Unit converter

Every unit on the site is stored as its size in one base unit for its quantity: meters for length, kilograms for mass, liters for volume, pascals for pressure, joules for energy. A conversion takes your value to the base unit and then out to the target unit, so the formula is always value × factor of the starting unit ÷ factor of the target unit.

Converting 12 inches to centimeters, for example, works as 12 × 0.0254 m ÷ 0.01 m = 30.48 cm. Going through one base unit means every pair of units in a category uses the same two numbers, and no conversion depends on another rounded conversion.

Temperature is the exception. Celsius, Fahrenheit and kelvin have different zero points, so they convert with a multiplication and an offset — the temperature page explains why that trips people up.

From definitions, not measurements. The international yard and pound agreement of 1959 fixed the inch at exactly 25.4 mm and the pound at exactly 0.45359237 kg, and the National Institute of Standards and Technology publishes those values in Special Publication 811. Since the 2019 revision of the SI, the kilogram, kelvin and other base units are themselves defined by fixed physical constants, as set out in the BIPM’s SI Brochure. A factor like 2.54 is therefore exact, not an approximation that might be refined later.

Sometimes a spacecraft is lost. In 1999 NASA’s Mars Climate Orbiter was lost on arrival at Mars because one team’s software reported thruster impulse in pound-force seconds while the navigation team’s software expected newton-seconds — a factor of about 4.45. NASA’s Mishap Investigation Board named the unit mismatch as the root cause.

Most mistakes are smaller and more ordinary: a US cup used in a metric recipe, a tire pumped to 2.2 psi instead of 2.2 bar, 5.10 typed for five feet ten inches. Each category page on this site names the error most common for that quantity.

Results are rounded for display, and very large or very small numbers switch to scientific notation — 9.46 × 10¹⁵ meters in a light-year is easier to read than sixteen digits. You can type a comma or a period as the decimal separator. The value you enter is added to the page address, so copying the link shares the exact conversion you are looking at. For lengths you can measure against your screen, there is also a calibrated online ruler.