Estimated Tax Safe Harbor vs. a Real Estimate: Which to Pay
The estimated tax safe harbor keeps you out of the underpayment penalty. It does nothing to make the number right. Pay 100% of last year’s tax across the four quarters (110% if your income was over $150,000) and the IRS won’t penalize you, no matter what this year’s tax turns out to be. A real estimate, built from closed books each quarter, does the other job: it keeps you from an April surprise in one direction, and from making the government an interest-free loan in the other.
We run an accounting firm, and the fourth-quarter estimate, due January 15, 2027, is the payment where the two approaches diverge most. Here’s how each works and why we build the real one.
What is the safe harbor rule for estimated taxes?
The underpayment penalty applies when you haven’t paid enough tax during the year, through withholding and estimates, by the time each quarterly deadline passes. The safe harbor is the rule that defines “enough” without reference to what you’ll actually owe.
You’re safe if your payments during the year total at least the smaller of:
- 90% of this year’s tax, which you can’t know until the year is over, or
- 100% of last year’s tax, or 110% if last year’s adjusted gross income was more than $150,000.
Almost everyone uses the second one, because last year’s tax is a known number sitting on a filed return. Your preparer divides it by four, prints vouchers, and you pay them. It’s simple, it’s defensible, and for a business whose profit is about the same every year, it’s fine.
The trouble is that small-business profit is rarely the same every year.
Two owners, same profit
Take two owners whose businesses each earn about the same this year. Round numbers, to make the arithmetic visible.
The first owner had a better year last year. A big project, a one-time gain, whatever it was. Last year’s tax was high. The safe harbor vouchers were built from it, and the owner paid them all, including the one in January. When the return is done in March, the refund is $9,000. That sounds like good news. It’s nine thousand dollars that sat with the Treasury for most of a year, interest-free, while the owner was paying interest on a line of credit to cover December payroll. The money was never gone. It was just unavailable at the moment it mattered.
The second owner had a worse year last year, and a better one this year. Last year’s tax was low, the vouchers were small, and the safe harbor was satisfied by mid-summer. No penalty. Then the return is done in March and the balance due is $14,000, on April 15, with no warning, in the middle of the spring cash dip. The safe harbor worked exactly as designed. The owner still got a bill they had no plan for.
Neither owner did anything wrong. Both were protected from a penalty. Both were unprotected from the number.
How do I calculate my quarterly estimated tax payment?
The real estimate takes about twenty minutes a quarter, once the books are closed. Four steps:
- Year-to-date profit, from the closed months. For the fourth-quarter estimate, that’s January through October, or through November if the close is quick.
- Project to year end. Add the remaining months at a reasonable run rate, adjusted for what you know: the December bonus payroll, the equipment placed in service, the slow last two weeks.
- Run it through the tax. Add the owner’s other income, apply the qualified business income deduction, the standard or itemized deductions, self-employment tax if it applies, and state tax. This is the step your accountant does, and it’s the step that takes fifteen of the twenty minutes.
- Subtract what’s been paid. Withholding from any W-2 in the household, the three estimates already made. What’s left is the fourth-quarter payment.
That’s the number. If it’s lower than the safe harbor voucher, you pay the lower number and keep the difference, as long as the payments still satisfy the safe harbor test or you’re comfortable that 90% of the actual tax is covered. If it’s higher, you pay the higher number, or at least you know in December that April will need it.
The catch
Step one requires closed books. Not “the bank feed is caught up.” Closed: every account reconciled to its statement, loans and payroll tied to outside documents, owner draws booked as draws and not as expenses. A profit number from unreconciled books is a guess, and an estimate built on a guess is a safe harbor payment with extra steps.
This is why most firms default to the safe harbor. They can’t build the real estimate because they don’t have closed months to build it from. The vouchers aren’t a tax strategy. They’re what’s left when the bookkeeping doesn’t support anything better.
Is it better to overpay estimated taxes?
Overpaying is safe, and safe isn’t the same as good. A large refund is a loan you made to the government at zero interest, usually while paying interest on something else. Underpaying by a little is fine if the safe harbor is met, but underpaying by a lot leaves a bill you didn’t plan for.
The goal isn’t a refund and it isn’t a balance due. It’s a January 15 payment that equals the actual remaining tax, within a reasonable margin, so April is a formality.
What a CFO does with this number
A fractional CFO doesn’t build the fourth-quarter estimate in isolation. It’s the last line of the November planning meeting, and it moves with everything above it. Place the truck in service before December 31, and the profit number drops, and the estimate drops with it. Run the S-corp salary catch-up on the final payroll, and the mix between wages and distributions changes the calculation. Fund the retirement plan, and it changes again.
So the CFO runs the December moves and the January payment as one decision, in one hour, from October’s closed books. Then the estimate is right because it reflects what’s actually going to happen, and the cash for it gets placed on the 13-week forecast as a real line on January 15, next to the year-end payroll taxes and the W-2 filing.
Where we come in
For our clients, the estimate is delivered in the second week of the last month of each quarter: mid-March, mid-May, mid-August, and mid-December, each built from books closed by the 10th. It’s a number with a one-paragraph explanation, not a voucher from last year’s return. In December it comes with the planning meeting, so the payment on January 15 already reflects the year-end moves. If you’ve been paying the safe harbor because nobody’s offered anything better, reach out. We’ll build the real one from your numbers.
Frequently asked questions
What is the estimated tax safe harbor?
Paying at least 100% of last year’s tax (110% if income was over $150,000) through withholding and estimates avoids the underpayment penalty.
How is a real estimate calculated?
Year-to-date profit from closed books, projected to year end, run through the tax, minus what’s already been paid.
Is overpaying bad?
It avoids penalty but lends the government money at zero interest; the goal is a January payment equal to the actual remaining tax.
