The Quarterly Tax Safe Harbour, With the Actual Arithmetic
In short
You owe no underpayment penalty if you pay either 90% of this year's total tax or 100% of last year's — 110% if last year's adjusted gross income was over $150,000. Whichever is smaller is your target. Hitting it means a surprise profitable year costs you nothing in penalties.
Why the rule exists
The US tax system is pay-as-you-go. Employees satisfy that through withholding; the self-employed satisfy it through four estimated payments a year. The obvious problem is that you cannot know your annual profit in April, and the IRS knows this. The safe harbour is the accommodation: hit one of two targets and you are protected, regardless of how wrong your estimate turns out to be.
The two targets
| Test | What you must pay across the four quarters | When it is the better test |
|---|---|---|
| Current-year test | 90% of this year's total tax | Income is falling, or this is your first year self-employed |
| Prior-year test | 100% of last year's total tax | Income is rising — the target is fixed and known in advance |
| Prior-year test, higher earners | 110% of last year's total tax | Same, but required if last year's AGI exceeded $150,000 |
You only have to satisfy one. In practice the prior-year test is the one to use whenever your income is growing, because it is a number you already know on 1 January — last year's Form 1040, line for total tax — while the current-year test requires predicting a year you have not lived yet.
A worked example of how much underpayment it permits
Suppose you owed $14,000 in total tax last year on an AGI of $95,000. This year your freelance business takes off and you end up owing $31,000.
- Prior-year test: AGI was under $150,000, so the target is 100% of $14,000 = $14,000, or $3,500 per quarter.
- You pay $3,500 four times. Total paid: $14,000.
- Actual tax: $31,000. You owe $17,000 on 15 April.
- Underpayment penalty: none. You satisfied the safe harbour.
You underpaid by $17,000 across the year and the IRS charges you nothing for it. This is the single most valuable thing to understand about estimated taxes, and it is why a sudden good year does not have to be a penalty event. You still owe the $17,000 — the safe harbour defers the payment, it does not reduce the tax — but it defers it interest-free.
What the penalty actually costs if you miss
The underpayment penalty is not a flat fine. It is interest, charged quarter by quarter on the amount you were short, at the federal short-term rate plus three percentage points. It accrues from each quarter's due date until the shortfall is paid.
Two consequences follow. First, the penalty on a small shortfall is genuinely small — people routinely overestimate it and panic. Second, because it is charged per quarter, paying a large catch-up amount in Q4 does not undo the Q1 shortfall. The IRS treats the quarters separately unless you use the annualised income method on Form 2210.
The annualised income method, and when it is worth the trouble
The default assumption is that your income arrived evenly across the year, so the required payment is the same each quarter. For many freelancers that is simply false — a consultant who closes one large project in November earned nothing in Q1 and should not be penalised for not having paid tax on it.
Schedule AI of Form 2210 lets you show the IRS when the income actually arrived and recompute the required payment for each quarter accordingly. It is genuinely tedious — you are effectively doing four partial-year tax returns — but it can eliminate a penalty entirely when your income is seasonal or lumpy. Worth doing when the penalty is meaningful; not worth it for $40.
The deadlines, and the one that catches people out
| Quarter | Income period | Due |
|---|---|---|
| Q1 | 1 January – 31 March | 15 April |
| Q2 | 1 April – 31 May | 16 June |
| Q3 | 1 June – 31 August | 15 September |
| Q4 | 1 September – 31 December | 15 January of the following year |
The quarters are not quarters. Q2 covers two months, Q3 covers three, and Q4's payment is due in January. The June deadline is the one people miss, because it arrives only two months after April rather than three.
Frequently asked questions
Frequently Asked Questions
What is the safe harbour rule for estimated taxes?
Do I have to pay quarterly taxes?
What happens if I miss a quarterly payment?
Can I just pay it all in one go in January?
Related
Sources
- IRS Revenue Procedure 2025-32 — 2026 inflation adjustments — tax brackets, standard deduction, QBI thresholds.
- IRS Schedule SE instructions — Self-employment tax: the 92.35% adjustment, the Social Security wage base, the Medicare surtax.
- IRS Form 1040-ES — Estimated tax deadlines and the safe-harbour rules used for the quarterly schedule.
- IRS Publication 334 — Tax guide for small business — deductible expenses and net profit.
- IRS Social Security wage base — The $184,500 Social Security cap applied for 2026.
How these numbers are produced
Calculations follow the order of operations on the IRS forms themselves: net profit, the 92.35% adjustment, self-employment tax, the 50% deduction against AGI, the standard deduction, the §199A QBI deduction with its phase-out, then the federal brackets. State tax is computed separately on state AGI less that state's own standard deduction, because no state allows the federal QBI deduction and almost none allow the federal standard deduction.
What it does not cover. The calculators model a single filer or a married couple filing jointly with self-employment income as the main source. They do not model local income taxes, W-2 wages held alongside freelance work, itemized deductions, the alternative minimum tax, or credits. Where a local income tax exists, it is described on that state's page.
Not advice. SelfTaxCalc produces estimates, not tax advice. No one here is a CPA, an enrolled agent or a tax attorney, and nothing on this site creates a professional relationship. Estimates are a starting point for planning and for a conversation with a professional — they are not a substitute for one.