NumberTally Tools is a free library of 128 calculators covering finance (mortgage, loan, amortization, compound interest, credit-card payoff), math, fitness (BMI, body fat, calories), time & pay (work hours, overtime, paycheck), and home DIY (concrete, paint, area). No signup, no paywall, works on any device. Every calculator has a clean embeddable version (?embed=1) that bloggers, teachers, and site owners can drop into their own pages with one paste of code.