When you work for an employer, you split Social Security and Medicare taxes with them. When you're self-employed, you pay both halves yourself — that's self-employment (SE) tax, and it surprises a lot of new freelancers.
The 15.3%
SE tax is 15.3%: 12.4% for Social Security plus 2.9% for Medicare. It's calculated on 92.35% of your net self-employment profit (the 7.65% reduction approximates the employer-side deduction a regular employee never sees).
Social Security stops at an annual wage base; Medicare has no cap and keeps applying to every dollar. If you also have a W-2 job, the wages already taxed there count toward the Social Security wage base.
The Self-Employment Tax Calculator breaks down the Social Security and Medicare portions and the deductible half.
The deductible half
You can deduct half of your SE tax "above the line" on your 1040, which lowers your income tax (though not the SE tax itself). And because this income usually has no withholding, you'll likely owe quarterly estimated payments to avoid a penalty.