Self-Employment Tax
Also called: SE tax, class 4 national insurance
Self-employment tax is both halves of FICA — 15.3% — paid by people with no employer to cover the other half. It applies to net business profit, not revenue, and half of it is deductible against income tax as a partial offset. It's the single biggest surprise for new freelancers, who budget for income tax and then discover another 15% owed on the same money.
Formula
SE tax = 15.3% × 92.35% of net self-employment profit