If we compare lengths of things that use the same unit of measurement, we only need to compare the numbers, to see which is shorter or longer. So far, we've looked at comparing lengths using centimetres (cm) and metres (m).

Smaller lengths

We can use the same process with smaller lengths. So when we need to see which weed is taller, we can compare measurements in millimetres (mm) and centimetres (cm) by converting one of our measurements.

Worked Example

Question 1

A ladybeetle is $4$4 mm long and a bull ant is $7$7 mm long.

Mark these two lengths on the number line.
Which insect is $longer$ ?
$Ladybeetle$
A
$Bull Ant$
B

Converting units

What happens if we need to compare with different units of measurement? This is where we need an extra step to convert units.

Click on the image to make it larger

Let's work through an example where we need to convert one of our measurements. You may also need to remember how to multiply and divide numbers by powers of 10.

Worked Examples

Question 2

Let's look at the three lengths $5.2$5.2 cm, $54$54 mm, and $4.5$4.5 cm and think about how we can order them from largest to smallest.

Before we can order them, it will be easier to convert them all to have the same unit.

Convert $54$54 mm into centimetres.
Choose the correct option that shows the three lengths in order from largest to smallest.
$54$54 mm, $5.2$5.2 cm, $4.5$4.5 cm
A
$54$54 mm, $4.5$4.5 cm, $5.2$5.2 cm
B
$4.5$4.5 cm, $54$54 mm, $5.2$5.2 cm
C
$4.5$4.5 cm, $5.2$5.2 cm, $54$54 mm
D

Question 3

Let's look at the three lengths $9.78$9.78 km, $8450$8450 m, and $9980$9980 m and think about how we can order them from smallest to largest.

Before we can order them, it will be easier to convert them all to have the same unit.

Convert $9.78$9.78 km into metres.
Choose the correct option that shows the three lengths in order from smallest to largest.
$9980$9980 m, $9.78$9.78 km, $8450$8450 m
A
$9.78$9.78 km, $8450$8450 m, $9980$9980 m
B
$8450$8450 m, $9980$9980 m, $9.78$9.78 km
C
$8450$8450 m, $9.78$9.78 km, $9980$9980 m
D

Remember!

The first step to comparing lengths is to make sure we are comparing the same unit of measurement. If they are not the same, then we need to convert one (or more) of them first.

Order and compare objects by length, including conversion

Comparing lengths