📖 Guide

Weight Units Explained: Grams, Pounds, Troy Ounces and Stone

Metric vs U.S. weight units, why gold uses troy ounces (31.1 g), the British stone (14 lb), and when weight means mass vs force.

A Troy Ounce and a Regular Ounce Differ by 10%

Gold trades at a price per troy ounce. A troy ounce equals 31.103 grams. A standard (avoirdupois) ounce equals 28.350 grams. The difference is 9.7%. If you buy one "ounce" of gold expecting 28.35 grams and receive 31.10 grams, you got a better deal than you thought. If a jeweler quotes gold per standard ounce but prices it per troy ounce, you pay 9.7% more than you expected. The troy/avoirdupois distinction costs investors money every year.

The troy ounce survived into the modern precious metals market because it was the standard unit in medieval European trade fairs, particularly in Troyes, France. Coin minters, assayers, and bullion dealers adopted it and never switched. Today every gold, silver, platinum, and palladium price on any commodities exchange anywhere in the world quotes troy ounces, regardless of whether the country otherwise uses metric or imperial units.

This guide covers metric mass units (mg, g, kg, tonne), U.S. customary weight units (ounce, pound, short ton), the troy system for precious metals, the British stone for body weight, and the physics distinction between weight and mass that matters whenever you move something off Earth.

The Metric System: Mass in Powers of Ten

The metric system anchors mass to the kilogram. The 2019 SI redefinition fixed the kilogram by specifying the exact value of Planck's constant: h = 6.62607015 × 10⁻³⁴ joule-seconds. Before 2019, the kilogram was defined by a physical platinum-iridium cylinder stored in Sèvres, France. The new definition, reproducible anywhere from fundamental physics constants, eliminated the risk of the artifact changing mass over time (it had been drifting by about 50 micrograms per century).

The metric mass ladder:

1 microgram (μg) = 0.000001 g (pharmaceutical dosing, airborne particles)
1 milligram (mg) = 0.001 g (medicine, jewelry gemstones)
1 gram (g) = 1,000 mg (food nutrition labels, small items)
1 kilogram (kg) = 1,000 g (human body weight, groceries, engineering)
1 tonne (metric ton) = 1,000 kg = 1,000,000 g (shipping, industrial materials)

The metric tonne (t) equals 1,000 kg or approximately 2,204.6 lb. The U.S. "short ton" equals 2,000 lb (907.2 kg). The UK "long ton" equals 2,240 lb (1,016 kg). All three are called "ton" in everyday speech, which creates systematic confusion in shipping, mining, and agricultural trade.

U.S. Customary Units: Ounces, Pounds, and Tons

U.S. weight units use the avoirdupois system, named from the Old French "avoir du poids" (goods of weight). Since 1959, the pound is fixed by treaty: 1 pound (lb) = 453.59237 grams exactly. All other avoirdupois units derive from this:

1 ounce (oz) = 1/16 lb = 28.3495 g
1 pound (lb) = 16 oz = 453.592 g
1 short ton = 2,000 lb = 907.185 kg

Converting pounds to kilograms: multiply by 0.453592. Converting kilograms to pounds: multiply by 2.20462. Quick mental estimate for body weight: multiply kg by 2.2, or divide lb by 2.2. A person weighing 70 kg weighs about 70 × 2.2 = 154 lb. A person at 150 lb weighs 150 ÷ 2.2 = 68.2 kg.

Converting ounces to grams: multiply by 28.35. Converting grams to ounces: multiply by 0.03527. A 16-oz ribeye steak weighs 453 g (about 1 pound exactly, which is where the 16-oz pound relationship shows up on menus).

The Troy System for Precious Metals

The troy weight system uses different units from avoirdupois, despite sharing some names:

1 troy grain = 64.799 mg (same grain as in avoirdupois)
1 troy pennyweight (dwt) = 24 grains = 1.5552 g
1 troy ounce (ozt) = 20 pennyweights = 480 grains = 31.1035 g
1 troy pound = 12 troy ounces = 373.242 g

Gold prices quoted at $2,000 per troy ounce mean $2,000 per 31.10 g, or $64.32 per gram. A 1 kg gold bar contains 32.15 troy ounces. At $2,000/ozt, a 1 kg bar is worth $64,300. The troy pound (12 troy oz) is smaller than the avoirdupois pound (16 av oz × 28.35 g = 453.6 g vs 12 × 31.10 = 373.2 g for troy).

Gemstone weight uses a separate unit: the carat. One carat (ct) = 0.2 grams = 200 milligrams. A 1-carat diamond weighs 0.2 g. The carat has no relationship to the karat (kt or k) used for gold purity (24k = pure gold, 18k = 75% gold).

Common Misconceptions

Weight and mass are the same thing. Mass is the amount of matter in an object, measured in kilograms. Weight is the gravitational force on that mass, measured in newtons. On Earth, a 70 kg person weighs about 686 newtons (70 × 9.81 m/s²). On the Moon (1/6 gravity), the same person has the same 70 kg mass but weighs only 114 newtons. Bathroom scales measure weight but display mass numbers calibrated for Earth's gravity. In everyday use, "weight" and "mass" are treated as synonymous; in physics, they are not.
A troy ounce is the same as a regular ounce. A troy ounce is 31.1035 g; a standard ounce is 28.3495 g. They differ by 9.7%. Gold, silver, platinum, and palladium prices always quote troy ounces. Anything else (food, medicine, postal weight) uses avoirdupois ounces. The word "ounce" alone is ambiguous in precious metals contexts.
A metric ton and a U.S. ton are the same. A metric tonne is 1,000 kg (2,204.6 lb). A U.S. short ton is 2,000 lb (907.2 kg). A UK long ton is 2,240 lb (1,016.0 kg). All three appear in shipping contracts. A cargo described as "500 tons" could mean 454, 500, or 508 metric tonnes depending on which ton the contract uses.
Stone is an old-fashioned UK unit no one uses anymore. UK doctors, gyms, and bathroom scales still commonly display body weight in stones and pounds. One stone equals 14 lb or 6.350 kg. A person who weighs "11 stone 4 pounds" weighs (11 × 14) + 4 = 158 lb = 71.67 kg. The unit remains in active use throughout the UK and Ireland for personal body weight.
The carat and the karat measure the same thing. Carat (ct) is a unit of gem mass: 1 ct = 0.2 g. Karat (k or kt) is a measure of gold purity: 24k = pure gold, 14k = 58.3% gold. A 2-carat 18-karat gold ring has a stone weighing 0.4 g and a gold setting that is 75% pure. They are unrelated measures that happen to sound identical.

Worked Example: Buying Gold Across Different Weight Systems

James calculates the true cost of a gold coin purchase

James wants to buy a 1-ounce American Gold Eagle coin. The spot price of gold is $2,150 per troy ounce. The coin has a premium of 4% over spot. He wants to know the total cost in dollars, the coin's weight in grams, and the price per gram.

Step 1: Total cost. Spot price + 4% premium: $2,150 × 1.04 = $2,236. James pays $2,236 for one coin.

Step 2: Weight in grams. The American Gold Eagle contains exactly 1 troy ounce of gold (31.1035 g), but the actual coin weighs 33.931 g total because it contains small amounts of silver and copper for hardness. The gold content is 31.10 g.

Step 3: Price per gram of gold. $2,236 ÷ 31.1035 g = $71.89 per gram of gold. Cross-check with spot: $2,150 ÷ 31.1035 = $69.12 per gram at spot. The 4% premium adds $2.77 per gram.

James also checks how this compares to buying gold in Europe, where his contact quotes gold at €1,980 per troy ounce. At an exchange rate of 1.08 USD/EUR, that is $2,138.40 per troy ounce, or $0.48/g cheaper than the U.S. spot price before any premium. The European premium adds 3%, so James pays €2,039.40 = $2,202.55 for the same amount of gold, saving about $33.50 per troy ounce.

When Standard Conversions Break Down

Apparent weight vs true weight in liquids. An object submerged in water experiences buoyancy equal to the weight of the displaced water. A 100 g rock submerged in water may read only 65 g on an underwater scale if it displaces 35 g of water. Archimedes's principle causes all underwater weighing to read less than true mass. Mineral assayers use this to calculate specific gravity and purity.
Tare weight in shipping and commerce. A truck carrying goods has a gross weight (vehicle + cargo) and a tare weight (empty vehicle). Net weight (the goods alone) = gross − tare. Food labeling requires net weight on the container. Converting "gross weight 500 lb" to kilograms without subtracting tare weight conflates the vehicle with the product.
Apothecary weights in pharmacy. Historical pharmaceutical measures used a third system: apothecary ounces (the same as troy ounces at 31.1 g) and apothecary grains (64.799 mg, also the same as troy grains). Old prescriptions written before the 1970s may reference "drachms" (1/8 apothecary oz = 3.888 g) or "scruples" (1/24 oz = 1.296 g). Modern pharmacy uses grams and milligrams exclusively.
Weight in altitude vs standard conditions. Standard gravity is defined as 9.80665 m/s². Gravity varies from 9.832 m/s² at the poles to 9.780 m/s² at the equator, and decreases with altitude. A precision balance weighing pharmaceutical compounds at different altitudes must use buoyancy and gravitational corrections. A bathroom scale at 5,000 m altitude reads about 0.16% lower than at sea level.
Wet vs dry weight in agriculture and food. Grain moisture content affects mass. Wheat traded at 13.5% moisture reads differently on the scale than the same grain dried to 11%. Agricultural commodity contracts specify moisture content alongside weight, because buyers pay for dry grain, not water. Converting metric tonnes to pounds without noting moisture content misrepresents the actual commodity volume.

Quick Reference: Weight Conversion Table

Unit	Grams	Ounces (av)	Pounds	Kilograms
1 mg	0.001	0.0000353	0.0000022	0.000001
1 g	1	0.03527	0.002205	0.001
1 troy oz	31.1035	1.09714	0.06857	0.03110
1 av oz	28.3495	1	0.0625	0.02835
1 lb	453.592	16	1	0.4536
1 stone	6,350.3	224	14	6.350
1 kg	1,000	35.274	2.2046	1
1 short ton	907,185	32,000	2,000	907.185
1 metric tonne	1,000,000	35,274	2,204.6	1,000

Frequently Asked Questions

How many grams are in a pound?

One pound equals exactly 453.59237 grams. This exact value has been fixed by international treaty since 1959. For practical use: 1 lb ≈ 454 g; 2.2 lb ≈ 1 kg. To convert pounds to grams, multiply by 453.592. To convert grams to pounds, multiply by 0.002205.

What is a troy ounce in grams?

One troy ounce equals 31.1035 grams. This is about 10% heavier than a regular (avoirdupois) ounce of 28.3495 grams. Gold, silver, platinum, and palladium are always priced and traded in troy ounces. Food, shipping, and body weight use avoirdupois ounces.

What is a stone in kg and pounds?

One stone equals exactly 14 pounds or 6.35029 kilograms. The stone is used for body weight in the United Kingdom and Ireland. A person who weighs 12 stone weighs 168 lb (76.2 kg). A person at 75 kg weighs 75 ÷ 6.350 = 11.81 stone, or 11 stone 11.3 pounds.

What is the difference between weight and mass?

Mass is the amount of matter in an object, measured in kilograms. Weight is the gravitational force on that mass, measured in newtons (kg × m/s²). On Earth, a 1 kg mass weighs 9.81 newtons. On the Moon, the same 1 kg mass weighs 1.62 newtons. Your bathroom scale reads mass (calibrated for Earth gravity) and labels it "kg" or "lb," even though it technically measures force.

How many pounds are in a metric ton?

One metric tonne (1,000 kg) equals 2,204.62 pounds. A U.S. short ton equals exactly 2,000 pounds (907.185 kg). The metric tonne is about 10% heavier than the short ton. The UK long ton (2,240 lb = 1,016 kg) sits between the two.

How many ounces are in a pound?

Sixteen avoirdupois ounces equal one pound. This is the everyday ounce used for food, shipping, and body weight. The troy pound contains 12 troy ounces, but the troy pound is rarely used in commerce; gold trading quotes individual troy ounces, not pounds.

How do I convert kg to lbs?

Multiply kilograms by 2.20462. Example: 80 kg × 2.20462 = 176.4 lb. Quick mental estimate: multiply by 2.2. For the reverse, divide by 2.20462 or multiply by 0.453592. A 200 lb person weighs 200 × 0.453592 = 90.7 kg.