A kilogram (kg) is the base unit of mass in the International System of Units (SI). For everyday purposes, the kilogram is also used as a unit of weight, even though it is actually a measurement of mass; for the purposes of this site, we will use "weight" as its common definition meaning mass . As an SI base unit, it is widely used all over the world.

As a unit of weight, the kilogram is one of the most common measurements, alongside other units such as the gram (g) and metric ton. As a comparison, pound (lb), ounce (oz), and ton (t) are units of weight in the US customary and imperial systems of measurement.

Below are some equivalences between kilograms and other measurements of mass.

1 kg = 1,000 g

1 metric ton = 1,000 kg

1 kg = 2.205 lb

1 kg = 35.274 oz

907.184 kg = 1 ton

Kilograms and SI prefixes

SI Prefixes denote multiples or fractions of a unit of measurement. SI uses a base 10 system, so all prefixes denote fractions or multiples of powers of 10. For example, a milligram is of a gram (or 10-3) and a kilogram is 103 grams.

1 kilogram = 1000 grams

Based on this, we can see that SI prefixes are actually used to modify the unit of grams rather than that of kilograms. This is because in SI, it is not possible for a unit of measurement to have more than one prefix. For example, it is not possible to have a "kilomilligram." Thus, even though the kilogram was chosen as the base unit in SI, it is the gram that SI prefixes are applied to.

The kilogram is unique in this respect as all of the other base units in SI do not have prefixes. SI prefixes work the same way for all SI units except that in the case of the kilogram, we need to remember that it is the gram that the prefix is applied to.

Using the meter as an example, for every other base unit in SI, of the unit can be indicated using the prefix "milli." This is not the case with the kilogram, because again, we cannot have a "millikilogram." of a kilogram is actually simply a gram.