Last updated: 2026-04-30

Mass and Weight

Mass is the amount of matter in an object — intrinsic and unchanged whether the object is on Earth or the Moon. Weight is the gravitational force acting on that mass, which varies with location. In everyday use the two terms are interchangeable, and every reading from a kitchen or bathroom scale is technically a mass, calibrated to standard Earth gravity. This converter treats them as one quantity, which covers almost every practical use.

Two unit systems are in common use: the metric system (kilogram, gram, metric ton — base-10) and the imperial / US-customary system (pound, ounce, ton — historical). The carat is a separate international standard used specifically for gemstones and pearls.

Conversion Reference

All values are computed against the SI kilogram. The kilogram was redefined in 2019 in terms of Planck's constant, anchoring it to a fundamental physical constant rather than a physical artifact.

From	To kilograms	Notes
1 pound (lb)	0.453 592 37	Exact, by 1959 international agreement
1 ounce (oz)	0.028 349 523 125	1/16 lb
1 stone	6.350 293 18	14 lb — UK body weight
1 US ton (short ton)	907.184 74	2,000 lb
1 UK ton (long ton)	1,016.046 908 8	2,240 lb
1 metric ton	1,000	Exact by definition
1 carat	0.000 2	Exact (200 mg)

Common Usage by Unit

Grams and kilograms are the standard for kitchen, grocery, and shipping contexts — used by virtually everyone outside the US for everyday weights, and universally in scientific and medical contexts.

Pounds and ounces are standard in US grocery stores, postal rates, and everyday body-weight references. Stones are the conventional British body-weight unit ("I'm 12 stone 4") — in common use within the UK and rarely seen outside it. Short tons are the US shipping and freight standard; long tons appear in older British references and in naval displacement figures for older vessels.

Carats are the unit of the gemstone and pearl trade. One carat is 200 mg exactly, divided into 100 "points" — so a 0.5 ct stone is 50 points or 100 mg.

Frequently Asked Questions (FAQ)

What is the difference between a US ton and a UK ton?

A US (short) ton is 2,000 lb ≈ 907 kg. A UK (long) ton is 2,240 lb ≈ 1,016 kg. The metric ton is 1,000 kg exactly and sits between them. When a freight quote says "ton" without qualification, it is almost always the short ton in the US and the metric ton everywhere else.

Are pounds and kilograms exact conversions?

Yes. Since the 1959 International Yard and Pound Agreement, 1 pound = 0.453 592 37 kg exactly. The kilogram itself was redefined in 2019 in terms of Planck's constant, so both endpoints of the conversion are now fixed by international agreement.

How heavy is a one-carat diamond?

Exactly 200 mg, by international metric carat standard. A carat is divided into 100 "points," so a 0.5 ct stone is 50 points (100 mg). The carat is unrelated to the karat (with a K) used for gold purity.

Is mass the same as weight?

Physically, no — mass is the amount of matter and weight is the force of gravity acting on that mass. In everyday use they are treated as identical, and every consumer scale calibrates its reading to give a "mass" assuming standard Earth gravity (9.806 65 m/s²). This converter follows that convention.