Mass and weight


The SI unit of mass is the kilogram, symbol kg.

The kilogram was originally defined as being equal to the mass of one cubic decimetre of water.

The modern definition of the kilogram is more precise. However, for most practical purposes, one litre of water can be regarded as having a mass of one kilogram.

The tonne, symbol t, is the special name for the megagram, symbol Mg. It is equal to 1000 kilograms. One cubic metre of water has a mass of one tonne.

1000 milligrams = 1 gram
1000 grams = 1 kilogram
1000 kilograms = 1 tonne

For historical reasons, the name of the base unit of mass includes the prefix kilo. The multiples and submultiples of the kilogram are formed by attaching prefix names to the unit name “gram”, and prefix symbols to the unit symbol “g”. Thus 10-6 kg is written as milligram, symbol mg, and not as microkilogram, µkg.

Orders of magnitude

 1000 μg = 1 mg = 0.000 001 kg = 10-6 kg 
 1000 mg = 1 g = 0.001 kg = 10-3 kg 
 1000 g = 1 kg = 1 kg = 100 kg 
 1000 kg = 1 Mg = 1000 kg = 103 kg 
 1000 Mg = 1 Gg = 1 000 000 kg = 106 kg 


When the word “weight” is used, the intended meaning should be clear. In science and technology, weight is a force, for which the SI unit is the newton. However, in commerce and everyday use, weight is usually a synonym for mass, for which the SI unit is the kilogram.