Why Quarterly Payments Matter

The IRS requires self-employed individuals to pay taxes as they earn income. If you don't pay enough throughout the year, you may face an underpayment penalty — even if you pay your full tax bill by April 15.

The IRS "safe harbor" rule: you won't owe penalties if you pay at least 90% of your current year tax OR 100% of last year's tax (110% if your AGI exceeds $150,000).

How This Calculator Works

It works the way the IRS does: from the bottom up, on what you have actually earned so far this year rather than on a guess made in January.

Your year-to-date gross income minus your business expenses gives net profit. Self-employment tax applies to 92.35% of that figure — the adjustment exists because an employee's half of FICA is not itself taxed, and this keeps the self-employed on the same footing. The rate is 15.3% (12.4% Social Security plus 2.9% Medicare) up to the Social Security wage base, which is $184,500 for 2026; above that, only the 2.9% Medicare portion continues. Income tax is then estimated on top, and anything you have already paid this year is subtracted before the figure is divided across the remaining quarters.

The Safe Harbour Is the Number That Actually Protects You

Underpayment penalties are not charged because you guessed your income wrong. They are charged because you paid in too little, too late. Two thresholds end that exposure:

• 90% of this year's total tax, or
• 100% of last year's total tax — 110% if your adjusted gross income exceeded $150,000.

The second is the useful one, because last year's number is already known and cannot move. If your income is climbing and you have no idea where it will land, paying to the prior-year figure buys certainty: you may owe a balance in April, but you will not owe a penalty for how you got there.

Five Things That Catch People Out

1. A spouse's withholding counts, and it counts generously. Tax withheld from a paycheque is treated as having been paid evenly across the whole year, whatever month it was actually withheld. If you are married filing jointly and your spouse has a job, increasing their withholding late in the year can repair an underpayment that estimated payments no longer can.

2. The quarters are not quarters. The payment periods run three, two, three and four months. A Q2 payment on 15 June covers April and May only. Budgeting in even thirds across the year will leave you short in June.

3. Uneven income has its own method. If you earned almost nothing until September, the default calculation will penalise you for not having paid in June. The annualised income installment method — Schedule AI of Form 2210 — recalculates each period against what you had actually earned by then. It is more work and it is usually worth it for seasonal or project-based income.

4. The penalty is interest, not a fine. It accrues at the federal short-term rate plus three points, charged from each missed due date until payment. A late payment is therefore always better than a skipped one, and a partial payment is better than nothing.

5. Deadlines shift. When the 15th falls on a weekend or a federal holiday, the due date moves to the next business day.

What This Calculator Cannot See

It does not know about your state or city estimated payments, which most states require on their own schedule. It does not model the qualified business income deduction, credits, capital gains, or a spouse's separate self-employment income. It assumes the standard deduction rather than itemised deductions. Treat the figure as a well-grounded starting point for setting money aside — not as a filing position.

Primary sources: IRS Publication 505, Tax Withholding and Estimated Tax · IRS Topic 751, Social Security and Medicare withholding rates · SSA contribution and benefit base. Figures checked against these sources on 11 September 2026.