Why the tax system wants money four times a year
The United States runs a pay-as-you-go tax system. An employee satisfies that automatically through withholding. Anyone with income that carries no withholding — a sole trader, a landlord, an investor, a retiree drawing from an IRA without an election — has to send the money in themselves, in four instalments under IRC §6654.
Miss them and the consequence is not a fine but interest: the IRS charges the federal short-term rate plus three points on each underpaid instalment, running from that instalment's due date until the tax is paid. Because it is computed period by period, paying the whole balance in April does not cure a shortfall that arose in the first quarter.
The statute also gives you a way out that does not require a perfect forecast. Meet either safe harbour and no penalty applies, however wrong the projection turns out to be. That is the single most useful thing to know about estimated tax, and it is why the calculator applies both tests and reports the cheaper one.
Note what the safe harbour does and does not do. It removes the penalty. It does not settle the tax — any shortfall is still payable with the return, so the calculator also shows the larger instalment that would leave nothing owing.
Building the projection, then applying the two tests
First, project the tax. Self-employment tax comes first because half of it is an adjustment to income: net profit times 0.9235 gives net earnings, 12.4 percent applies up to the Social Security wage base and 2.9 percent to everything, and half the result comes off adjusted gross income. Subtract the standard deduction to get taxable income, apply the graduated rate schedule, and add the self-employment tax back to reach total tax. The self-employment tax calculator shows that half of the chain in more detail.
Second, apply the 90 percent test. Paying 90 percent of the current year's actual tax avoids the penalty. It is the cheaper target when income is falling, and the risk is obvious: it depends on a projection you are making before the year has happened.
Third, apply the prior-year test. Paying 100 percent of last year's total tax also avoids the penalty, rising to 110 percent if last year's AGI exceeded $150,000 — $75,000 if married filing separately. This target is a known number rather than a forecast, which makes it the safer choice in a year when income is rising sharply.
Fourth, take the lower target, subtract expected withholding, divide by four. Withholding counts toward the requirement and, unlike an estimated payment, it is treated as paid evenly across the four periods whenever it actually happened. That asymmetry is worth exploiting: extra withholding in December can cure a first-quarter shortfall, an extra December instalment cannot.
Two eligibility conditions apply to the prior-year test. You must have filed a return covering a full 12 months, and the prior year must have shown a tax liability. A first-year business has no prior-year harbour and must rely on the 90 percent test.
Worked example: $80,000 of projected profit with $12,000 of tax last year
A single filer expects $80,000 of Schedule C profit in 2026, no other income and no withholding. Last year's return showed $12,000 of total tax on an AGI below $150,000.
- Net earnings. 80,000 × 0.9235 = $73,880.00.
- Self-employment tax. 73,880 × 0.153 = $11,303.64, all of it below the wage base.
- Adjusted gross income. 80,000 − (11,303.64 ÷ 2) = 80,000 − 5,651.82 = $74,348.18.
- Taxable income. 74,348.18 − 16,100 standard deduction = $58,248.18.
- Income tax. 0.10 × 12,400 = 1,240.00; 0.12 × 38,000 = 4,560.00; 0.22 × (58,248.18 − 50,400) = 0.22 × 7,848.18 = 1,726.60. Total $7,526.60.
- Projected total tax. 7,526.60 + 11,303.64 = $18,830.24.
- The two targets. 90 percent of 18,830.24 = $16,947.22. 100 percent of last year's tax = $12,000.00.
- Required annual payment. The lesser, $12,000.00, so $3,000.00 a quarter.
Paying $3,000 four times keeps you out of penalty territory, but it leaves 18,830.24 − 12,000 = $6,830.24 payable when the return is filed. To owe nothing in April instead, pay 18,830.24 ÷ 4 = $4,707.56 a quarter. Both numbers are shown above so you can choose deliberately rather than discover the gap in April.
How to read the result
Decide which of the two instalment figures you want before the first due date. The safe-harbour figure minimises what leaves your bank account during the year and leaves a balance in April. The full-liability figure is larger each quarter and leaves nothing. Neither is more correct; the safe harbour is a floor, not a target.
Recheck the projection at least twice during the year. A quarter that runs far ahead of forecast changes the 90 percent target but not the prior-year target — which is exactly why the prior-year harbour is the safer basis in a growth year. If you are relying on the 90 percent test and income accelerates, you are underpaid retroactively for every earlier period.
If income is genuinely seasonal, use the annualised income instalment method. Schedule AI of Form 2210 lets you match instalments to when the income actually arose, so a business that earns nothing until September does not have to pay in April. It requires the schedule to be filed and it is more work, but for lumpy income it is often worth several hundred dollars.
Withholding is the most flexible lever you have. If you have a job alongside the business, or a spouse who does, raising withholding late in the year cures an earlier shortfall because withholding is deemed paid evenly. A one-off withholding election on an IRA distribution does the same thing.
State estimated tax is separate. Most states with an income tax run their own instalment regime with its own due dates and its own safe harbour percentages, and several do not follow the federal 110 percent rule at all. Run the state calculation independently.
Estimated tax due dates and the periods they cover
| Instalment | Income period covered | Due date | Months of income covered |
|---|---|---|---|
| 1 | 1 January – 31 March | 15 April | 3 |
| 2 | 1 April – 31 May | 15 June | 2 |
| 3 | 1 June – 31 August | 15 September | 3 |
| 4 | 1 September – 31 December | 15 January of the following year | 4 |
The instalments are equal under the standard method but the periods are not equal in length, which is what makes the annualised method useful for seasonal income. A due date falling on a weekend or federal holiday moves to the next business day.
Mistakes that cost money
- Forgetting self-employment tax in the projection. At 14.13 percent of profit it is frequently larger than the income tax, and it is the single most common reason a first-year business under-pays.
- Assuming April settles everything. The penalty is computed per period, so a late payment does not undo an early shortfall.
- Relying on the 90 percent test in a rising year. If actual income beats the projection, the target rises retroactively and every instalment was short.
- Using the prior-year harbour without checking eligibility. It requires a full 12-month prior year with a tax liability, so it is unavailable in a first year of filing.
- Missing the 110 percent step. Prior-year AGI above $150,000 raises the harbour by a tenth; paying 100 percent leaves you 10 percent short all year.
- Ignoring a large one-off event. A share sale, a Roth conversion or a business sale can multiply the liability in a single quarter — price it with the capital gains tax calculator and add an instalment for that period.
- Treating the instalments as deductible. Federal estimated tax payments are not a business expense. They are payments against a personal liability and never touch Schedule C.
The penalty is interest, and it is charged period by period
The underpayment charge under §6654 is computed on each instalment separately, at the federal short-term rate plus three percentage points, running from that instalment's due date to the earlier of the payment date and the filing deadline. Three consequences follow. Paying the whole year's tax in the fourth quarter does not fix a first-quarter shortfall. Overpaying one quarter does not automatically offset an earlier one, although the excess does carry forward to later periods. And there is no penalty at all if the balance due at filing is under $1,000, or if you had no tax liability at all in the prior year and that year covered 12 months.
How this fits with the rest of the year
Estimated tax sits between two other calculations. Upstream, the projection needs a credible profit figure and a credible self-employment tax figure; the self-employment tax calculator and the taxable income calculator build them. Downstream, the return reconciles what you paid against what you owed; the income tax refund estimator closes that loop.
Deductions that reduce Schedule C profit reduce both taxes at once, so they are worth revisiting before you fix the instalment amount. The home office deduction and the business mileage deduction are the two most commonly under-claimed, and each dollar they remove from profit saves income tax plus about 14.1 points of self-employment tax.
Payment mechanics are simpler than they used to be. Instalments can be made through IRS Direct Pay from a bank account, through EFTPS, or by card for a fee; the payment must be designated to the correct tax year and to Form 1040-ES so it is credited to the right period. Keep the confirmation numbers — reconciling misapplied estimated payments is one of the more tedious things a return preparer does.
One planning note that changes the shape of the year. Because withholding is treated as paid evenly and estimated payments are not, a household with any wage income has a strictly more flexible instrument available than one without. If you are choosing between raising a spouse's withholding and making a fourth-quarter instalment of the same size, the withholding route covers earlier periods as well — the instalment only covers its own.