Quarterly Estimated Tax Calculator

Quarterly estimated tax is how people without an employer pay the IRS: four dated payments covering income tax plus self-employment tax on money nobody withholds from. You owe them if you expect to be short by $1,000 or more after withholding and refundable credits. The amount you must send isn't your tax bill — it's the smaller of the two safe harbors, and choosing the right one is usually worth thousands in cash flow.

The safe harbor is the number that matters

The IRS does not ask you to predict your year correctly. It asks you to clear one of two bars, and you only have to clear the lower one:

Harbor B is the one people miss. It is fixed on the day you file last year's return — a number you already know — and it doesn't move no matter how good this year gets. Double your income and harbor B doesn't budge; you settle the rest, penalty-free, when you file in April.

The four 2026 due dates

They are not evenly spaced, and the third gap is three months while the second is two. Each covers the period ending just before it:

None of the four lands on a weekend or holiday in 2026, so none of them shifts. You can skip the January payment entirely if you file your 2026 return and pay the balance by February 1, 2027 — the ordinary January 31 deadline pushed to the next business day.

What underpaying actually costs

The underpayment charge is interest, not a flat fine. Each missed instalment accrues from its own due date until the money arrives or April 15, 2027, whichever comes first — so the first quarter's shortfall is charged for a full year while the fourth's runs only 90 days. Form 2210 computes each rate period as simple interest:

2026 is not a single-rate year. The section 6621 rate for non-corporate underpayments was 7% in the first quarter, dropped to 6% for the second, and returned to 7% for the third and fourth. Anyone applying one annual rate across the whole year gets the answer wrong:

Skipping all four payments in the example above costs about $843 — roughly 9% of the $9,000 that should have been paid, because most of it sat outstanding for the better part of a year. The charge applies per period even if your finished return shows a refund.

Self-employment tax is most of the bill

For anyone under six figures, self-employment tax usually exceeds income tax, and it starts at the first dollar of profit — there is no standard deduction shielding it:

• A day job uses up the wage base first. If W-2 wages already cover the $184,500 Social Security base, freelance profit on top pays only the 2.9% Medicare half — which is why the calculator asks about wages separately.
• Half of it is deductible against income tax (not against SE tax itself), which is already applied above before the brackets run.
• The 0.9% Additional Medicare surcharge is not deductible — it sits outside the one-half deduction.
• The Schedule SE you download may print last year's wage base. The 2026 figure is $184,500; forms circulating early in the year often still show $176,100.

Two ways to stop making quarterly payments

• Withhold instead. Tax withheld from a W-2 job, a pension or an IRA distribution is treated as paid evenly across all four periods no matter when it was actually taken. A December withholding top-up can retroactively cure a shortfall from April — an estimated payment in December cannot.
• Have a spouse over-withhold. On a joint return their W-4 withholding counts toward the household's required annual payment, so one job can carry the other's freelance income without either of you writing a quarterly check.
• Watch the first-year trap. Harbor B needs a prior-year return covering a full 12 months. In your first year of self-employment there is no harbor B, so 90% of the current year is the only bar — the year you can least afford to guess is the year you have to.