CalculatorPlanet is a free, growing library of calculators across 18 categories Tax & Salary, Health, Finance, Academic, Construction, Gaming, Agriculture, and more. Every tool shows its formula, works a real example, and cites a source for anything beyond arithmetic. No signup, no paywall. State paycheck calculators compute real local and county tax, not a flat federal estimate. New calculators and deeper guides ship inside these categories regularly, not new categories tacked on.