This blends federal income tax, state tax if applicable, and the 15.3% self-employment tax into one rate, for a rough estimate only.
This is an estimate to help you spot deductions worth tracking and asking a tax professional about, it is not tax advice and doesn't account for phase-outs, AMT, or state-specific rules. For the real filing, work with a CPA or enrolled agent, or use the quarterly tax estimate tool alongside your actual records.