ConvertThemAll is a free set of unit converters and calculators. It covers length, weight, temperature, volume, data sizes, money, percentages, dates and more, in 20 languages.
Every unit factor is stored as an exact fraction taken from its legal or scientific definition, for example 1 inch = 0.0254 metres exactly. Results are worked out with those fractions, so they are exact; when a result has to be rounded for display, the page shows ≈ instead of =.
Each tool shows its formula and a worked example, so you can check the answer yourself. Calculations run in your browser. There is no account to create.
Exchange rates come from published central-bank reference rates and always show their date and source. They are for information, not for transactions.
Sources
The International System of Units (SI Brochure, 9th edition), the 1959 international yard and pound agreement, and national standards bodies for customary units.