Unit Converter SI/US Calculator

Most unit conversions work because the quantities involved sit on ratio scales, meaning zero in one unit is zero in every other. Length, mass, volume, pressure and energy all behave this way, so each unit needs only a single multiplicative factor to its SI base and any pair converts by multiplying by the source factor and dividing by the target factor. A point worth knowing is that many US customary factors are exact by definition rather than measured. The international yard and pound agreement of 1959 fixed one inch at exactly 2.54 cm and one pound at exactly 0.45359237 kg, from which one foot equals exactly 0.3048 m and one mile equals exactly 1609.344 m. The US gallon is defined as exactly 231 cubic inches, or 3.785411784 L, while the imperial gallon is 4.54609 L, roughly 20 percent larger, a mismatch that quietly corrupts recipes and fuel economy figures. Temperature is the exception because Celsius and Fahrenheit are interval scales with arbitrary zero points, so conversion requires both a scaling term and an offset: C equals (F minus 32) times 5/9, and K equals C plus 273.15. Only kelvin has a true zero, which is why thermodynamic formulas demand it. A few other definitions deserve care. One standard atmosphere is exactly 101,325 Pa, the thermochemical calorie is exactly 4.184 J while the international steam table calorie is 4.1868 J, and the BTU has several competing definitions clustered near 1,055 J. Conversion also cannot manufacture precision: converting a value measured to two significant figures does not justify reporting six.

The metric system was a product of the French Revolution. The revolutionary government commissioned a rational replacement for a chaotic patchwork of regional measures, and a law of 1795 established the metre and the gram. The metre was originally defined as one ten-millionth of the distance from the North Pole to the equator along the meridian passing through Paris, and in 1799 a platinum bar known as the metre des Archives was deposited as the physical standard. International governance followed with the Metre Convention, signed in Paris on 20 May 1875, which created the International Bureau of Weights and Measures and the General Conference on Weights and Measures. The name International System of Units, abbreviated SI in all languages, was adopted at the 11th General Conference in 1960. The United States has a longer and more ambivalent record. The Metric Act of 1866 made metric units legal for trade, and the Mendenhall Order of 1893 went further by defining the customary yard and pound in terms of metric standards, so US units have been legally metric-derived for well over a century. The Metric Conversion Act of 1975 declared metric the preferred system but left adoption voluntary, which is why customary units persist in daily commerce. The most recent change came on 20 May 2019, when the kilogram, ampere, kelvin and mole were redefined in terms of fixed values of fundamental constants, including the Planck constant, retiring the last physical artifact standard.