Four estimated tax payments, three different lengths of year: what the IRS actually measures

The four federal estimated tax dates for 2026 are April 15, June 15, September 15, and January 15, 2027. They are called quarterly, but the periods they cover are not quarters. And if you want to sit inside the IRS safe harbor, each of the four payments is a quarter of the required annual amount, whatever the profit did that period.

*Figures are for tax year 2026. The safe-harbor percentages and thresholds match those verified against IRS Publication 505 on 22 September 2026 for the linked guide. Dates were computed against the 2026 calendar on 28 September 2026.*

The periods are 3, 2, 3 and 4 months

The IRS ties each installment to a payment period: January 1 through March 31, April 1 through May 31, June 1 through August 31, and September 1 through December 31. That is three months, two months, three months and four months. The due dates fall after each period closes, and the last one falls after the year does. Source: IRS, Form 2210 instructions and Publication 505.

The 2026 dates land on a Wednesday, a Monday, a Tuesday and a Friday. None of the four moves for a weekend this year.

The safe harbor is four equal parts

To avoid the underpayment penalty, the rule most owners use is the smaller of two figures: 90% of this year's tax, or 100% of last year's tax. That second figure becomes 110% if last year's adjusted gross income was over $150,000 ($75,000 if married filing separately). Source: IRC Section 6654(d); IRS Publication 505.

Divide that required annual amount by four. Each installment owes 25% of it on its due date. The penalty is worked out one installment at a time, so paying the whole amount on January 15 does not erase a late April payment.

Two owners, worked through

Brightline Heating & Air (the Dayton, Ohio S corporation with six vans). Last year's federal tax was $88,000 and last year's AGI was $345,000. That is over $150,000, so the test is 110%.

- $88,000 × 1.10 = $96,800 for the year. - $96,800 ÷ 4 = $24,200 on each of the four dates. - A flat 25% of an expected $380,000 profit is $95,000. That is $1,800 short of $96,800.

A one-person consulting practice. Last year's federal tax was $40,000 and last year's AGI was $120,000. That is at or under $150,000, so the test is 100%.

- $40,000 × 1.00 = $40,000 for the year. - $40,000 ÷ 4 = $10,000 on each of the four dates. - If this year's profit doubles, the safe harbor amount does not change. The tax bill in April will be larger, and the difference is due then. It is not a penalty.

The second case is why the prior-year test is popular. It is a number you can know today. The 90% test cannot be known until the year is over.

What changes the figure

Tax withheld from wages counts as paid in equal parts on each due date, no matter when it was actually withheld, unless you elect otherwise. If you owe less than $1,000 after withholding and credits, there is no penalty at all. Source: IRC Section 6654(e)(1) and (g). Both change the cash you have to send. Neither changes the four-part split.

Uneven income has its own method, the annualized income installment method, filed on Form 2210. It matches each payment to the profit earned to date. It is a different calculation and it is not shown here.

This rule is for people whose business profit lands on their personal return: sole proprietors, partners, S corporation owners. A C corporation pays its own estimated tax under a different rule.

Where the tool fits

We built a check that runs the prior-year test on your figures: what you set aside, against what the IRS safe harbor requires. It lives at unfoldcfo.com/tools/tax-reserve-calendar, and the Brightline run above is written up at unfoldcfo.com/guides/tax-reserve-calendar. It does not test the 90% leg, it does not compute the underpayment penalty itself, and it does not do the annualized method. It says which of those it skipped.

Sources. IRS Publication 505, Tax Withholding and Estimated Tax; IRS Form 2210 and instructions; Internal Revenue Code Section 6654 (irs.gov, law.cornell.edu). Brightline and the consulting practice are illustrations, not customers. General information, not tax advice; your CPA owns the judgment calls.