Estimated
quarterly
tax.

Why this one catches everybody

Employment hides tax by design. It is removed before the money lands, in small amounts, by somebody else’s software, and most people go a decade or more without ever having to think about the timing of it.

Self-employment removes the machinery and keeps the obligation. The money now arrives whole, which feels like more of it, and the obligation arrives separately on a schedule nobody mentioned. The surprise is almost never about the rate. It is about the calendar.

The four dates, and why they are not quarters

They are called quarterly payments and the periods are not quarters. This is a real source of error, because a reader who assumes equal three-month blocks will be late on the second payment and early on the fourth. From the IRS:

• Income earned 1 January to 31 March, payment due 15 April.
• Income earned 1 April to 31 May, payment due 15 June. Two months.
• Income earned 1 June to 31 August, payment due 15 September. Three months.
• Income earned 1 September to 31 December, payment due 15 January of the following year. Four months.

If a due date lands on a Saturday, Sunday or legal holiday, the IRS treats payment as on time if it is made on the next day that is none of those.

Why a refund does not rule out a penalty

The penalty is not simply a charge for owing tax at the end of the year. It can apply because you did not pay enough during each payment period. That means you can be due a refund overall and still owe a penalty for paying too little earlier in the year.

Anyone intending to settle up once, in April, on the basis that it all comes out even, is operating on a model of the system that does not match it.

The three ways out of the penalty

The IRS describes the relief as: “most taxpayers will avoid this penalty if they owe less than $1,000 in tax after subtracting their withholdings and credits, or if they paid at least 90% of the tax for the current year, or 100% of the tax shown on the return for the prior year, whichever is smaller.”

The third of those is the one worth understanding, because it is the only one you can act on with certainty. The current-year figure is unknowable until the year ends. Last year’s total is a number you already have. Paying against a known figure converts an estimate into arithmetic, which is why it is the route most often recommended to people whose income varies.

Special rules apply to farmers, fishermen and higher-income taxpayers, and those live in IRS Publication 505 rather than on this page, partly because they are genuinely more complicated and partly because the thresholds involved move.

What to do with this, practically

The simplest habit is to set money aside as you earn it instead of trying to find it when payment is due. The right percentage depends on your income, state, and deductions, but the four federal due dates are published and easy to add to your calendar now.

One more thing worth knowing: the obligation is not triggered by being incorporated or by having a business name. It is triggered by expecting to owe. Someone with a salaried job and a side trade can land in this regime without ever having thought of themselves as a business.