Measure & compare length (metric units)
Theory
Length is measured in millimetres (mm), centimetres (cm), metres (m) and kilometres (km). This Year 5 maths page shows how to choose a sensible unit, read a ruler to the nearest millimetre (even when the object does not start at \(0\)), change between units, and compare and order lengths.
Choosing a unit: millimetres for very small lengths (the thickness of a phone), centimetres for things you can hold (a book), metres for rooms, buildings and sports fields, kilometres for distances between places.
\(1\text{ cm} = 10\text{ mm}\), \(1\text{ m} = 100\text{ cm}\) and \(1\text{ km} = 1000\text{ m}\).
On a ruler, the numbered marks are centimetres and each small mark is a millimetre. Halfway between two centimetre marks is \(5\) mm, or half a centimetre.
If an object does not start at \(0\), the length is end reading minus start reading.
To compare lengths, write them in the same unit first, then compare the numbers.
Metric units of length:
How to measure with a ruler
- Place one end of the object on \(0\) (or note the start reading).
- Read the whole centimetres at the other end, then count the small millimetre marks past it.
- If it did not start at 0, subtract the start reading.
- Write the unit.
(i) The width of a library book
(ii) The thickness of a phone
(iii) The length of a football field
(i) Centimetres
(ii) Millimetres
(iii) Metres
| \(7.5 - 2\) | \(=\) | \(5.5\text{ cm}\) |
\(5.5\) cm (or \(55\) mm).
| \(4 \times 10 + 3\) | \(=\) | \(43\text{ mm}\) |
Key: \(43\) mm. Pen: \(86\) mm is \(8.6\) cm, which is \(9\) cm to the nearest centimetre.
| \(37\) | \(<\) | \(70 < 73 < 107\) |
\(37\) cm, \(70\) cm, \(73\) cm, \(107\) cm.
| \(312 - 178\) | \(=\) | \(134\text{ cm}\) |
\(134\) cm longer.
(i) Was his estimate too long or too short?
(ii) By how much?
| \(150 - 136\) | \(=\) | \(14\text{ cm}\) |
(i) Too long
(ii) \(14\) cm
| \(1\text{ m}\) | \(=\) | \(100\text{ cm}\) |
\(100\text{ cm} > 95\text{ cm}\), so \(1\) m is longer.
| \(10 - 4\) | \(=\) | \(6\text{ cm}\) |
He read the end without subtracting the start. The pencil is \(6\) cm long.
Common pitfalls
Frequently asked questions
Which unit should I use?
mm for tiny lengths, cm for small objects, m for rooms and fields, km for distances between places.
How many millimetres are in a centimetre?
\(10\). So \(4\) cm \(3\) mm is \(43\) mm.
How do you measure if the object does not start at 0?
Subtract the start reading from the end reading: \(7.5 - 2 = 5.5\) cm.
How do you compare lengths in different units?
Change them to the same unit first: \(1\) m \(= 100\) cm.
How do you round a length to the nearest centimetre?
Look at the millimetres. \(8.6\) cm has \(6\) mm, which is more than half a centimetre, so it rounds up to \(9\) cm.