Free unit converters that actually get the factors right.

Every modern conversion traces back to the seven SI base units — metre, kilogram, second, ampere, kelvin, mole, candela. Since 2019 these are no longer defined by artifacts; the kilogram is fixed by the Planck constant, and the metre by the speed of light (the distance light travels in vacuum in 1/299,792,458 of a second).

Imperial-to-metric factors are also exact, not approximations. Per the 1959 international yard and pound agreement: 1 inch = 25.4 mm, 1 foot = 0.3048 m, 1 mile = 1,609.344 m, 1 pound = 0.45359237 kg. The numbers look messy precisely because they are exact.

Where conversions get tricky is when systems collide: a US gallon (3.785 L) is 20% smaller than a UK gallon (4.546 L), aviation uses knots because one nautical mile equals one minute of latitude, and binary KiB (1024 bytes) versus decimal KB (1000 bytes) explains why a 1 TB drive shows as 931 GB in Windows. The 17 tools below cover length, weight, temperature, volume, area, energy, speed, time, data size, time zones, cooking, Morse code and Roman numerals.