Weight & Mass

Mass measures the quantity of matter in an object; weight is the gravitational force on that mass. In everyday usage, the two terms are used interchangeably, and the units described here are used for both. The kilogram is the SI base unit and is now defined through Planck's constant — a fixed value from quantum mechanics — rather than a physical object. The pound is the primary unit in the US customary system. Accurate mass measurement is fundamental to cooking, medicine, commerce, science, and manufacturing worldwide.

Units

Kilograms (kg): The SI base unit of mass, defined since 2019 by fixing Planck's constant to exactly 6.626 070 15 × 10⁻³⁴ J·s. Scale: Roughly the mass of a litre of water at 4°C. Example: A large pineapple weighs about 1 kilogram.
Grams (g): One thousandth of a kilogram (0.001 kg). The everyday unit for food portions, medicine doses, and small quantities. Scale: Approximately the mass of a standard paper clip. Example: A teaspoon of sugar weighs about 4 grams.
Pounds (lb): Exactly 0.453 592 37 kilograms (fixed internationally in 1959). The primary unit of weight in the United States. Scale: Roughly the weight of a large apple. Example: A typical loaf of bread weighs about 1 pound.
Ounces (oz): One sixteenth of a pound — exactly 28.349 5 grams. Used for food portions and small weights in the US and UK. Scale: About the weight of four standard office paper clips. Example: A standard chocolate bar is often 3.5 oz (100 g).
Milligrams (mg): One thousandth of a gram (0.001 g). Used for pharmaceutical doses and trace measurements. Scale: One grain of salt weighs approximately 1 milligram. Example: A standard aspirin tablet contains 325 mg of active ingredient.
Tons (t (metric)): 1 000 kilograms (one metric tonne). The SI unit for large masses. Note: the US short ton (907 kg) differs from the metric tonne. Scale: Approximately the weight of a small car. Example: A fully loaded semi-truck may weigh up to 40 metric tons.

Key Facts

The kilogram is the last SI unit to have been redefined (in 2019) — moving from a physical cylinder in Paris to a quantum constant.
One litre of water at 4°C weighs almost exactly one kilogram — this was the original design goal of the metric system.
The troy ounce (31.1 g) used for precious metals differs from the avoirdupois ounce (28.35 g) used for everyday goods.
Pound (lb) abbreviation comes from the Latin libra pondo — 'a pound by weight.'
A human hair weighs approximately 0.06–0.10 milligrams per millimetre of length.

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