Metric Length: mm, cm, m and km

What you will learn

• Name the metric units of length and say what size each one is.
• Choose a sensible unit for what you measure.
• Convert between millimetres, centimetres, metres and kilometres.

Before you start: you should be able to measure with a ruler and multiply or divide by $10$, $100$ and $1000$. See Measuring Length with a Ruler.

The idea

The metric system uses units that are linked by tens. The small units are millimetres (mm) and centimetres (cm). The bigger ones are metres (m) and kilometres (km). The prefixes tell you the size: milli is a thousandth, centi is a hundredth and kilo is a thousand.

Choosing a unit

Pick the unit that gives a sensible number. A pencil is about $15$ cm, not $0.00015$ km. A road trip is measured in kilometres.

Reading a ruler in mm and cm

On a ruler, the small marks are millimetres and the numbered marks are centimetres. A length of $13.5$ cm is $13$ cm and $5$ mm, which is $135$ mm.

Converting

• Going to a smaller unit: multiply (more of them). $2$ m is $200$ cm.
• Going to a bigger unit: divide (fewer of them). $350$ cm is $3.5$ m.

Worked examples

Example 1: Metres to centimetres

Change $3.2$ m to centimetres.

1. $1$ m is $100$ cm.
2. $3.2 \times 100 = 320$.

Answer: $320$ cm

Example 2: Centimetres to metres

Change $750$ cm to metres.

1. $100$ cm make $1$ m, so divide by $100$.
2. $750 \div 100 = 7.5$.

Answer: $7.5$ m

Example 3: Adding lengths in different units

Add $2$ m $30$ cm and $85$ cm.

1. Change both to centimetres: $230$ cm and $85$ cm.
2. $230 + 85 = 315$ cm.
3. As metres and centimetres: $3$ m $15$ cm.

Answer: $315$ cm, or $3$ m $15$ cm.

Tip: always put lengths in the same unit before you add, subtract or compare them.

Why it matters

Building, sewing, sport, maps and journeys all use metric lengths. Knowing the links between the units lets you read a plan, buy the right amount of material and judge whether an answer is sensible.