Mass Conversion Tools

Mass measures the amount of matter in an object. Convert between kilograms, pounds, ounces, grams, and more.

Popular Mass Converters

Kilogram to Pound
kg → lb
Pound to Kilogram
lb → kg
Gram to Ounce
g → oz
Ounce to Gram
oz → g

All Mass Converters

Kilogram to Pound
(kg to lb)
Gram to Ounce
(g to oz)
Pound to Kilogram
(lb to kg)
Ounce to Gram
(oz to g)

Mass conversions cover kilograms, grams, pounds, ounces, and stones, the units used for body weight, cooking, shipping, and gym equipment. The kilogram itself was redefined in 2019, and the everyday word 'ounce' actually refers to two different units depending on whether you're weighing food or gold.

Mass and weight are not the same thing

Mass measures how much matter an object contains, and it does not change no matter where the object is. Weight measures the force gravity exerts on that mass, and it does change: an astronaut has the same mass on the Moon as on Earth but weighs about a sixth as much, because lunar gravity is weaker. A kitchen scale technically measures weight and then displays it as mass, which works fine on Earth's surface because gravity there is close enough to constant for everyday purposes.

In casual use, kilograms and pounds get treated as weight units, and for anything happening on the ground that is a harmless simplification. The distinction only matters in physics and space travel, where the same object can weigh very different amounts while its mass stays fixed.

The 2019 kilogram redefinition

For 130 years, the kilogram was defined by a single platinum-iridium cylinder kept in a vault outside Paris, nicknamed Le Grand K. Every other kilogram in the world was ultimately calibrated against that one object, an odd way to define a unit of science, because the cylinder's mass could theoretically drift from contamination or cleaning.

In May 2019, the kilogram was redefined using the Planck constant, a fixed number from quantum physics, measured with a device called a Kibble balance. The physical cylinder was retired as the official standard, and the kilogram is now reproducible in a lab anywhere in the world without reference to a single object in France.

Avoirdupois ounces, troy ounces, and the stone

The everyday ounce used for food, body weight, and general retail is the avoirdupois ounce, equal to exactly 28.349523125 grams. Precious metals use a different, heavier unit, the troy ounce, equal to 31.1034768 grams, about 10 percent more. Gold and silver have priced by the troy ounce since medieval trade fairs, and the convention never switched over even after avoirdupois became the everyday standard.

The stone, equal to exactly 14 avoirdupois pounds or about 6.35 kilograms, is still the standard way body weight is spoken about in the UK and Ireland, where someone might describe themselves as '11 stone 4' rather than giving a figure in kilograms or pounds directly.

Frequently asked questions

What's the actual difference between mass and weight?
Mass is a fixed property of an object, the amount of matter it contains, while weight is the force gravity applies to that mass and changes depending on where the object is. On Earth's surface the two track closely enough that people use the words interchangeably, but a scale on the Moon would show a much smaller number for the same object.
Why did the kilogram need redefining in 2019?
The old standard was a single physical cylinder in France, and every kilogram measurement in the world traced back to it. Because a physical object can theoretically gain or lose mass from handling or contamination, scientists replaced it with a definition based on the Planck constant, a fixed number from quantum physics that never changes and can be measured anywhere.
Why is gold priced in troy ounces instead of regular ounces?
The troy system has weighed precious metals since medieval trade, and it stuck even after most everyday weight moved to the avoirdupois system used for food and general goods. A troy ounce is about 10 percent heavier than the everyday ounce, so applying the wrong one to a gold price understates it noticeably.
How many pounds is a stone, and where is it still used?
A stone equals exactly 14 pounds, or about 6.35 kilograms. It survives specifically for describing body weight in the UK and Ireland; it is not used for parcels, produce, or general retail weight in either country.