Standard units

Handspans give different answers. Long ago, different parts of the world also had their own systems of units. When people began to travel and trade, this caused a lot of confusion. So countries agreed on one set of standard units: the International System of Units, or SI units.

The SI unit of length is the metre. Its symbol is m. A metre is divided into 100 equal parts; each part is a centimetre (cm).

Look at the 15-cm scale in your geometry box. Between two big marks, such as 1 and 2, the length is 1 cm. Each centimetre is divided into 10 smaller parts; one part is a millimetre (mm). 1 mm is the smallest length this scale can measure, and 1 mm = 0.1 cm.

For long distances we use the kilometre (km), which is 1000 metres. Remember:

• 1 km = 1000 m
• 1 m = 100 cm
• 1 cm = 10 mm

Two worked examples:

• 50,000 m into km: divide by 1000, so 50,000 m = 50 km. And 1.5 km = 1.5 × 1000 = 1500 m.
• 2 m into mm: 2 m = 200 cm, and 200 cm = 200 × 10 = 2000 mm.

Watch out: "1 km = 100 cm" is wrong. 1 km is 1000 m, and every one of those metres is 100 cm.

Some scales also show inches: 1 inch = 2.54 cm. The inch and the foot were used in earlier days, and some people still use them. A handspan is not a standard unit, because it changes from person to person.