Pick a category, enter a value, and choose the units to convert from and to.
Length, weight/mass, volume, area, speed, time, and digital storage each have a full set of common units — for example length covers meters, kilometers, centimeters, millimeters, miles, yards, feet, inches, and nautical miles. Temperature converts between Celsius, Fahrenheit, and Kelvin using their proper formulas rather than a simple ratio.
Every unit in a category is converted to a common base unit first (meters for length, kilograms for weight, liters for volume, and so on), then converted from that base into your target unit — so any combination within a category works, not just fixed pairs. Temperature is the one exception, since Celsius, Fahrenheit, and Kelvin scales don't share a zero point, so it uses direct formulas instead.
Some conversion factors are internationally defined exact values (1 inch = 2.54cm exactly), while others involve rounding depending on the standard used (e.g. US survey feet vs. international feet differ by a tiny fraction) — for everyday use these differences are negligible, but they can matter in scientific or legal contexts.
No — unit conversion only works within the same physical quantity (length to length, weight to weight), since converting across categories (like meters to kilograms) isn't mathematically meaningful without additional context like density.
Worked example: 1 mile = 1609.34 meters exactly as used here, so 5 miles converts to 8,046.7 meters, or 8.047 km — try entering 5 with 'mi' as the From unit and 'km' as the To unit above.