About Convertly

Convertly is a free measurement conversion reference covering 202 units across 21 categories, with 492 dedicated conversion pages. There is no sign-up, no usage limit and no paywall.

How conversions are calculated

Every unit is defined relative to a base unit for its category — the metre for length, the kilogram for mass, the second for time. A conversion converts the input to that base unit and then out to the target unit, in full double precision. Rounding happens only when the answer is displayed, so chained conversions do not accumulate error.

Exact vs approximate factors

Some relationships are exact by international definition: one inch is exactly 25.4 millimetres, one pound is exactly 0.45359237 kilograms, one US liquid gallon is exactly 3.785411784 litres. Where a factor is exact we say so on the page. Where it is not — such as astronomical or historical units — the displayed factor is the best accepted value.

US customary vs Imperial

Gallons, pints, fluid ounces, tons and several other units differ between the United States and the United Kingdom. Convertly keeps them as separate units rather than silently picking one, because a 20% error in a recipe or a fuel calculation matters. Pages for these units always spell out which system is in use.

Corrections

If you spot a factor you believe is wrong, tell us on the contact page. Accuracy reports are prioritised over everything else.