How this works

Length and weight conversions use fixed, internationally agreed conversion factors. The exact values were established through international treaty and are not approximations. Temperature is different: Celsius, Fahrenheit, and Kelvin each define "zero" differently, so converting between them requires both scaling and shifting — not a simple multiplication.

Length conversions and their exact values

1 inch = 25.4 millimeters (exact)

Defined exactly in 1959 by international agreement between the US, UK, Canada, Australia, New Zealand, and South Africa.

1 foot = 0.3048 meters (exact)

Derived from the exact inch definition: 12 inches × 25.4mm = 304.8mm = 0.3048m.

1 mile = 1,609.344 meters (exact)

5,280 feet × 0.3048m/ft = 1,609.344m. The international mile used in road distances worldwide.

1 yard = 0.9144 meters (exact)

3 feet × 0.3048m/ft. Used in American football field dimensions, fabric, and some construction contexts.

Weight conversions

1 pound = 0.45359237 kilograms (exact)

The international avoirdupois pound, defined exactly in 1959. Used for body weight and most everyday weight in the US.

1 ounce = 28.349523125 grams (exact)

One sixteenth of a pound. Note that troy ounces (used for precious metals) are different: 1 troy ounce = 31.1035g.

1 stone = 6.35029318 kilograms

14 pounds. Used for body weight in the UK and Ireland, rarely elsewhere.

Temperature conversion formulas

Celsius to Fahrenheit: °F = (°C × 9/5) + 32

Water freezes at 0°C / 32°F and boils at 100°C / 212°F at standard pressure.

Fahrenheit to Celsius: °C = (°F − 32) × 5/9

Body temperature is approximately 37°C / 98.6°F. A useful shortcut: doubling a Celsius temperature and adding 30 gives a rough Fahrenheit equivalent.

Celsius to Kelvin: K = °C + 273.15

Kelvin is the SI base unit of temperature. Zero Kelvin (absolute zero) is the theoretical point at which all thermal motion stops. Water freezes at 273.15K and boils at 373.15K.

Frequently asked

Are these conversions exact?

Length and weight conversions use their internationally defined exact ratios. Temperature conversions are exact in formula but limited by floating-point arithmetic, so results beyond 8 or so decimal places may carry small rounding errors — negligible for any practical purpose.

Why is temperature conversion different from the others?

Celsius, Fahrenheit, and Kelvin each define zero differently. Kelvin's zero is absolute zero; Celsius's is water's freezing point; Fahrenheit's is the temperature of a specific salt-ice mixture. Because the zero points differ, you can't just multiply — you have to shift as well as scale, which is why the formulas involve addition and subtraction.

What's the difference between mass and weight?

Technically, kilograms and pounds measure mass (the amount of matter), while newtons and pound-force measure weight (the gravitational force on that mass). In everyday use — including this converter — "weight" and "mass" are used interchangeably, since Earth's gravitational pull is essentially constant at the surface.