Hello and thank you for your question. Self employment tax is calculated on business net income (gross income less cost of goods sold less expenses) As a side note Gross Income is total income. Once Net Income is calculated then that amount is used to calculate self employment tax. SE tax is 15.3% (medicare and social security) and computed on Schedule SE. I have inserted a link below from the IRS that has more detail and instructions plus the forms to review. In addition, I will add links to a Tax Estimator and 1040ES for estimated tax payments. I hope this answered your question and have a wonderful day.
IRS SE Tax
Tax Estimator
IRS 1040ES
