Blog · 5 November 2026 · Tax year 2026

Uneven income? The annualized income installment method lets quarterly taxes follow when you actually earn

The IRS default assumes your income arrives evenly all year: four equal quarterly payments, each a quarter of the annual target. For a freelancer whose year does not look like that — a quiet spring, a huge autumn contract, a seasonal business — equal payments mean overpaying in slow months and, worse, being charged an underpayment penalty for a quarter in which you had barely earned anything yet. The annualized income installment method exists for exactly this case: it re-figures each required payment from the income actually earned up to that point in the year.

Do estimated tax payments have to be equal?

No — not if you can show the IRS why they are unequal. The standard method divides the required annual payment by four and demands each instalment by its due date. The annualized method replaces that with four cumulative calculations, one per instalment period. In each period you annualize the income earned so far — that is, you scale it up as if it continued all year — work out the tax on that annualized figure, and take the required percentage of it. If your early-year income was small, your early required payment is small. The catch, and it is a real one, is paperwork: you must prove the timing by filing Form 2210 with Schedule AI attached to your annual return. Pay uneven amounts without filing Schedule AI and the IRS simply applies the equal-instalment method and assesses the penalty anyway.

The four periods, factors and percentages

Schedule AI does not use calendar quarters. Each period starts on 1 January and runs cumulatively to a cut-off date, and each is tied to one payment due date for tax year 2026:

The factors are just the fraction of the year each period covers, inverted: three months is a quarter of the year, so × 4; five months is 5/12, so × 2.4; eight months is 8/12, so × 1.5. The percentages are the 90% current-year safe harbor split across the year: 22.5%, 45%, 67.5%, 90%. Each instalment you actually pay is the cumulative required amount minus what you have already paid in earlier periods. And note the uneven period lengths — the second period ends 31 May, not 30 June, which is why the due dates (April, June, September, January) do not fall at neat three-month intervals.

Worked example: a seasonal freelancer

Single filer, 1099 profit, tax year 2026, on the same simplified assumptions as the rest of this site (standard deduction, simplified 20% QBI, no credits, no state tax — see the About page). Full-year profit is $60,000, so total federal tax is $12,037.20 and the equal-instalment route under the 90% safe harbor demands $2,708.37 every quarter. But the profit arrives unevenly: $5,000 by 31 March, $15,000 cumulative by 31 May, $35,000 cumulative by 31 August, $60,000 for the full year.

Check the arithmetic: period (a) is 22.5% × $3,024.87 = $680.60; period (b) is 45% × $6,504.88 = $2,927.20, less $680.60 already paid = $2,246.60; period (c) is 67.5% × $10,308.35 = $6,958.14, less $2,927.20 = $4,030.94; period (d) is 90% × $12,037.20 = $10,833.48, less $6,958.14 = $3,875.34. Total paid through the year is $10,833.48 — exactly the 90% safe-harbor total, just redistributed: $680.60 in April instead of $2,708.37, with the difference caught up in September and January when the money had actually arrived. The remaining 10% of the tax ($1,203.72) is settled with the return, as with any 90% safe-harbor plan.

If you are not sure you owe estimates at all, run the $1,000 test first; if you want your equal-instalment number for comparison, run the quarterly estimated tax calculator on your full-year profit.

How to claim it — and the traps

• File Form 2210, Schedule AI, with your return. Check box C in Part II of Form 2210 and attach the completed Schedule AI. This is the step that makes uneven payments legitimate; without it, the IRS defaults to equal instalments.
• Keep period-by-period records. Schedule AI needs income, deductions and credits for each period, not just year-end totals. A freelancer who only totals up in January cannot reconstruct a 31 May figure convincingly. Track profit monthly, at minimum.
• Annualize deductions too, not just income. The worksheet annualizes the period's deductions and credits the same way — using full-year deductions against three months of income would understate the early periods and overstate the saving.
• It can also work against you. If your income is front-loaded — a big January, then nothing — annualizing the early period can demand more than the equal instalment would. Run both methods and pay the smaller required amount for each period; Schedule AI itself takes the smaller of the annualized and regular instalment.
• Compare with the prior-year safe harbor first. If last year's tax was low, four equal payments at 100% (or 110%) of last year's tax may already be cheaper and need no Schedule AI at all. Annualizing is for when neither safe harbor fits your cash flow.
• Withholding is different. Tax withheld from a W-2 job is generally treated as paid evenly across the year unless you elect otherwise, so it cannot be timed the way estimated payments can — relevant if you mix freelance income with a salaried job.

Bottom line

Uneven income does not have to mean an underpayment penalty, and it does not have to mean lending the IRS money in months you have not earned it. The annualized income installment method matches each payment to the income earned by that period's cut-off — 31 March, 31 May, 31 August, 31 December — at the cost of one extra schedule with your return. If your income swings by season, by launch, or by a single large contract, work the four periods before you pay the next instalment, and pay it by the usual route (see how to pay estimated taxes online) — the method changes how much is due, not how or when you pay it.

Estimate only — not tax advice. Figures are for federal tax, tax year 2026, under the simplified assumptions on the About page (no credits, no state tax, simplified QBI). Your actual obligation depends on your full return, including withholding and credits. If a figure here ever disagrees with IRS.gov, IRS.gov is right.

Sources: IRS.gov — Publication 505 (2026), Tax Withholding and Estimated Tax (annualized income installment worksheet: periods ending 31 Mar, 31 May, 31 Aug, 31 Dec; factors 4, 2.4, 1.5, 1; applicable percentages 22.5%, 45%, 67.5%, 90%); IRS Form 2210 and Schedule AI (check box C, Part II; attach Schedule AI to the return); IRS.gov — Estimated Tax for Individuals. Tax figures from this site's engine, checked 5 Nov 2026.