IRS Form 2210 Schedule AI: the Annualized Income Installment Method explained

The IRS expects estimated tax to be paid as you earn, not in a lump sum at filing time. Its default assumption is that your income arrived in four equal pieces, so it expects four equal payments. For a salaried filer that assumption is roughly true. For a consultant who bills nothing in the spring, or an investor who sells stock in November, it is badly wrong — and being wrong in the IRS's favour means an underpayment penalty for quarters when the money had not arrived yet.

Schedule AI, the second page of IRS Form 2210, Underpayment of Estimated Tax by Individuals, Estates, and Trusts, replaces the assumption with your actual figures. You report income, deductions and withholding for each cumulative period of the year, and each quarter's required installment is recalculated from what you had genuinely earned by that date.

The periods are cumulative — each one starts on January 1, not at the end of the previous quarter. That surprises people: the "second quarter" figure on Schedule AI covers five months, not three.

Multiplying the period's income by its factor produces "annualized income": what you would earn for the full year if the rest of the year matched the pace so far. Tax is then computed on that annualized figure, which is why a strong first quarter can produce a startlingly large annualized income — it assumes the pace continues.

1. Total the income for the period. Wages, self-employment income, interest, dividends and capital gains actually received between January 1 and the period cutoff.
2. Subtract deductions for the period. Itemized deductions are annualized the same way; the standard deduction is not — it is applied in full against annualized income.
3. Annualize. Multiply by the period factor (4, 2.4, 1.5 or 1).
4. Compute the tax on the annualized amount. Ordinary brackets, plus long-term capital gains rates, self-employment tax, the 3.8% Net Investment Income Tax and the 0.9% Additional Medicare Tax where they apply.
5. Apply the cumulative installment percentage. 22.5% for the first period, 45% for the second, 67.5% for the third and 90% for the fourth. This is the year-to-date share of the annualized tax the IRS wants paid by that due date.
6. Deduct what you have already paid. Withholding to date plus earlier estimated payments. The remainder is the installment due.

Those cumulative percentages are the 90% current-year safe harbor spread across the four due dates. See the safe harbor guide for why 90% rather than 100%.

A freelance designer, filing single, earns nothing in Q1, $20,000 by May 31, $40,000 by August 31 and $150,000 by December 31 after a large year-end contract.

• Equal installments:the IRS treats the $150,000 as $37,500 per quarter and expects roughly a quarter of the year's tax by April 15 — on income that did not exist.
• Schedule AI: the first period annualizes $0 to $0, so the first installment is $0 and no penalty accrues for that quarter. The large payment lands in the fourth installment, where the income actually occurred.

Same total tax; the penalty exposure for the first three quarters disappears. That gap is the entire reason Schedule AI exists.

One asymmetry is worth knowing: the IRS treats tax withheld from a paycheck as paid evenly across the year regardless of when it was actually withheld, while an estimated payment counts on the date you make it. So if you discover in December that you are badly underpaid, increasing withholding on a year-end paycheck or an IRA distribution can retroactively cover earlier quarters in a way a December estimated payment cannot.

You can elect to have withholding counted when it was actually withheld instead, but the election applies to all four quarters — it is only worth it when your withholding was front-loaded.

• It is more work. You need income, deduction and withholding totals at three mid-year cutoffs, not just an annual total.
• It requires records. If your bookkeeping cannot say what you earned by May 31, you cannot complete the form honestly.
• It does not help steady earners. With even income, annualizing produces the same four installments as the default method.