244 free calculators, each one built to be checked rather than trusted. You get the result, the formula that produced it, a worked example with every step, the source the formula was checked against with the date, and a short list of cases where the formula stops giving the right answer. Behind every calculator sits a file of hand-worked test values the site must match before it can deploy. Topics run from US paycheck and mortgage maths to BMI, concrete volume, unit conversion, statistics and chemistry. Free to use with no sign-up, and paid for by a fixed set of labelled sponsor slots rather than an ad network.
Most calculator sites give you one number and no way to check it. I wanted each answer to come with the formula, a worked example you can follow by hand, the source it was checked against, and the cases where it goes wrong. Every calculator also has hand-worked test values, so a wrong constant fails the build instead of reaching a visitor.