If your income is not fully covered by payroll withholding, the IRS expects you to pay taxes throughout the year through quarterly estimated payments. Miss a payment or underpay a quarter and you may face a penalty, even if you are owed a refund at filing. For taxpayers with variable or investment income, getting those payments right every quarter can be difficult. That is where safe harbor comes in.

Safe harbor is a straightforward way to protect yourself from underpayment penalties regardless of how your current year income plays out. Instead of estimating what you will owe this year, you base your payments on what you already know: your prior year tax return. If you pay an amount equal to 100% of last year’s tax liability, spread across the four quarterly due dates, the IRS cannot assess an underpayment penalty for the current year. It does not matter if your income went up significantly or if you end up owing a large balance in April. The penalty protection holds.

There is one important threshold to know. If your prior year adjusted gross income exceeded $150,000 (or $75,000 if you are married filing separately), the safe harbor amount increases to 110% of your prior year tax rather than 100%. For higher income taxpayers, paying exactly what you paid last year is not enough. That distinction catches people off guard and is worth confirming with your CPA before making your first quarterly payment of the year.

Safe harbor is particularly useful in years when income is hard to project. A business sale, a large capital gain, a strong year in a variable compensation structure. These are situations where estimating current year liability accurately is genuinely difficult. Anchoring your payments to a known prior year number removes the guesswork and keeps you protected while you and your advisor plan around the actual outcome.

A quick word on the penalty itself and the deadlines

The underpayment penalty is not a flat fee. The IRS calculates it as interest on the shortfall, based on the federal short term rate plus three percentage points, and that rate is reviewed and can change each quarter. In other words, an underpaid quarter does not just carry a symbolic cost. It compounds like real interest for as long as the shortfall goes uncorrected, which is one more reason to get ahead of it rather than true it up at filing.

For calendar year filers, the four estimated payment due dates fall on April 15, June 15, and September 15 of the current year, and January 15 of the following year. If a due date lands on a weekend or federal holiday, the payment is considered on time if made by the next business day. Missing even one of these dates can trigger a penalty for that quarter alone, even if you catch up later in the year.

You don’t have to navigate this alone or guess at the right number. The IRS publishes Form 1040-ES, which includes a worksheet for calculating your estimated tax, along with the safe harbor rules directly on its estimated taxes page. For the full rules on penalty calculations and the exceptions the IRS allows, its underpayment penalty page walks through when the penalty applies and when it can be reduced or waived.

Helpful IRS resources: Estimated Taxes Underpayment of Estimated Tax by Individuals Penalty Topic no. 306, Penalty for Underpayment of Estimated Tax

At MTA, we help clients think through estimated payments before each due date. If you are unsure what your safe harbor amount is or whether your current payments are on track, that is exactly the kind of conversation we are here for. Our team has been helping Houston’s high net worth individuals and business owners navigate complex tax situations since 1985.