Strictly, mass is the amount of matter in an object while weight is the force gravity exerts on it, but in everyday use the words are interchangeable. The kilogram is the SI base unit; since 2019 it has been defined through the Planck constant rather than a physical prototype.

The pound (0.45359237 kg exactly) and ounce are the common imperial units, and the stone (14 pounds) is still used for body weight in the UK and Ireland. A handy anchor: one litre of water weighs almost exactly one kilogram.

Handy Weight reference points

1 kg ≈ 2.205 lbKilogram to pound
1 lb = 16 ozPound to ounces
1 st = 14 lbStone to pounds
≈ 1 kgWeight of one litre of water
≈ 3.3 kgAverage newborn baby

Supported Weight units

Milligrams (mg) — One thousandth of a gram, used for medication doses.
Grams (g) — One thousandth of a kilogram.
Kilograms (kg) — The SI base unit of mass.
Metric tons (t) — The metric tonne, 1,000 kilograms.
Ounces (oz) — One sixteenth of a pound (about 28.35 g).
Pounds (lb) — The pound, exactly 0.45359237 kg.
Stones (st) — The stone, 14 pounds, used for body weight in the UK.

This tool is provided for general information only. Verify important figures independently. · Last reviewed: August 25, 2026