Contractors and other self-employed workers have a quarterly deadline coming up that's easy to lose track of between jobs: the third-quarter estimated tax payment is due September 15, 2026.
What's trending
Unlike a W-2 employee, contractors and other 1099 workers don't have taxes withheld automatically, so the IRS collects estimated tax in four payments across the year. For 2026, those dates are April 15, June 15, September 15, and January 15, 2027 for the final quarter. Anyone who expects to owe $1,000 or more for the year after withholding is generally required to make these payments, and self-employment tax — 15.3% for Social Security and Medicare — is calculated on top of regular income tax, since a self-employed worker is effectively paying both the employer and employee share.
The safest way to avoid an underpayment penalty is the safe harbor method: paying either 90% of the current year's estimated tax or 100% of the prior year's total tax liability (110% if last year's income was higher), split evenly across the four payments.
Why it's resonating
For contractors, Q3 lands at an awkward time — right in the middle of the busy season, when it's easiest to let paperwork slide. But an underpaid Q3 estimate doesn't just create a bigger bill in January; it can trigger a penalty even if the full year's taxes eventually get paid, because the IRS looks at whether each quarter was covered on time, not just the annual total.
The other common trap is basing the estimate on a rough guess instead of actual numbers. A contractor who's had a strong summer season and simply pays what they paid last quarter can end up underpaying by a wide margin if income was uneven across the year.
Getting the number right
The estimate is only as good as the records behind it — income received, materials and job expenses paid, and mileage driven to job sites all factor into what's actually owed. Contractors who track invoices, job expenses, and mileage as they go, rather than reconstructing three months of receipts the week before the deadline, tend to land on a far more accurate Q3 number. Apps built for that kind of day-to-day tracking, like the Small Business Finance Tracker on Etsy, keep invoices, job expenses, and mileage logs in one place specifically so that number is easy to pull together instead of guessed at.
The bottom line
September 15 isn't far off, and for contractors mid-way through a busy build season, it's worth pulling actual income and expense numbers now rather than waiting until the week of the deadline. A few minutes of bookkeeping this month is a lot cheaper than an underpayment penalty in January.