Measure length in centimetres & metres
Theory
Measuring length in centimetres and metres is a Year 3 measurement skill: choosing the right unit, reading a length from a ruler by lining up the zero mark, and using the fact that 1 metre = 100 centimetres to change between units. This page explains centimetres (cm) and metres (m), how to read a ruler, how to compare and order lengths, and how to add and subtract lengths in everyday problems. Curriculum: VC2M3M02 / AC9M3M02.
Length is how long, tall or wide something is. We measure it with a ruler or a tape measure. Two units are used most in Year 3: the centimetre and the metre.
A centimetre (cm) is small, about the width of your fingernail. We use centimetres for small things like a spoon, a book or a hand. A metre (m) is much longer, about one big step. We use metres for long things like a fence, a hallway or a garden bed.
To measure with a ruler, line up the left end of the object with the zero mark, then read the centimetre mark at the other end. The two units are joined by one key fact: 1 metre is the same as 100 centimetres, so \(1\text{ m} = 100\text{ cm}\).
Measuring length has no long formulas, but these facts help you swap between centimetres and metres.
How to measure and compare a length
- Pick the unit. Use centimetres for small things and metres for long things.
- Line up the zero. Put the left end of the object on the 0 mark, then read the centimetre mark at the other end.
- Match the units. To convert or compare, use \(1\text{ m} = 100\text{ cm}\): multiply metres by 100, or divide centimetres by 100.
- Work it out. Joining lengths end to end: add. Cutting a piece off or finding how much longer: subtract. Equal lengths in a row: multiply. All the way around a shape: add every side.
- Check. Does the answer have a unit (cm or m), and does it make sense next to a benchmark like a ruler (\(30\) cm) or a tall adult (a bit under \(2\) m)?
A book is small, about the length of your hand. Metres are for long things like a room.
Use centimetres (about 25 cm).
| \(\text{start}\) | \(=\) | \(0\text{ mark}\) |
| \(\text{end}\) | \(=\) | \(9\text{ mark}\) |
The twig is \(9\) cm long.
| \(1\text{ m}\) | \(=\) | \(100\text{ cm}\) |
| \(7 \times 100\) | \(=\) | \(700\) |
| \(7\text{ m}\) | \(=\) | \(700\text{ cm}\) |
So \(7\) m is \(700\) cm.
| \(1\text{ m }35\text{ cm}\) | \(=\) | \(100 + 35\) |
| \(1\text{ m }35\text{ cm}\) | \(=\) | \(135\text{ cm}\) |
| \(140\) | \(>\) | \(135\) |
So \(140\) cm is longer.
| \(800 \div 100\) | \(=\) | \(8\) |
| \(800\text{ cm}\) | \(=\) | \(8\text{ m}\) |
| \(460\) | \(=\) | \(400 + 60\) |
| \(460\text{ cm}\) | \(=\) | \(4\text{ m }60\text{ cm}\) |
Every \(100\) cm makes \(1\) m. The hundreds give the metres and what is left is the centimetres.
| \(4\text{ m}\) | \(=\) | \(400\text{ cm}\) |
| \(400 - 170\) | \(=\) | \(230\) |
| \(\text{left}\) | \(=\) | \(230\text{ cm}\) |
Change metres to centimetres first, then subtract. \(230\) cm of tape is left (that is \(2\) m \(30\) cm).
| \(70 + 70 + 70 + 70\) | \(=\) | \(280\) |
| \(4 \times 70\) | \(=\) | \(280\text{ cm}\) |
| \(280\text{ cm}\) | \(=\) | \(2\text{ m }80\text{ cm}\) |
Joining lengths end to end means adding them. The row is \(280\) cm, which is \(2\) m \(80\) cm.
| \(1\text{ m }5\text{ cm}\) | \(=\) | \(105\text{ cm}\) |
| \(1\text{ m }50\text{ cm}\) | \(=\) | \(150\text{ cm}\) |
| \(95 < 105 < 110\) | \(<\) | \(150\) |
Change them all to centimetres, then order: \(95\) cm, \(1\) m \(5\) cm, \(110\) cm, \(1\) m \(50\) cm.
| \(5 + 5\) | \(=\) | \(10\text{ m}\) |
| \(2 + 2\) | \(=\) | \(4\text{ m}\) |
| \(10 + 4\) | \(=\) | \(14\text{ m}\) |
A rectangle has two long sides and two short sides. Add all four: it is \(14\) m all the way around.
Common pitfalls
Frequently asked questions
How do you find how much longer one thing is?
Write both lengths in the same unit, then subtract the shorter from the longer. For a rope \(1\) m \(15\) cm long and a rope \(85\) cm long: \(115 - 85 = 30\), so the first rope is \(30\) cm longer.
How do you measure length with a ruler?
Line up the left end of the object with the zero mark on the ruler. Then read the centimetre number at the other end of the object. That number is the length in centimetres.
How many centimetres are in a metre?
There are 100 centimetres in 1 metre, so \(1\text{ m} = 100\text{ cm}\).
How do you change metres to centimetres?
Multiply the number of metres by 100. For example, \(5\text{ m} = 5 \times 100 = 500\text{ cm}\).
When should you use metres instead of centimetres?
Use metres for long things like a hallway, a fence or a garden bed. Use centimetres for small things like a spoon, a book or a hand.
How do you compare two lengths in different units?
Change both lengths to the same unit first, usually centimetres. For example, \(1\) m \(35\) cm is \(135\) cm, so \(140\) cm is longer.
What is 340 cm in metres and centimetres?
Split off the hundreds. \(340 = 300 + 40\), and \(300\) cm is \(3\) m, so \(340\) cm is \(3\) m \(40\) cm.