Weight & Mass Converter

Convert between kilograms, pounds, ounces, stone, and tonnes, with the difference between weight and mass explained clearly.

Weight and Mass Are Not Actually the Same Thing

Everyday conversation treats weight and mass as interchangeable, but they measure genuinely different things: mass is the amount of matter in an object, unchanging wherever the object goes, while weight is the force gravity exerts on that mass, which does change depending on location — an object weighs slightly less at high altitude and considerably less on the Moon, even though its mass hasn’t changed at all. For everyday and commercial purposes, this distinction rarely matters because we’re always measuring at Earth’s surface, which is why kilograms and pounds get used as if they were weight units even though a kilogram is technically a unit of mass. The kilogram itself was, until 2019, defined by a physical platinum-iridium cylinder kept in France, and is now defined through a fixed value of the Planck constant, making it one of the most precisely defined units in existence. The pound, by contrast, is now defined in terms of the kilogram, at exactly 0.453592 kilograms, so every pound-to-kilogram conversion today ultimately traces back to that single fixed ratio. Stone, still common in the UK for body weight, equals fourteen pounds, a leftover from an older wool-trading measurement system.