self-employment tax (Social Security and Medicare). This is the piece people most often underestimate.
What Happens If You Miss the Deadline
The IRS charges an underpayment penalty calculated on a quarterly basis, currently based on the federal short-term rate plus 3%. It compounds daily. The penalty applies from the date the payment was due — not from when you eventually file your return — so waiting until April to “catch up” doesn’t erase the Q3 penalty.
The good news: it’s not usually a business-ending amount, but it’s real money that’s completely avoidable with basic planning.
Common Q3 Mistakes to Avoid
- Paying based on last year’s numbers when this year is dramatically different — either overpaying and hurting cash flow, or underpaying and getting hit with penalties
- Forgetting state and local estimated payments — Pennsylvania and Philadelphia have their own estimated payment requirements separate from federal
- Waiting until September 14 — payment processing and confirmation can take time; don’t cut it close
- Not adjusting after a strong Q2 or Q3 — if business picked up mid-year, your Q3 and Q4 payments should reflect that, not just mirror Q1 and Q2
The Bottom Line
Estimated taxes are one of the easiest things to fall behind on because there’s no single moment that forces you to deal with them — no W-2, no year-end statement, just a date on the calendar. The business owners who avoid penalties are the ones who treat it like any other recurring bill: reviewed and paid on schedule, not calculated in a scramble on September 14.
Not sure what your Q3 payment should be, or whether you’re on track for the rest of the year? JD Tax & Accounting Advisors helps Philadelphia freelancers, S-corp owners, and small businesses calculate accurate quarterly payments — so April never comes with a surprise. Book a free consultation before the September 15 deadline.