Weight and mass are concepts so fundamental to everyday life that most people never stop to question why there are so many different units for measuring the same thing. Kilograms in a science class, pounds on a bathroom scale in the US, stone at a UK doctor's office, troy ounces on a gold price chart, carats at a jeweler - each unit comes from a different historical tradition, serves a specific community, and persists because the people who use it find it practical. This guide explains every major weight unit, where it came from, and when to use it.
Weight vs Mass - Is There a Difference?
Technically yes - mass is the amount of matter in an object and is constant regardless of location. Weight is the force exerted by gravity on that mass and varies slightly with location (you weigh slightly less at the top of a mountain than at sea level). In everyday usage the distinction is almost never meaningful - when you step on a scale it measures weight (force) but displays mass (kg or lbs) assuming standard gravity. In scientific contexts, especially in physics, the distinction matters and mass in kilograms (SI unit) is used. For practical purposes this guide uses weight and mass interchangeably as everyday language does.
The Metric System - Grams and Kilograms
The metric system defines the gram as the base unit of mass - originally defined as the mass of one cubic centimeter of water at 4°C. The kilogram (1,000 grams) became the SI base unit and is now defined by fixing the value of the Planck constant. The metric system's power-of-ten structure makes arithmetic simple: 1 kg = 1,000 g = 1,000,000 mg. The metric tonne (1,000 kg) is used for large masses. Milligrams are standard in medicine and pharmacy. Micrograms appear in nutrition labels and atmospheric science. Metric weight units are used in all scientific contexts worldwide and in everyday life in most countries.
Imperial and US Customary - Pounds and Ounces
The imperial system evolved from older English units with roots going back to Roman and Anglo-Saxon measurements. The avoirdupois system (used for everyday goods) defines: 16 drams in an ounce, 16 ounces in a pound, 14 pounds in a stone, 8 stones (112 lbs) in a hundredweight, and 20 hundredweights (2,240 lbs) in a long ton. The US customary system diverged slightly - the US short ton is 2,000 lbs rather than 2,240. The stone is used in the UK and Ireland for body weight but is not used in the US. Pounds and ounces remain standard in the US for almost all everyday weight measurements.
The single most useful conversion to memorize: 1 kg ≈ 2.205 lbs. For a quick mental estimate double the kg value and add 10% - 70 kg × 2 = 140, +10% = 154. Close to the exact 154.3 lbs.
The Stone - A Uniquely British Unit
The stone (14 lbs = 6.35 kg) is one of the most regionally specific weight units still in common use. It is used almost exclusively in the UK and Ireland for expressing human body weight. A British person might describe their weight as "11 stone 4" meaning 11 stones and 4 pounds - equivalent to 158 lbs or 71.7 kg. Despite the UK's official adoption of metric, the stone remains deeply embedded in everyday British culture for personal weight. UK bathroom scales almost universally show stone and pounds alongside kilograms.
Troy Weight - Gold, Silver, and Gemstones
- The troy system originated in Troyes, France, a major medieval trading center, and was adopted for precious metals because merchants needed a consistent, trusted standard.
- A troy ounce (31.1035g) is about 10% heavier than an avoirdupois ounce (28.3495g) - a critical distinction when buying or selling precious metals.
- All gold, silver, platinum, and palladium prices are quoted per troy ounce globally.
- A troy pound (373.24g) is lighter than an avoirdupois pound (453.59g) - troy has 12 ounces per pound while avoirdupois has 16.
- The carat (0.2g) is a metric unit used for gemstone weight - not troy. A 1-carat diamond weighs exactly 0.2 grams. This is different from karat (purity of gold alloys).
When buying or selling precious metals always verify whether prices are quoted per troy ounce or regular ounce. A troy ounce is about 10% heavier - confusing the two can cause significant pricing errors.