Guide to Weight and Mass Units: Grams, Pounds, and Beyond

Introduction

Weight and mass are two of the most frequently confused concepts in everyday measurement. Markets sell produce "by the kilogram," doctors record patient "weight" in pounds, and chemists measure compounds in grams—yet the underlying physics distinguishes these quantities in ways that matter in certain contexts. This guide explains the distinction, surveys every major unit system, and provides the exact conversions you need.

For instant calculations, visit the UnitFYI weight and mass converter.

Weight vs. Mass: Why It Matters

Mass is a measure of the amount of matter in an object. It does not change regardless of location—a 70 kg person has 70 kg of mass on Earth, on the Moon, and in deep space.

Weight is the gravitational force acting on that mass. It is expressed in Newtons (SI) or pounds-force (imperial). On Earth's surface, where gravitational acceleration is approximately 9.81 m/s², weight and mass are numerically close enough that everyday usage treats them interchangeably.

The practical consequence: a scale that reads kilograms is technically measuring force and converting to mass using Earth's gravity. A spring scale taken to the Moon would give a reading about one-sixth as large, because lunar gravity is roughly 1.62 m/s²—even though the object's mass is unchanged.

In this guide, "weight units" refers to units used in everyday weighing contexts, acknowledging the common conflation.

The Metric System

Standard Metric Units

The gram (g) is the practical base unit for small-scale measurement. A standard paperclip weighs about 1 g. A US nickel coin weighs exactly 5 g.

The kilogram (kg) is the SI base unit of mass. One liter of pure water at 4°C has a mass of almost exactly 1 kg. The international prototype kilogram—a platinum-iridium cylinder kept in France—served as the definition of the kilogram until 2019, when the definition was redefined in terms of the Planck constant.

The metric tonne (t)—also spelled "tonne" to distinguish it from the short ton—equals 1,000 kg. A mid-size passenger car weighs roughly 1.5 tonnes. Shipping container cargo is specified in tonnes.

The milligram (mg) is common in pharmacy and nutrition. A standard aspirin tablet contains 325 mg of active ingredient. Vitamin D supplements are often dosed in micrograms (µg), where 1 µg = 0.001 mg.

The Imperial and US Customary System

Units from Small to Large

Ounce (oz) The avoirdupois ounce is the standard ounce for everyday goods: food packaging, postal limits, beverage containers. 1 oz = 28.3495 grams.

Pound (lb) 1 pound = 16 ounces = 453.592 grams (commonly rounded to 453.6 g). This is the everyday weight unit in the United States. A standard loaf of bread weighs about 1 lb (454 g). Body weight is expressed in pounds throughout the US.

Stone (st) Used primarily in the United Kingdom and Ireland for body weight. 1 stone = 14 pounds = 6.35029 kg. A 75 kg person weighs approximately 11 stone 11 pounds in British usage.

Short ton (US ton) 1 short ton = 2,000 pounds = 907.185 kg. Used in the United States for industrial and freight applications.

Long ton (Imperial ton) 1 long ton = 2,240 pounds = 1,016.05 kg. Used historically in the UK; now largely superseded by the metric tonne.

Key Metric-Imperial Conversions

• 1 oz = 28.3495 g
• 1 lb = 453.592 g = 0.453592 kg
• 1 kg = 2.20462 lb
• 1 short ton = 0.907185 t (metric tonnes)
• 1 metric tonne = 2,204.62 lb = 1.10231 short tons

Use the kg to lb converter or the lb to kg converter for quick results.

Troy Weight System

The troy system is used exclusively for precious metals (gold, silver, platinum, palladium) and gemstone trade.

Troy Units

The troy ounce (31.1035 g) is heavier than the avoirdupois ounce (28.3495 g) by about 9.7%. Gold spot prices are always quoted per troy ounce. A standard 400 troy ounce gold bar held in central bank reserves weighs 12.4 kg.

The grain is shared between the troy and avoirdupois systems—both define exactly 7,000 grains to the avoirdupois pound and 5,760 grains to the troy pound. Ammunition propellant charges are measured in grains.

Carat

The carat (ct) is the standard unit for gemstone mass: 1 carat = 0.2 grams (200 mg). The term originates from carob seeds, which were historically used as counterweights due to their near-uniform mass. The 530.4-carat Cullinan I, the largest gem-quality diamond ever cut, weighs 106.08 grams.

The carat should not be confused with karat (kt), which is a measure of gold purity (24 karat = pure gold, 18 karat = 75% gold).

Specialty and Regional Units

Quintal: In the metric world, 1 quintal = 100 kg. Used in agriculture across South America, Europe, and parts of Asia.

Picul (or Pikul): A traditional unit used in Southeast and East Asian trade, typically 60 kg in China, varying by region. Appears in historical commodity records.

Tola: A traditional South Asian unit for precious metals. 1 tola = 11.6638 g (exactly 3/8 troy ounce). Gold is still sold by the tola in India and Pakistan.

Real-World Reference Points

Choosing the Right Unit

For cooking and nutrition, grams and ounces dominate. For body weight, kilograms (most of the world) or pounds and stones (US, UK) are standard. For precious metals, always use troy ounces. For large cargo and industrial loads, metric tonnes are universal in global trade while short tons persist in US domestic contexts.

Explore all UnitFYI weight converters for every combination of units covered in this guide.