Safe Harbor Tax Calculator

Two routes, and you only need one

The underpayment penalty is not a punishment for owing tax — it is a charge for paying too little during the year. The rules give you two ways out, and clearing either is enough. Pay at least 90% of what you will owe this year, or pay 100% of what you owed last year — rising to 110% if your prior-year adjusted gross income exceeded $150,000. Meet one of those and you are safe even if a spectacular December leaves you owing far more in April.

For freelancers with rising or unpredictable income, the prior-year route is usually the better bet, because last year's tax is a fact rather than a forecast. You can compute it exactly from a return you have already filed, whereas 90% of this year's tax depends on a number you will not know until the year ends. Base your payments on the prior-year figure, and any additional tax simply becomes due at filing without penalty.

What the penalty actually costs

The charge works like interest rather than a fine: it accrues daily on the shortfall from each quarterly due date until the money is paid, at the federal short-term rate plus three points. That rate has been 6% for the first two quarters of 2026 and 7% from July, compounded daily. On a $3,000 shortfall outstanding for roughly half a year, the cost is around $100 — irritating rather than ruinous.

That proportionality has a practical implication people often miss: if you are behind, paying something immediately is much better than waiting for the next due date, and far better than doing nothing until April. The clock runs continuously, so a payment made today stops the meter on that portion from today.

Where the quarters catch people out

The four periods are not equal. The first covers three months, the second covers only April and May, the third covers three months and the fourth covers four. Payments are due on 15 April, 15 June, 15 September and 15 January. Anyone assuming payments fall neatly every three months tends to be late in June — the shortest and most frequently missed period of the year. Our deadlines page lists every date for 2026 and 2027, with a calendar file.

When your income is lumpy

The default assumption is that you earned evenly across the year and should therefore pay in four equal instalments. Freelancers rarely do. If most of your income arrived in the second half, paying a quarter of the annual total in April can be genuinely difficult — and unnecessary. Form 2210's annualised income instalment method lets you pay based on income actually received in each period, which often eliminates a penalty entirely. It requires more record-keeping, and it is worth it in a year with a very uneven shape.

One exception worth knowing

If you owed no tax at all in the prior year, were a US citizen or resident for the whole twelve months, and that year covered twelve months, there is no penalty for the current year regardless of what you pay. That covers many people in their first full year of freelancing — but the tax itself still falls due at filing, so setting money aside remains essential even when the penalty risk is zero.

Frequently asked questions