Ask a contractor when taxes are due and most will say "April 15." That's the deadline that gets all the attention — and it's only one of them. For business owners, the IRS wants its money four times a year, and the penalties for missing those dates compound quietly in the background.
September 15 is a triple deadline for many owners: third-quarter estimated payments, extended S-corp and partnership returns, and a checkpoint for everything else. Miss the estimate and penalties start compounding. Miss the extended return and you're late, extension or not.
Here's the full picture — the dates, the math, and the system that turns this from a panic into a process.
Why do contractors owe quarterly estimates in the first place?
Employees have it easy: their employer withholds tax from every paycheck and sends it to the IRS. Business owners have no employer. Nobody withholds anything. So the IRS requires you to send in your tax yourself, in four installments across the year.
If you're a sole proprietor, partner, or S-corp owner who'll owe $1,000 or more at filing time, you're generally expected to be paying as you go. Wait until April to settle up and you'll owe the tax *plus* penalties and interest for paying late all year.
When are estimated payments actually due?
Not quarterly in the calendar sense — the IRS runs its own schedule. For most owners:
- 1st payment: April 15
- 2nd payment: June 15
- 3rd payment: September 15
- 4th payment: January 15 of the following year
And those aren't the only dates on a contractor's calendar. March 15 is the S-corp and partnership filing deadline. April 15 covers individual and C-corp returns. October 15 is the extended individual deadline. Put them all on the calendar once, and tax season becomes a series of small tasks instead of one big panic.
What happens if you underpay?
The underpayment penalty is interest-based — it accrues on what you should have paid, for the period you should have paid it. The math adds up faster than owners expect:
Underpaying by $20,000 across the year can trigger penalties and interest approaching $1,600 — on top of the $20,000 you still owe in April. That's a $1,600 fee for the privilege of paying late, plus the cash-flow shock of a five-figure surprise.
There is a safe harbor, though. You generally avoid the penalty if you pay at least 100% of last year's total tax (110% if your income is over $150,000) or 90% of this year's tax, spread across the four payments. The safe harbor is the simplest target — but note it's based on last year's numbers, which only works if this year looks like last year.
How do you figure out the right amount?
The trap most contractors fall into: they set their estimates in January based on last year's profit, then never touch them again. But construction profit swings — a big job lands, a quarter goes slow, you buy equipment — and the January guess goes stale.
The book's mid-year rule applies here directly. Say your profit is running $60,000 ahead of last year. At roughly a 30% combined marginal rate, that's $18,000 of extra tax coming. Catch it in July and it's four manageable payments. Catch it in April and it's an $18,000 surprise plus penalties and interest.
The fix is a 30-minute check every quarter — or at minimum every July. Three questions:
- How does actual profit compare to the January estimate?
- Are my estimated payments on track with reality?
- Does anything big (equipment purchase, entity change, new hire) change the math?
What if you already missed a payment?
Pay it now. The underpayment penalty is time-based — it accrues for the period the money was late — so every week you wait makes it worse. Sending a late payment today stops the bleeding on that portion immediately. You don't need to wait for the next due date to catch up.
And if your income was uneven — a huge Q4 after three quiet quarters, for example — know that the penalty can be annualized. Form 2210's Schedule AI lets you match your payments to *when* the income actually arrived, which can shrink or eliminate the penalty when a late-year windfall caused the underpayment. It takes extra paperwork, but for contractors with lumpy income, it's often worth it.
What's the system that makes this painless?
Four habits, none of them complicated:
- Calendar every deadline once — all four estimates plus your entity's filing dates, for the full year ahead.
- Separate the money when it arrives. The owners who never get surprised move a percentage of every deposit into a tax account immediately. What percentage? Roughly your effective rate — 25–35% for most profitable contractors. It's not the IRS's money yet, but it's not yours to spend either.
- Run the quarterly check. 30 minutes, the three questions above. Adjust the next estimate up or down.
- Use the IRS Business Tax Account (irs.gov/businesses) to see what the IRS thinks you've paid. Owners who log in quarterly catch misapplied payments and missing estimates before penalties pile up.
Your move
Put every tax deadline for next year on your calendar right now — all four estimates plus your filing dates. It takes ten minutes and it kills an entire category of surprise.
And if your estimates were set once in January and never revisited, that's a mid-year check worth doing before the next payment date. Book a discovery call here and we'll re-run your numbers together.
*This article is for education only and isn't tax advice for your specific situation — talk to a qualified tax professional about your estimated payments.*