Since the ratio for units of length in the SI system is ten, the ratio for units of area [[$ \text{m}^2 = \text{m} \cdot \text{m} $]] must be [[$10\cdot10 = 100$]]. The table shows the most common units of area.

Symbol	Name	In basic units
km2	square kilometre	1 000 000 m2
ha	hectare	10 000 m2
a	are	100 m2
m2	square metre	1 m2
dm2	square decimetre	0,01 m2
cm2	square centimetre	0,0001 m2
mm2	square millimetre	0,000001 m2

As with lengths, a smaller unit can be converted into a larger one by dividing its numerical value by the ratio. Conversely, a larger unit can be converted into a smaller unit by multiplying its numerical value by the ratio.

Example 2

Convert:

a) [[$ \text{3,2} $]] square kilometres to hectares. 

[[$\begin{align} \text{3,2 km}^2 = 100 \cdot \text{3,2 ha} = \text{320 ha} && \text{Convert to one unit smaller.} \end{align}$]]

b) [[$ 16200 $]] square centimetres to square metres.

[[$\begin{align} 16200 \text{ cm}^2 = \dfrac{16200}{100 \cdot 100} \text{ m}^2 = \text{1,62 m}^2 && \text{Convert to two units greater.} \end{align}$]]

Like in length unit conversions, we can take advantage of the knowledge that the number notation system we use is decimal and has a base number of [[$10$]]. Since the area unit conversion ratio is [[$100$]], each unit conversion always moves the decimal point by two steps.

Example 3

Convert the areas a) [[$240$]] mm² and b) [[$\text{3,2}$]] ha to square metres.