If you’re self-employed or own a business that doesn’t withhold taxes from your pay, the IRS expects you to make quarterly estimated tax payments. When those payments fall short, the IRS charges a penalty that functions like interest on the amount you should have paid but didn’t.

The penalty applies per quarter, not just at year end. Each quarterly payment has its own due date (April 15, June 15, September 15, and January 15 of the following year), and the IRS calculates the underpayment for each period separately. So even if you catch up later in the year, you can still owe a penalty for the earlier quarters you missed.

There are two safe harbor rules that protect you from the penalty. If you paid at least 90% of your current year tax liability through estimated payments and withholding, you’re fine. Alternatively, if you paid at least 100% of your prior year’s total tax liability, you’re also protected regardless of how much you owe this year. That second threshold jumps to 110% if your adjusted gross income was over $150,000. For most trades business owners whose income fluctuates year to year, the prior-year safe harbor is the easier one to rely on because you know exactly what number to hit.

California has its own estimated tax penalty with similar rules. The state expects 30% of your annual payment by the first quarter, 40% by the second, no payment due in the third, and 30% by the fourth. It’s a different schedule than the federal equal-quarter approach, and it catches people off guard.

The penalty rate changes quarterly and is tied to the federal short-term interest rate plus 3%. In recent years that has been in the 7% to 8% range. It’s not catastrophic for a single year, but it adds up if you’re consistently underpaying. And it’s money you’re handing over for nothing.

The reason so many contractors and trades business owners run into this is straightforward. Income is uneven. You might have a slow first quarter and a huge third quarter. Tax strategy and planning that accounts for seasonal swings makes a real difference because you can use the annualized income installment method on Form 2210 to reduce or eliminate the penalty when your income genuinely varied throughout the year.

Going forward, the simplest fix is to set aside a percentage of every deposit into a separate account for taxes. For most self-employed people in California, 25% to 30% covers federal income tax, self-employment tax, and state income tax. Make your quarterly payments from that account on time and you avoid the penalty entirely.

If you’re behind on estimated payments right now, making a payment today is still better than waiting until you file. The penalty stops accruing on the portion you pay, so every day you delay costs a little more. And if your books are messy or nonexistent, it’s hard to even know what your estimated payments should be. Getting bookkeeping for trades businesses current is the first step toward accurate quarterly estimates and no more surprises at tax time.