Estimated taxes.
In plain English
The tax system is pay-as-you-go. Employees prepay through withholding; self-employed people and those with significant non-wage income (freelance, investments, rental) prepay through four estimated payments a year. Safe-harbor rules prevent penalties: generally pay at least 90% of this year's tax or 100% of last year's (110% at higher incomes) and you're protected even if you owe more in April.
01Why it matters
The first year of side income is where this bites. Nothing was withheld, the April bill includes income tax plus self-employment tax, and an underpayment penalty rides on top. The fix is a calendar and a percentage, not heroics.
02The math, step by step
A W-2 employee earns $12,000 from freelance work on the side. Setting aside roughly 25% to 30% ($3,000 to $3,600) and paying it across the quarterly deadlines covers income plus self-employment tax and avoids penalties. Under the statutory safe harbor (IRC section 6654), you generally avoid a penalty by paying the lesser of 90% of this year's tax or 100% of last year's (110% if your prior-year adjusted gross income was over $150,000, or $75,000 if married filing separately), with no penalty if you owe less than $1,000. The quarterly due dates are April 15, June 15, September 15, and January 15.
03What this is NOT
Estimated taxes are the same income tax, prepaid manually. A refund-every-year W-2 spouse's withholding can sometimes be increased to cover a side hustle instead, which is the lazy and legitimate alternative.
04Receipts
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Plain-English answers from our glossary. Receipts included. Never advice.
Educational tool. Answers come only from ClearMoneySchool's published glossary and are not advice. Why we never give advice