How do I avoid an underpayment penalty on estimated taxes?

Pay at least 90 percent of this year's tax, or 100 percent of last year's tax (110 percent if your prior-year adjusted gross income was above 150,000 dollars) - whichever is lower. Meeting either test is enough.

What are the quarterly due dates?

Generally April 15, June 15, September 15 and January 15 of the following year. If a date falls on a weekend or holiday it moves to the next business day.

Why are the four payments not equal to a quarter of my tax?

Income is often uneven, and the safe-harbour test is based on the total paid across the year rather than each instalment being exactly a quarter. You can annualise using Schedule AI if your income is seasonal.

Quarterly estimated tax

Quarterly estimated tax calculator

Four payments a year, and a penalty that only applies if you fall short of a specific threshold. This tool tells you the target, the amount per quarter and the dates - then you can set a reminder and stop thinking about it.

Estimate only, not tax advice. State estimated-tax rules, the annualised income instalment method (Schedule AI) and wage withholding are not modelled. Tax year .

Payment per quarter

Estimated annual tax

Safe harbour target

Payment schedule

Payment	Due date	Amount

If a due date falls on a weekend or federal holiday it moves to the next business day. Confirm with the current IRS instructions.

The penalty is avoidable, and the rule is mechanical

Underpayment penalties are not a judgement call. You avoid them by paying enough, on time, against one of two tests:

90% of this year's tax - hard to do when you do not know the year yet
100% of last year's tax - 110% if your prior-year AGI was above $150,000

Meeting either is enough, which is why the second test is the freelancer's friend: it is knowable in January, and for a rising income it is usually the cheaper target. That is what the calculator uses when you enter last year's number.

Why four dates instead of monthly

The system is quarterly by design - think of it as a settlement of the year so far, not a monthly withhold. That is also why uneven income is a problem: in a seasonal business you can end up owing a penalty even if the annual total was plenty. The fix is the annualised income instalment method (Schedule AI), which this calculator does not model.

The three ways people get this wrong

Only paying on profit. The safe harbour is based on total tax - it includes self-employment tax and the income tax on it.
Forgetting state estimates. Many states run their own quarterly system with different dates and thresholds.
Stopping after a low year. If income drops, the prior-year safe harbour protects you - but you have to elect it deliberately.

Questions

Do I owe estimated tax if I also have a W-2 job?

Only if the withholding does not cover the extra tax from your self-employment income. The cleanest fix is usually to increase the withholding on your W-2 rather than sending quarterly payments.

Can I pay all four quarters at once?

You can, but paying late for an earlier quarter still accrues interest for that period. Estimating four equal payments by the first deadline is the simple version most people use.

What if my income is seasonal?

Use the annualised method (Schedule AI) rather than equal quarters. This calculator assumes equal instalments, which will overstate the required payment in a back-loaded year.

Deadline reminders

One email per deadline, with the four dates and your amounts.