The International System of Units

Why a common system?

If every country or person used their own units, scientists would be unable to understand one another or compare their measurements. That is why the whole world (with very few exceptions) uses the International System of Units, known as SI.

The basic units to know at secondary school

Quantity	SI unit	Symbol
Length	metre	m
Mass	kilogram	kg
Time	second	s
Temperature	kelvin (or degree Celsius)	K (or deg C)

Some derived units are based on these base units, such as speed in m/s (metres per second) or area in m² (square metres).

Multiplicative prefixes

To express very large or very small values, prefixes are placed before the unit:

Prefix	Symbol	Factor
kilo	k	× 1000
hecto	h	× 100
deka	da	× 10
(unit)	-	× 1
deci	d	/ 10
centi	c	/ 100
milli	m	/ 1000

Example: 1 km = 1000 m, and 1 cm = 0.01 m.

Common pitfalls

• Confusing the gram (g) and the kilogram (kg): 1 kg = 1000 g – a common mistake in calculations.
• Using non-SI units without converting them (miles, inches, pounds): always convert to SI units before carrying out a calculation.
• Forgetting that a unit symbol never takes an ‘s’ in the plural: we write ‘5 kg’, never ‘5 kgs’.