Unit Converter
Convert length, weight, and temperature between metric and US/imperial units using the exact SI conversion factors fixed by the 1959 international yard-and-pound agreement and published by NIST.
How the conversions work
Each length is first expressed in metres and each weight in kilograms, then multiplied by the exact factor for the unit you want. For example, 1 inch is defined as exactly 0.0254 m and 1 pound as exactly 0.45359237 kg. Because these factors are fixed by definition rather than measured, converting between metric and imperial units introduces no measurement error — only the rounding you choose when the result is displayed.
Temperature is different: instead of a single multiplier it needs an offset, because the Celsius and Fahrenheit scales start at different zero points. Celsius to Fahrenheit is (°C × 9/5) + 32, and Celsius to kelvin is °C + 273.15. Rounding is the main practical limit for every conversion — showing only a couple of decimals can nudge a value slightly, which can matter for recipes, medication amounts, or engineering tolerances. This tool keeps full precision internally and rounds only what you see.
For measurements that carry legal, medical, or safety weight — medication doses, structural or mechanical engineering, trade and customs, or laboratory work — rely on a calibrated instrument and the primary standard rather than a web converter. National metrology institutes such as NIST publish the authoritative factors, and legal-for-trade measurements must be made with certified, traceable equipment.
Sources: NIST — Guide to the SI, Appendix B (conversion factors)
Frequently asked questions
Are these conversion factors exact or approximate?
The length and weight factors are exact by definition. Since the 1959 international yard-and-pound agreement, 1 inch is defined as exactly 25.4 mm and 1 pound as exactly 0.45359237 kg, so the conversions carry no measurement uncertainty. The only rounding is whatever you apply to the displayed answer.
Why does temperature use a formula instead of a simple factor?
Length and weight scales share the same zero point, so you convert by multiplying. Temperature scales do not: 0 °C is the freezing point of water while 0 °F is much colder, so you have to both scale and shift the value. Celsius to Fahrenheit is (°C × 9/5) + 32, and Celsius to kelvin is °C + 273.15.
How many pounds are in a kilogram?
One kilogram is about 2.20462 pounds, found by dividing 1 by 0.45359237. In the other direction, one pound is exactly 0.45359237 kg. An avoirdupois ounce is 1/16 of a pound, or exactly 28.349523125 grams, and a stone is 14 pounds (6.35029318 kg).
Do the US and British imperial units differ in this tool?
For the length and weight units here — inch, foot, yard, mile, pound, ounce, and stone — US customary and British imperial values are identical, because both derive from the 1959 agreement. The two systems only diverge for volume (a US gallon differs from an imperial gallon), which this length, weight, and temperature converter does not cover.
