I built ChargeWhat to solve a problem: freelancers massively undercharging because they forgot to account for self-employment tax, health insurance, and the fact that you can't bill every hour you work.
The calculator works backwards from your target take-home pay and accounts for SE tax (15.3% on 92.35% of net profit), your state income tax across all 50 states, realistic billable hours, business expenses, health insurance, and a profit buffer for slow months.