Guide
If you run a plumbing, HVAC, electrical, cleaning, landscaping, or handyman business as a sole proprietor or LLC, the third-quarter estimated tax payment is due September 15, 2026. Here's how to figure what you owe without guessing.
Most trade businesses have their best months between June and September. That means the September 15 payment usually covers your highest-earning stretch of the year — which also means it's the easiest one to underestimate. Owners who set aside a flat percentage all spring get caught out when a busy July and August push net profit well past what they budgeted for.
The IRS wants four payments a year: April 15, June 15, September 15, and January 15. September 15 is the one that lands right after peak season cash flow, so it's worth treating differently than the others.
If you expect to owe $1,000 or more in tax for the year after subtracting any withholding (rare for most owner-operators, common if you or a spouse also has a W-2 job), you're generally expected to pay estimated tax quarterly rather than in one lump sum next April. This applies whether you're a sole proprietor, single-member LLC, or S-corp owner paying yourself a mix of salary and distributions.
This isn't tax advice — every setup is different once you add spouses, other income, or an S-corp election. Confirm your specific number with your accountant. What follows is the general shape of how it works so you're not walking into that conversation blind.
The rule of thumb most bookkeepers use for owner-operators: 25–30% of net profit for federal income tax plus self-employment tax combined, adjusted up or down based on your state and your actual bracket. Net profit means after materials, subcontractor costs, vehicle expenses, insurance, and other write-offs — not your top-line revenue from jobs booked.
Quick gut check: if your business nets $90,000 this year after expenses, a 25–28% set-aside is roughly $22,500–$25,200 for the full year, or about $5,600–$6,300 per quarter. Your actual number depends on filing status, other income, and deductions — this is a planning estimate, not a filing number.
The IRS gives you a safe harbor to avoid a penalty even if your estimate is off: pay at least 90% of what you'll owe this year, or 100% of what you owed last year (110% if last year's adjusted gross income was over $150,000, or $75,000 if married filing separately). Hitting either number keeps you out of penalty territory even if your final bill is higher than expected.
Say a two-truck HVAC business nets $18,000 in profit for the July–September stretch after payroll, parts, and overhead. Run that through a job profitability calculator job by job during the quarter and you'll have a real net number instead of a guess when September 15 rolls around — most owners who get surprised by a big tax bill were tracking revenue, not profit, all quarter.
At a 27% set-aside, that's roughly $4,860 for the quarter. If you've been pulling a flat percentage from every invoice into a separate savings account as it's paid — not waiting until the deadline to figure it out — you're not scrambling to find the cash in September.
If your pricing has been inconsistent job to job, it's worth running your numbers through a break-even calculator first. A business pricing too close to break-even has a much smaller cushion to pull tax money from than one pricing with a healthy margin built in — and the calculator will show you exactly where that line sits.
Miss or shortchange a quarterly payment and the IRS charges interest on the underpaid amount from the due date until you pay it. That rate moves every quarter — it's running at 7% annually for Q3 2026 — so don't treat any number here as fixed; check irs.gov/payments/quarterly-interest-rates for the current rate before you do the math. The penalty compounds daily, but it's still far cheaper than not paying at all, so a late or partial payment beats skipping the quarter entirely.
The owners who never stress about September 15 do one thing differently: they move money the day they get paid, not the week before the deadline. A simple version that works for a one- or two-truck operation:
If quoting, invoicing, and this kind of admin work keep sliding to the bottom of the list because you're out running calls, that's exactly the recurring back-office work we take off owners' plates — see what that looks like at Hire us. For a lighter-weight fix, the Operations Kit ($19) includes simple tracking templates that make the monthly check-in above a five-minute task instead of a spreadsheet project.
Generally yes, if you expect to owe $1,000 or more in tax for the year after subtracting any withholding. Most sole proprietors, single-member LLC owners, and 1099 subcontractors fall into this bucket once the business is making real money. Check with your accountant to confirm your specific situation.
You generally avoid an underpayment penalty if you pay at least 90% of what you owe for the current year, or 100% of what you owed last year (110% if last year's adjusted gross income was over $150,000, or $75,000 married filing separately) through withholding and estimated payments combined.
The IRS starts charging interest on the unpaid amount from the due date until you pay it, at a rate set quarterly. For Q3 2026 that rate is 7% annually, but it can change each quarter, so confirm the current rate at irs.gov before assuming a number. Paying late is still better than not paying at all.