Start with 15.3% self-employment tax on 92.35% of your net profit — that piece is fixed IRS math, not a guess: 12.4% for Social Security and 2.9% for Medicare. Then add your federal and state income-tax bracket on top of that same net-profit number. The IRS doesn't publish one blended percentage because your bracket depends on your total income and deductions, but once both pieces are stacked, a contractor netting a real profit after paying themselves commonly lands around 25-30% of net profit set aside. The only way to know your real number is to run last year's actual return through the math once, not guess at tax time.
You know the number exists somewhere, and you know it's bigger than you want it to be, but nobody ever sat you down and showed you where it comes from. Every contractor forum has a version of the same thread: someone had a good year, didn't set enough aside, and got hit with a bill in April that also came with a penalty for not paying quarterly. This isn't a tax-prep article and it isn't a substitute for your CPA. It's the actual IRS mechanics behind the number, laid out so you can run it yourself off a real profit figure instead of a feeling.
What exactly is self-employment tax, and why is it separate from income tax?
Self-employment tax is your version of the Social Security and Medicare tax a W-2 employee splits with an employer — as a self-employed contractor, you cover both halves yourself. Per the IRS's own Self-Employment Tax page: "The self-employment tax rate is 15.3%. The rate consists of two parts: 12.4% for social security (old-age, survivors, and disability insurance) and 2.9% for Medicare (hospital insurance)."
It isn't applied to your full net profit, either. Per IRS Topic 554: "Generally, the amount subject to self-employment tax is 92.35% of your net earnings from self-employment." Income tax is calculated separately, on top of that same underlying net-profit figure, at your regular federal and state bracket.
Do I actually have to pay the IRS four times a year?
If you expect to owe it, yes. Per the IRS's own Estimated Taxes page: "Individuals, including sole proprietors, partners, and S corporation shareholders, generally have to make estimated tax payments if they expect to owe tax of $1,000 or more when their return is filed." That $1,000 threshold is easy to hit for a contractor netting real profit, and it's on top of, not instead of, your April filing.
Per the IRS's own estimated tax FAQ, the four payment due dates fall on April 15, June 15, September 15, and January 15 of the following year, each covering roughly the prior three months of income. Miss a quarter and the IRS doesn't just wait for April — it can charge a penalty for that specific quarter even if the full year's tax gets paid on time.
| Payment period | Due date |
|---|---|
| January 1 – March 31 | April 15 |
| April 1 – May 31 | June 15 |
| June 1 – August 31 | September 15 |
| September 1 – December 31 | January 15 (following year) |
Source: IRS, "Estimated Tax" FAQ (irs.gov, 2026); dates shift to the next business day if they fall on a weekend or legal holiday. One-line takeaway: a contractor who only thinks about taxes in April is already three payments behind by the time they start.
What happens if I guess wrong and underpay?
The IRS charges an underpayment penalty, but it has a built-in escape hatch called the safe harbor rule. Per IRS Topic 306, most taxpayers avoid the penalty if they "paid withholding and estimated tax of at least 90% of the tax for the current year or 100% of the tax shown on the return for the prior year, whichever is smaller."
There's one wrinkle for a shop that had a strong prior year: per the IRS's own Form 2210 instructions, "If your adjusted gross income for [the prior year] was more than $150,000 ($75,000 if your filing status is married filing separately), substitute 110% for 100%." In plain terms: pay based on last year's actual tax bill, times 1.0 or 1.1 depending on your prior-year income, divided into four payments, and you're safe from the penalty even if this year turns out to owe more.
Does the 20% small-business deduction (QBI) lower what I need to set aside?
Only on the income-tax side, and this is the mistake that trips up a lot of owners who've heard about it secondhand. Per the IRS's own Section 199A FAQ, eligible taxpayers can deduct up to 20% of qualified business income — but "the QBID does not reduce net earnings from self-employment, under section 1402." It also doesn't reduce net investment income tax under section 1411.
So the 14.13% self-employment-tax floor from earlier stays exactly the same whether or not you qualify for the QBI deduction. The deduction only shrinks the income-tax portion of the bill, and only after your actual return confirms you qualify for it — don't build a 20% discount into your quarterly set-aside math ahead of time.
So what's a real number I can actually pull from every deposit?
The example below is illustrative, built to show the mechanism on round numbers, not a filed return.
| Net profit subject to SE tax (92.35%) | $9,235 |
| Self-employment tax (15.3% of that) | $1,413 |
| Illustrative blended federal + state rate | ~13% |
| Illustrative income tax on $10,000 | ~$1,300 |
| Combined, this example | ~$2,700–$2,900 |
Where should that money actually sit so I don't touch it?
In an account you never look at for anything else. The pattern that actually holds up across shops that stop getting surprised in April: open a separate savings account, and the day a job payment clears, sweep the set-aside percentage into it before the money has time to sit in the operating account long enough to feel spendable. Owners who set the number aside from the whole deposit, not "whatever's left after bills," are the ones who aren't scrambling to find cash on January 15.
This is also exactly the piece that gets missed when the books happen at the kitchen table on a Sunday night instead of the day the money lands. By the time anyone sits down to look, three different deposits have already blended into the operating balance and there's no clean way to tell what was job revenue and what was supposed to be next quarter's tax payment.
How does Top Builder AI help without me hiring a CPA?
It doesn't replace your CPA, and it doesn't file anything — the set-aside math above is yours to run, or your CPA's to confirm. What it does is keep the number you're running that math against honest: real job margin and net-profit figures computed straight from your actual ServiceTitan job data, not a guess made three months later during a bookkeeping cleanup. The Financial agent tracks earned revenue, cost-to-date, and margin per job, deterministically, so the profit figure you're applying 15.3% plus your bracket to is a real number as of today, not a number you half-remember from last quarter.
- No filed returns, no tax advice. This is the mechanism your CPA already knows — the agent's job is an accurate profit number, not tax prep.
- No invented figures. Every margin and profit number comes from your real ServiceTitan job data, computed deterministically.
- Nothing posts itself. The numbers are decision-support; the operator approves every action.
- Delivered as a configured install, set up against your real ServiceTitan job data from day one.
See your real margin, not a guess
Run the math above against your actual net profit for the quarter, not a number you're estimating from memory. A fit call walks through what your real ServiceTitan job data shows.
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