Self-employment tax
Social Security and Medicare tax for people who work for themselves. It is 15.3% on 92.35% of net profit, on top of income tax.
Employees split Social Security and Medicare tax with their employer. Freelancers, contractors and gig workers pay both halves, which is called self-employment tax. It is 12.4% for Social Security and 2.9% for Medicare.
You owe it once your net earnings from self-employment reach $400 for the year. You figure it on Schedule SE, and you can deduct half of it when figuring adjusted gross income.
Example
On $42,000 of net profit: $42,000 × 92.35% = $38,787, and $38,787 × 15.3% = $5,934 of self-employment tax (rounded).