Weight & Mass Unit Converter

What is weight (really mass), and why are there so many units?

In everyday speech we say "weight" but what we actually convert is mass — the amount of matter in an object, measured in kilograms in the SI system. True weight is a force (mass × gravitational acceleration) and is measured in newtons. Because the Earth's gravity barely changes between continents, kitchen scales and bathroom scales are calibrated so that a 1 kg mass reads "1 kg", and the everyday distinction is invisible. The kilogram itself was redefined in 2019 against the Planck constant rather than the platinum-iridium prototype it replaced, so today the SI base unit for mass is anchored to a physical constant of nature.

The reason there are so many mass units is historical, not scientific. Britain shipped the avoirdupois pound — the modern lb, fixed by the 1959 international agreement at exactly 0.45359237 kg — to its colonies, where it persisted in the United States. Continental Europe adopted the gram and kilogram with the metric system from 1795 onwards. Jewellers kept the carat, originally a carob-seed weight, now standardised at exactly 200 mg. Pharmacists and ammunition makers still use the grain (1/7000 of a pound, exactly 64.79891 mg). Each industry locked in a unit before SI consolidation, and the switching cost is high enough that the old units persist on grocery labels, gym scales, jewellery counters, and freight manifests.

The mass units, explained

Kilogram (kg) and its decimal multiples — the SI baseline

The kilogram is the SI base unit for mass. Since 20 May 2019 it has been defined by fixing the numerical value of the Planck constant h at 6.62607015 × 10⁻³⁴ J·s. The gram (g, 10⁻³ kg) and milligram (mg, 10⁻⁶ kg) cover laboratory and pharmaceutical work; the metric tonne (t, 10³ kg) is universal in shipping, agriculture, and industrial bulk. The microgram (µg, 10⁻⁹ kg) is the everyday unit for vitamins and trace contaminants.

Pound (lb) and ounce (oz) — the avoirdupois system

Since 1959 the international avoirdupois pound is exactly 0.45359237 kg, and the avoirdupois ounce is exactly 1/16 of a pound = 0.028349523125 kg = 28.349523125 g. These are the units on US grocery labels, gym dumbbell sets, postal scales, and luggage allowances on US-domestic flights. The same names exist in the troy system used for precious metals (1 troy ounce = 31.1034768 g), which is a different unit despite the shared word.

Stone (st) — the British body-weight habit

One stone equals exactly 14 pounds = 6.35029318 kg. Although officially deprecated in the UK since 1985, it remains the dominant unit for personal body weight in everyday British speech and on bathroom scales sold in Britain and Ireland. "12 stone 4" means 12 × 14 + 4 = 172 lb ≈ 78.0 kg. It does not appear on scales sold in the US, the EU, or most of the Commonwealth.

Tons — short, long, and metric

Three different "tons" coexist and they are not interchangeable. The metric tonne (t) is exactly 1000 kg. The US short ton is 2000 lb = 907.18474 kg. The UK long ton (also called the imperial ton) is 2240 lb = 1016.0469088 kg. Mining tonnages, freight rates, and emissions reports usually mean the metric tonne, but US auto specs and US scrap yards still mean the short ton. Always check the spec sheet.

Carat (ct) and grain (gr) — small specialised units

The metric carat is exactly 200 mg, used worldwide in gemstone trading. A 1-carat diamond weighs 0.2 g; a 2-carat ring stone weighs 0.4 g. The grain is exactly 64.79891 mg = 1/7000 lb, used today mostly in ammunition (bullet weights), powder loads, and apothecary pharmacopoeias. Pharmacy moved to milligrams long ago, but "5-grain aspirin" (≈ 325 mg) survives as legacy product naming.

Atomic mass unit (u) and dalton (Da) — chemistry's unit

Although not in the converter above (the values are far too small for a general-purpose tool), it is worth knowing that molecular biologists and chemists use the unified atomic mass unit u, also called the dalton (Da). 1 u = 1.66053906660 × 10⁻²⁷ kg, defined as 1/12 of the mass of a free, neutral carbon-12 atom at rest in its ground state. A water molecule weighs about 18 Da; a typical protein weighs tens of kilodaltons (kDa).

Real-world applications and which unit to expect

• Personal body weight: The metric world reports body mass in kg (a 70 kg adult); the United States in lb (155 lb); the United Kingdom and Ireland in stone-and-pound ("12 st 4"). Medical charts, sports leagues, and clinical trials in non-US countries default to kg even when the patient "thinks in" pounds or stones.
• Cooking and baking: Modern recipes have moved from volume (cups) to mass (grams) for accuracy, especially in baking. 100 g of flour weighs about 3.5 oz; 250 g of butter ≈ 8.8 oz ≈ one US stick. Kitchen scales sold internationally toggle between g and oz at the press of a button.
• Postal and shipping: International parcel rates use kg (under 2 kg, 2–5 kg, 5–10 kg brackets are standard at most postal services). US domestic rates often use lb-and-oz for letter mail and lb for parcels. Aviation cargo uses the chargeable weight in kg, computed from the greater of actual mass and volumetric weight (cm³ ÷ 6000).
• Freight and bulk industry: Shipping containers list payload in metric tonnes; iron ore, grain, and crude oil are quoted in tonnes globally. The notable exceptions are US coal and US scrap metal markets, which still trade in short tons, and a handful of UK statutes that retain long tons.
• Jewellery and gemstones: Diamond and coloured-stone weights are quoted in carats (ct). Gold and other precious metals use the troy ounce (1 ozt = 31.1034768 g) — different from the everyday avoirdupois ounce. A 1-oz gold bullion coin contains 31.1 g of gold, not 28.3 g.
• Firearms and reloading: Bullet weights and powder charges are listed in grains. A 9 mm Luger pistol typically uses a 115 gr or 124 gr bullet (≈ 7.5–8.0 g); a .308 Winchester rifle round commonly uses a 168 gr match bullet (≈ 10.9 g). Reloading manuals never use grams.
• Pharmaceuticals and supplements: Active ingredient mass is given in mg or µg. Common doses: vitamin D 25 µg (1000 IU), aspirin 325 mg or 81 mg ("baby aspirin"), paracetamol 500 mg or 1 g. Pharmaceutical regulators worldwide require SI units on labels.

How much is 1 unit of each in kilograms?

Frequently asked questions about weight units

References

1. NIST Special Publication 811 — Guide for the Use of the International System of Units (SI)
2. BIPM SI Brochure (9th edition, 2019)
3. ISO 80000-4:2019 — Quantities and units, Part 4: Mechanics
4. NIST Handbook 44 — Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices

Dedicated converter pages