Best payroll calculator for solopreneurs

There are roughly 15-25 payroll calculators and tools competing in the solopreneur/freelancer space, from ADP Run to Guidepoint to niche players like Stride Health and Catch. The core problem isn't lack of options—it's that most are either over-engineered enterprise solutions with unnecessary features and pricing tiers, or barebones calculators that don't handle the specific complexity solopreneurs face: variable income, quarterly estimated taxes, self-employment tax optimization, and multi-state liability. The real gap sits between simplicity and depth. Solopreneurs need a tool that's dead-simple to input hours/rates but smart enough to auto-calculate federal + state + self-employment taxes, flag when quarterly payments are due, and give month-over-month income projections without requiring an accountant. Most existing tools solve for employees, not for people whose income swings 40% month-to-month. The opportunity is for a founder to build something that feels like a calculator but acts like a tax strategist—focused entirely on the solopreneur workflow, with integrations to Stripe/PayPal for real income data, and a mobile-first interface since these founders are never at a desk.