Maths & Numbers Calculators

12 free maths & numbers calculators, each explaining its formula.

Percentages, ratios, averages, geometry and algebra, each with the method written out so you can reproduce the answer without the calculator.

About this section

This section covers the mathematics that comes up outside a classroom: percentages that need reversing, ratios that need splitting, averages that need choosing between, and geometry that needs applying to a real wall or a real floor.

Most errors here are not arithmetic errors. They are errors of method. Removing a twenty per cent addition by subtracting twenty per cent overshoots, because the percentage was applied to a smaller base. Averaging two percentages without weighting them by their sample sizes gives a number that describes nothing. Reading a ratio of two to three as two thirds rather than two fifths is probably the single most common ratio mistake made.

Each page therefore explains the method rather than only producing the answer. The formula is written out, a worked example runs through it step by step, and the common failure modes are named explicitly. The intention is that you could reproduce the result on paper, which is both a check on our implementation and the thing that stops the same mistake happening next time.

Where a concept has more depth than the calculator needs — why n−1 appears in a sample standard deviation, why the discriminant tells you the shape of a parabola before you solve it — that explanation is on the page too.