Mass Converter
Mass conversions are exact, not approximate, because the imperial units are legally defined in terms of the kilogram: the international pound is precisely 0.45359237 kilograms. From that single definition every other factor follows, an ounce being a sixteenth of a pound and a stone fourteen pounds. Enter a value and choose its unit, and this converter expresses it in kilograms, grams, pounds, ounces, stones, and metric tonnes by first converting to kilograms and then out to each unit, so every result is internally consistent.
Mass conversion factors
1 pound = 0.45359237 kilograms (exact)
1 ounce = 1/16 pound
1 stone = 14 pounds
1 kilogram = 1,000 grams
1 metric tonne = 1,000 kilograms
The converter changes your input into kilograms using these exact factors, then converts from kilograms into every output unit, keeping all results consistent.
Mass units context
- The kilogram is the SI base unit of mass, defined since 2019 by fixing the Planck constant.
- The international avoirdupois pound is defined as exactly 0.45359237 kilograms.
- The stone (14 pounds) is still common for body weight in the UK and Ireland.
- A metric tonne (1,000 kg) differs from a US short ton (2,000 lb) and a UK long ton (2,240 lb).
- Mass is intrinsic to an object; weight is the gravitational force on that mass and varies with location.
Mass converter: frequently asked questions
How many pounds are in a kilogram?
One kilogram equals about 2.2046226 pounds, because one pound is defined as exactly 0.45359237 kilograms. This is an exact legal definition set by international agreement and adopted by NIST, not an approximation, so the conversion is precise to as many digits as you need.
What is the exact pound-to-kilogram factor?
The international avoirdupois pound is defined as exactly 0.45359237 kilograms. Every other imperial mass unit follows from it: an ounce is one sixteenth of a pound, and a stone is 14 pounds. This calculator uses these exact factors so conversions are reproducible.
What is the difference between mass and weight?
Mass is the amount of matter in an object and is the same everywhere; weight is the force gravity exerts on that mass and varies with location. These units (kilogram, pound, ounce) are units of mass. On Earth's surface people often use them loosely for weight, but the conversions here are mass-to-mass.
How big is a metric tonne?
A metric tonne is exactly 1,000 kilograms, about 2,204.6 pounds. It is not the same as a US short ton (2,000 pounds) or a UK long ton (2,240 pounds). This converter uses the metric tonne; if you need short or long tons, convert from kilograms or pounds accordingly.
Why convert through kilograms?
The calculator first converts whatever unit you enter into kilograms, then from kilograms into every other unit. Using a single base unit keeps the factors consistent and avoids rounding drift, so converting pounds to ounces gives the same answer as the direct relationship of 16 ounces per pound.
Official sources
- NIST: Weights and measures unit conversion.
- NIST Handbook 44: Mass and weight definitions.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.