The United States runs a pay-as-you-go tax system. Employees satisfy that through withholding. Business owners satisfy it through quarterly estimated payments — and if they underpay, they owe an interest charge even when the balance is settled in full by April.
That charge is not technically a penalty in the punitive sense; it is interest computed at the federal underpayment rate, applied quarter by quarter. But it is entirely avoidable, and the mechanism for avoiding it is more generous than most owners realize.
The due dates
For a calendar-year taxpayer, estimated payments are due:
| Period | Due |
|---|---|
| January 1 – March 31 | April 15 |
| April 1 – May 31 | June 15 |
| June 1 – August 31 | September 15 |
| September 1 – December 31 | January 15 (following year) |
Note the periods are not equal quarters. The second covers two months and the third covers three, which surprises people building payment schedules on a simple quarterly cadence. Dates shift to the next business day when they fall on a weekend or holiday.
The safe harbor
Here is the provision worth knowing. You avoid the underpayment penalty if you pay, through withholding and estimates, the lesser of:
- 90% of the tax shown on the current year’s return, or
- 100% of the tax shown on the prior year’s return — rising to 110% if your prior-year adjusted gross income exceeded $150,000 ($75,000 if married filing separately)
The second option is the useful one. It is based on a number you already know, and it holds regardless of how much you earn this year.
An owner whose income doubles can pay 110% of last year’s tax, owe a very large balance in April, and incur no underpayment penalty at all. The obligation to pay is unchanged — but the penalty exposure is removed, and the cash stays in the business in the meantime. Publication 505 sets out the rules in full.
The reverse case matters too: in a year when income falls sharply, the prior-year safe harbor may require paying far more than you will owe. There, the 90%-of-current-year test is the cheaper target — you just have to project accurately enough to rely on it.
Withholding has a timing advantage
Estimated payments are credited when made. Withholding is treated as paid evenly throughout the year, regardless of when it was actually withheld.
That asymmetry is exploitable. An owner who reaches December underpaid can ask a spouse’s employer to increase withholding sharply for the final pay periods, and the IRS treats those amounts as if they had been withheld all year — retroactively curing earlier quarters. The same money paid as a Q4 estimate would only cure the fourth quarter.
This is one of the few genuinely clean fixes available late in the year.
Uneven income and the annualized method
The standard calculation assumes income arrives evenly. Many businesses are seasonal — a landscaping company earning most of its profit between April and September, a tax practice front-loaded into the first quarter.
Paying four equal installments when income is concentrated late means overpaying early. The annualized income installment method, computed on Form 2210 Schedule AI, lets you match payments to when income was actually earned.
It requires closed books at each installment date, which is a real cost — you cannot annualize from records that are three months behind. For a seasonal business with a disciplined monthly close, the cash flow benefit usually justifies the effort.
Self-employment tax is part of it
Estimated payments must cover self-employment tax as well as income tax. At 15.3% on 92.35% of net earnings, it is frequently the larger of the two for owners in the lower brackets, and it is the most common reason a projection comes up short.
For 2026, the Social Security portion applies to earnings up to $184,500; the Medicare portion has no cap. Our quarterly estimated tax calculator computes both alongside the safe harbor comparison.
States run their own systems
Most states with an income tax impose their own estimated payment requirements, generally on the same dates but with their own safe harbor rules — and some are less forgiving than the federal version. Meeting the federal safe harbor says nothing about your state position.
The practical approach
Set aside a fixed percentage of every deposit into a separate account, and pay from that account rather than from operating cash. The exact percentage matters less than the discipline; a business that reaches April with the money set aside has a paperwork problem, while one that spent it has a financing problem.
Then compute both safe harbor figures at the start of the year, pick the lower, and revisit at midyear. That single review catches the case where a strong year makes the prior-year harbor a bargain, and the case where a weak year makes it an expensive default.