When you have a job, your employer quietly withholds tax from every paycheck and the whole thing is invisible. When you work for yourself, nobody does that — so the IRS asks you to pay your own tax four times a year, and charges a penalty if you underpay along the way.
The good news: the penalty is completely avoidable, and not by guessing well. There’s a rule that makes it mathematically impossible to owe one.
First, the dates (they’re not even quarters)
- April 15 — for January through March
- June 15 — for April and May (yes, only two months later)
- September 15 — for June through August
- January 15 — for September through December
Nobody said the “quarters” were equal. Put them on a wall calendar and half the problem disappears.
The safe harbor — the part that actually protects you
Here’s the rule worth memorizing. If you pay, across the year, at least:
- 90% of this year’s tax, or
- 100% of last year’s tax (110% if your income was over $150,000),
…then you owe no underpayment penalty — even if you end up owing more in April. That’s the magic of the “100% of last year” version: last year’s tax is a number you already know, so you can lock in penalty protection on day one and settle any difference in April without a penny of penalty.
The arithmetic is simple, but doing it under deadline pressure four times a year is where people slip. That’s exactly what the Quarterly Tax Autopilot in our Playbook is for: enter last year’s tax and this year’s expected profit, and it hands you your four safe-harbor payments and the dates they’re due. Penalty-proof by math, not by hope.
The Business Owner’s Tax Playbook
Six tools that run your entire tax year — deductions in depth, the structure decision, how to pay yourself, and a quarterly-tax autopilot that makes penalties mathematically impossible.
