The first year of a side business has a predictable plot twist. The work goes well, the money lands in the account, and then a tax bill shows up that is noticeably larger than the one a paycheck of the same size would have produced.
Nothing went wrong. The money was simply never withheld.
A W-2 paycheck has already had income tax, Social Security, and Medicare taken out before it reaches you. Side-business income does not. The full amount arrives, it sits in your account looking like yours, and the obligation attached to it comes due later — four times a year, on a schedule most people meet for the first time by missing it.
This is not a moral failing or a sign you are bad with money. It is a mechanical difference between two ways of getting paid. The mechanics of staying ahead of it are equally plain: what fraction of the profit is already spoken for, where it sits until it is due, and when the dates fall.
The two taxes, and why the total surprises people
Profit from a side business generally gets taxed twice over — not double taxation in the technical sense, but two separate taxes stacked on the same dollars.
Income tax. Your business profit adds to your other income and is taxed at your marginal rate — the rate on your next dollar of income, not your average. If a W-2 job already puts you in the 22% bracket, the side-business profit generally starts at 22%, not at zero. This is the part that catches people who mentally price their side income at “my tax rate” without noticing it stacks on top of everything else.
Self-employment tax. This is the one with no W-2 equivalent, and it is usually the real source of the surprise. On a paycheck, you pay 7.65% toward Social Security and Medicare and your employer quietly pays the matching 7.65%. When you work for yourself, you are both parties, so you pay both halves: 15.3% total — 12.4% for Social Security plus 2.9% for Medicare.
Two details soften it slightly:
- Self-employment tax applies to 92.35% of net profit — what is left after deductible business expenses — not to the whole thing. So the effective bite is closer to 14.1% of profit than the headline 15.3%.
- Half of the self-employment tax is deductible against your income tax, which claws a little back.
And a few thresholds shape the edges. Self-employment tax kicks in once net earnings clear $400 for the year. The 12.4% Social Security portion only applies up to an annual wage base — $184,500 for 2026 — above which that piece stops, though the 2.9% Medicare portion keeps going with no ceiling. High earners add another 0.9% Medicare surtax above $200,000 (single) or $250,000 (married filing jointly).
Stack the two together and the reason for the surprise is clear: a side business owes its income-tax bracket plus roughly 14% toward Social Security and Medicare — half of which a paycheck showed you as FICA, and half of which an employer was quietly paying on your behalf.
If you want to see the self-employment piece on your own numbers, the self-employment tax calculator runs the same arithmetic.
Where the “set aside 25–30%” rule comes from — and where it falls short
The common advice is to park somewhere between a quarter and a third of net profit. That range is not arbitrary, but it is also not universal, and it is worth seeing exactly which situation it describes.
Work it through for someone in the 22% federal bracket:
| Component | Rate on net profit |
|---|---|
| Self-employment tax (15.3% × 92.35%) | ~14.1% |
| Federal income tax at a 22% marginal rate, applied after the half-SE deduction (22% × 92.9%) | ~20.4% |
| Blended, before state tax and before QBI | roughly 34.5% |
A modeled comparison, not advice. Held fixed: a 22% federal marginal rate, no state income tax, and no QBI deduction.
Notice that 34.5% sits above the 25–30% rule. That is not a rounding quirk — it is the honest consequence of the assumptions listed under the table, and it is the direction that costs money. Two things move it back toward the familiar range:
- A lower bracket. Someone in the 12% bracket lands near 25% (14.1% + 12% × 92.9%), which is where the low end of the rule comes from.
- The QBI deduction. A sole proprietor under the §199A taxable-income threshold — $201,750 single / $403,500 married filing jointly for 2026 — generally deducts 20% of qualified business income. That pulls the 22%-bracket figure to roughly 30.5%, the top of the range rather than above it.
So the rule of thumb describes a filer in a lower bracket, or one taking the QBI deduction, with no state income tax. Change any of those and the number moves — and a state income tax moves it up again.
Which is worth saying plainly: any single percentage is a planning cushion, not a calculation of what you will owe. It is a way of making sure the money still exists when the real figure is known. The precise number comes from a return, prepared with full information — yours or your preparer’s.
A cushion that lands under the real bill is the failure mode that hurts, since the shortfall is discovered at filing, when the money has already been spent. That asymmetry is the argument for checking the rule against your own bracket rather than adopting it as a default.
The four dates
Because there is no employer withholding along the way, the IRS expects the money in installments rather than all at once in April. For a calendar-year filer, estimated payments are generally due:
- April 15 — for income earned January through March
- June 15 — for April and May
- September 15 — for June through August
- January 15 of the following year — for September through December
Each date shifts to the next business day when it lands on a weekend or holiday. The uneven spacing is genuinely odd-looking — the second “quarter” covers two months, the fourth spills into the next calendar year — and it trips up people who reasonably assume the dates fall every three months.
Generally, estimated payments are expected once you would owe $1,000 or more at filing after accounting for withholding. Below that, the requirement typically does not apply.
What “safe harbor” actually means
Here is the part that reframes the whole exercise, and it is the single most useful thing to understand about quarterly taxes.
The IRS penalty for underpaying is not triggered by owing money at filing. It is triggered by not having prepaid enough during the year. And “enough” has a defined bright line — the safe harbor. Meet it, on time and across all four dates, and the underpayment penalty generally does not apply, even if a substantial balance is still due in April.
The federal safe harbor is generally the lesser of:
- 90% of the tax you will owe for the current year, or
- 100% of the tax shown on last year’s return — rising to 110% if last year’s adjusted gross income (AGI) was above $150,000.
The second option is why this matters so much. Last year’s total tax is a number that already exists, printed on a form you already have. You do not need to forecast a volatile year of side-business income to aim at it — it is a fixed, known target.
For a growing business that is a meaningful advantage: a year where income doubles can still be penalty-safe on the strength of the prior year’s much smaller number. The extra tax is still owed at filing; it simply is not penalized.
The mechanics carry real caveats. The standard schedule assumes roughly even payments across the four dates; for income that arrives in lumps, the IRS offers an annualized-income method — a worksheet that matches each payment to the income actually earned in that period, instead of assuming four equal ones. The prior-year option needs a prior-year return, not prior-year business income: a first year of self-employment still leans on the 1040 you filed for last year, W-2 income and all — and a modest prior-year tax bill is exactly what makes that anchor generous. What actually removes the option is having no full prior year behind you: no return filed for last year, or a prior tax year shorter than 12 months. Married filing separately has a lower AGI trigger. And this is the federal safe harbor only — states set their own rules, thresholds, and dates.
Where GlidePath fits
GlidePath is a desktop app that keeps the business side and the household side in one picture, and it treats this problem as two distinct jobs — because they genuinely are.
A cushion, computed from your own books. Once your business transactions are mapped to Schedule C — the IRS form a sole proprietor files with their return — the app knows Line 31, the net-profit line at the bottom of it. It applies a flat set-aside percentage to that figure: 27% by default, adjustable anywhere from 0% to 60%. The result folds into your household cash flow, so the money is visibly spoken for rather than sitting in the balance looking spendable.
That 27% is a starting point, not a recommendation, and the section above is the reason to look at it: it sits toward the low end of the range, so a filer in the 22% bracket — or one paying state income tax — has a straightforward case for raising it. The app is deliberate about naming the figure what it is: a blended rule-of-thumb cushion, not a filing-grade calculation. When your preparer gives you a real effective rate, you can put that number in instead. (How the mapping works.)
A real safe-harbor read, once you supply the inputs. Enter last year’s total tax from your 1040, last year’s AGI, and the withholding you expect this year, and the app computes the prior-year bright line — applying the 100%-or-110% test, subtracting expected withholding, and splitting the remainder across the four dated installments. Leave the AGI blank and it assumes the more conservative 110%, and says so.
That figure is a penalty-safe ceiling, not the minimum you owe. Its whole appeal is that it needs no forecast: pay it and you’re safe regardless of how this year turns out. But if you expect to earn less than last year, it will ask for more than you need — so the app carries the other §6654 path too. An optional Base it on this year instead panel takes your own income and profit estimate, applies the 90%-of-this-year test, and uses whichever of the two is lower. The prior-year number stays penalty-safe either way; the projection is only ever as good as the estimate you put in.
A payment jar that fills from your own transactions. As you categorize estimated-tax payments in your ledger, they count toward the target, and the app shows whether you are ahead of or behind the reference pace. Nothing is pulled from the IRS or a bank connection — it reads the transactions you already have.
One pack for your accountant. The Schedule C breakdown joins depreciation, home office, mileage, and 1099 prep in a single export, so the hand-off is a file rather than a shoebox.
These four run entirely on your own computer, against files you can open yourself. Both numbers show their work: where the figure came from, what it assumes, and where it stops being reliable.
Where the numbers stop
Worth stating directly, because a planning tool that hides its edges is worse than no tool:
- The set-aside is a flat percentage, not a tax engine. It does not model your brackets, your filing status, or your spouse’s withholding.
- The safe-harbor calculation is federal only. State estimated taxes have their own rules, thresholds, and schedules.
- Nothing auto-populates from a prior return. Last year’s figures are numbers you type in from a form you already have.
- The quarterly rhythm assumes even payments on the calendar-year schedule. Lumpy income may call for the annualized method.
- None of this is tax advice, and none of it files anything. It is bookkeeping and planning that makes the filing conversation shorter.
A rhythm that makes it boring
The reason quarterly taxes feel stressful is almost never the arithmetic. It is that the question gets asked four times a year, from scratch, under time pressure.
Folding it into a monthly money close removes most of that. Once a month, in the same sitting where you verify balances and import transactions, four things are worth having in front of you:
- Profit so far, and whether the set-aside percentage still matches the year you are actually having.
- The next due date, and whether the target is on pace.
- Where the set-aside money is sitting. Many owners keep it in a separate savings account — money in a different account is harder to spend by accident.
- Whether income has stepped up enough that the percentage is worth another look.
Handled that way, the quarterly dates stop being events. They become a transfer that was already funded, on a date that was already known, for an amount that stopped being a surprise months ago.
That is the whole goal. Not a perfect number — a number that already exists when it is time to pay it.
This guide explains general tax mechanics for planning purposes. It is not tax advice, and the figures a tool produces are estimates rather than filed amounts. Rules change and individual situations vary — a tax professional confirms what actually applies to you.