About 1plus1

1plus1 is a free collection of calculators covering finance, health, dates, unit conversion, mathematics, geometry, physics, statistics, construction and everyday life. There is nothing to install and no account to create.

Every calculator shows more than a result. It names the formula it applies, lists the assumptions it makes and, where it helps, writes out the steps. Two calculators that disagree usually disagree about assumptions rather than arithmetic, so we state ours.

Each calculator is a small, declarative definition: its fields, its validation rules, its calculation function and its text. The calculation itself is a pure function covered by automated tests that compare its output against published reference values.

Financial calculators receive extra attention on rounding and on compounding conventions, because those two details account for most of the difference between an online estimate and a lender’s quote.

The site is published in English, French and Spanish. Each calculator is translated in full — interface, explanations, formulas, examples and questions. Nothing is machine-translated at page load.

Advertising, placed outside the calculation area so it never gets in the way of a result. There is no premium tier, no data broker and nothing to buy.