Convert pounds, kilograms, ounces, stone, grams and 10+ more units instantly. Includes a body weight display and a precious metals converter.
Weight conversion is one of the most common calculations people need daily — for body weight, cooking, shipping, fitness, science, and commerce. The world uses three primary systems: the metric system (kilograms, grams), the avoirdupois system (pounds, ounces — used in everyday US and UK life), and the Troy system (troy ounces, pennyweights — used exclusively for precious metals and gemstones). Understanding the differences between these systems, and when each applies, prevents costly measurement errors.
In everyday language, “weight” and “mass” are used interchangeably — but they mean different things in physics. Mass is the amount of matter in an object, measured in kilograms or grams. It never changes regardless of location. Weight is the gravitational force acting on that mass, measured in Newtons. On the Moon (gravity = 1/6 of Earth’s), your mass is identical but your weight is only 1/6. A 70 kg person weighs 686 Newtons on Earth but only 114 Newtons on the Moon — yet their mass stays 70 kg everywhere. This calculator converts mass units, which people commonly call “weight” in everyday usage.
The kilogram (kg) is the SI base unit of mass. Since 2019, it has been defined by fixing the value of Planck’s constant (h = 6.62607015 × 10⁻³⁴ J·s), replacing the old physical platinum-iridium prototype cylinder stored in Paris since 1889. This new definition makes the kilogram reproducible anywhere in the universe from fundamental constants. The gram (g) is exactly 1/1000 of a kilogram. The milligram (mg) = 1/1000 gram is used for medication doses. The microgram (μg) = 1/1,000,000 gram is used in nutritional labeling for vitamins and trace elements.
The avoirdupois system — the everyday weight system used in the United States and informally in the UK — is built around the pound. The word “avoirdupois” comes from Old French meaning “goods of weight.” Key relationships in the avoirdupois system:
| Unit | Abbreviation | Grams (exact) | Relationship |
|---|---|---|---|
| Grain | gr | 0.06479891 g | 7,000 grains = 1 lb |
| Ounce | oz | 28.349523 g | 16 oz = 1 lb |
| Pound | lb | 453.59237 g | 14 lb = 1 stone |
| Stone | st | 6,350.293 g | 2 stone = 1 quarter |
| Short Ton (US) | ton | 907,184.74 g | 2,000 lb |
| Long Ton (UK) | LT | 1,016,046.9 g | 2,240 lb |
| Metric Ton | t | 1,000,000 g | 2,204.62 lb |
The stone (14 pounds = 6.35 kg) is an ancient unit still used in everyday speech in the UK and Ireland for body weight, though it was officially dropped from legal use in 1985. A British person might describe their weight as “12 stone 4” (12 stone 4 pounds = 78.0 kg = 172 lb). The stone unit is never used in science, medicine, or industry — only for body weight in casual speech. Doctors in the UK record weight in kilograms; scales in gyms show both kg and stone. Understanding stone is essential for reading older British literature, medical records, and weight loss discussions on UK forums.
The Troy system is a completely separate weight system used exclusively for precious metals (gold, silver, platinum, palladium) and gemstones. It originated in Troyes, France, a medieval trading city. The Troy ounce (ozt) = 31.1035 grams — approximately 10% heavier than an avoirdupois ounce (28.35 g). This distinction matters enormously: when you see a gold spot price of “$2,000 per ounce,” that ounce is a Troy ounce. Buying or selling precious metals using the wrong ounce definition will result in a 10% pricing error.
The Troy system relationships: 24 grains = 1 pennyweight (dwt). 20 pennyweights = 1 troy ounce. 12 troy ounces = 1 troy pound. Note that the Troy pound (373.24 g, 12 troy oz) is lighter than the avoirdupois pound (453.59 g, 16 oz) — a counterintuitive fact that catches many people off guard.
The carat (ct) is the standard unit of mass for gemstones including diamonds, rubies, sapphires, and emeralds. One metric carat = exactly 200 milligrams = 0.2 grams. The carat for gemstone mass is completely different from the karat used to describe gold purity (24 karat = pure gold). A 1.00 ct round diamond is approximately 6.5 mm in diameter. The largest gem-quality diamond ever found, the Cullinan Diamond (1905), weighed 3,106.75 carats = 621.35 grams.
Several specialized weight units appear in specific industries. The short ton (2,000 lb = 907.18 kg) is used in the US for bulk commodities like coal, steel, and grain. The long ton (2,240 lb = 1,016.05 kg) is the UK equivalent, still used in shipping and some commodities. The metric ton (1,000 kg = 2,204.62 lb) is the international standard for heavy industry and cargo. A fully loaded Boeing 737 weighs approximately 79 metric tons. A standard shipping container (20 ft) holds approximately 28 metric tons of cargo.
Medicine and laboratory work demand precision at very small scales. Milligrams (mg) are the standard for drug dosages — aspirin tablets are 325 mg, acetaminophen standard dose is 500 mg. Micrograms (μg or mcg) appear on vitamin labels: Vitamin D is typically 25 μg (1,000 IU) per tablet. At the laboratory scale, nanograms (ng) and picograms (pg) are used in analytical chemistry, DNA quantification, and environmental testing. The transition from one prefix to the next is always 1,000× in the metric system, making scale conversions straightforward.
Understanding weight is essential for shipping. USPS First-Class Mail has a 3.5 oz (99 g) maximum for letters and 13 oz (368 g) for large envelopes. Priority Mail Express domestic limit is 70 lb (31.75 kg). FedEx and UPS domestic ground services cap individual packages at 150 lb (68 kg). International air freight uses kilograms for pricing, with typical per-kg rates that drop as weight increases. Always weigh packages on a calibrated scale before shipping — carrier scales at drop-off points are the final authority, and reweighs can add unexpected fees to pre-paid labels.
Understanding weight conversions has practical consequences across cooking, shipping, travel, medicine, and fitness. Here are the scenarios where weight conversion knowledge is most valuable.
European and UK recipes use grams; most US recipes use ounces and cups. The conversion is 1 oz = 28.35 g, making a 100 g measurement ≈ 3.53 oz. For baking precision, using a kitchen scale in grams is strongly preferred over volumetric measures (cups) because ingredient density varies dramatically. A cup of packed brown sugar weighs 220 g; a loosely spooned cup may weigh only 160 g. Professional bakers always work by weight. Key baking reference weights: 1 cup all-purpose flour ≈ 120-130 g, 1 cup granulated sugar ≈ 200 g, 1 stick butter = 4 oz = 113 g.
Shipping costs are weight-dependent, and carriers use different systems. US domestic carriers (FedEx, UPS, USPS) price by pounds. International carriers often use kilograms. Freight shipping uses metric tons. A common confusion: carriers use dimensional weight (volumetric weight) for large, light packages — calculated as L×W×H (inches) ÷ 139 for US domestic. If the dimensional weight exceeds the actual weight, you’re charged the higher figure. International air freight typically uses 166 cubic inches per kilogram as the dimensional weight divisor.
Different countries describe body weight in different units, which creates confusion when reading international health research, clothing size guides, or medical records. The United States uses pounds exclusively. Most of the world uses kilograms. The UK uses a mix — official medical records in kilograms, but casual conversation often in stone and pounds. A person who weighs 75 kg is 165.3 lb or 11 stone 11 lb. BMI is always calculated using kilograms and meters, regardless of which system you measure in — so always convert first before calculating BMI.
Gym equipment labeling differs by country and manufacturer. Barbells and plates in the US are typically marked in pounds; international and Olympic equipment is in kilograms. An Olympic barbell is 20 kg (44 lb). A standard plate is either 45 lb (20.4 kg) or 20 kg (44.1 lb) depending on the set. The difference (0.4 kg / 0.9 lb per plate) adds up with multiple plates on a bar. Competition powerlifting and weightlifting always uses kilograms for official records.