How to Estimate Quarterly Taxes With a W-2 and Side Business

If you have a W-2 job and earn extra income from a side business or LLC, you probably need to make quarterly estimated tax payments to the IRS. Miss them, and you'll owe underpayment penalties when you file. Here's how to figure out exactly what you owe each quarter.

Why quarterly payments exist

When you're a W-2 employee, your employer withholds taxes from every paycheck. But your side business income has no withholding — nobody takes taxes out before the money hits your account. The IRS expects you to pay as you earn, not all at once in April. That's what quarterly estimated payments are for.

The four quarterly deadlines

How to calculate your quarterly payment

Your quarterly payment covers two taxes on your self-employment income:

1. Self-employment tax (15.3%) — This is Social Security (12.4%) and Medicare (2.9%) on your net self-employment income. You get to deduct half of this.

2. Income tax — Your side business profit gets added on top of your W-2 income, so it's taxed at your marginal rate (the bracket your W-2 income already pushed you into).

The basic formula: take your estimated annual LLC profit, calculate SE tax + income tax on it, subtract what your W-2 employer already withholds, then divide by 4.

The safe harbor rule (avoid penalties)

You don't need to estimate your taxes perfectly. The IRS gives you a "safe harbor" — if you meet either of these thresholds, you won't owe penalties even if you underpay:

• 100% rule: Pay at least 100% of last year's total tax liability (110% if your AGI was over $150K per IRS Topic 306)
• 90% rule: Pay at least 90% of this year's actual tax liability

Most people with a W-2 and side business use the 100% rule because it's based on known numbers from last year's return.

Adjust your W-4 instead

Here's a strategy many people miss: instead of writing quarterly checks to the IRS, you can increase your W-4 withholding at your day job. The IRS treats W-2 withholding as if it was paid evenly throughout the year, even if you increase it in December. This means you can avoid quarterly payments entirely by having your employer withhold extra.

To use this approach, divide your estimated quarterly payment by the number of remaining pay periods and add that amount as "Extra withholding" on Line 4(c) of your W-4 form.

The easy way

Instead of doing all this math by hand, you can use our free quarterly estimated tax calculator. Enter your W-2 income, LLC income, filing status, and state — it calculates your quarterly payment, checks safe harbor compliance, and even tells you how to adjust your W-4 to avoid quarterly payments altogether.