Measure length in millimetres
Theory
Measuring length in millimetres is a Year 3 measurement skill: reading a ruler to the nearest millimetre, using 10 mm = 1 cm, and changing between millimetres and centimetres. This page explains the small ticks on a ruler, how to convert by multiplying or dividing by 10, and how to compare, add and subtract lengths. In the Australian Curriculum and the Victorian Curriculum this is Year 4 content; it is taught in Year 3 in Western Australia. Curriculum: VC2M4M01 / AC9M4M01.
A millimetre (mm) is a small unit of length. On a ruler it is one of the tiny ticks between the numbered centimetres. A centimetre (cm) is the wider space between two numbers.
Every centimetre is split into 10 equal millimetres, so \(10\text{ mm} = 1\text{ cm}\). To change centimetres into millimetres you multiply by 10, and to change millimetres into centimetres you divide by 10.
Use millimetres for tiny things or when you need an exact length, such as a bead, an ant, a screw or a stamp. Use centimetres for things like a pencil or a book, and metres for long things like a room.
Millimetres let you measure small things more exactly. A length like \(46\text{ mm}\) can also be written as \(4\text{ cm }6\text{ mm}\) — the same length, just split into whole centimetres and the millimetres left over.
Measuring in millimetres does not need a formula, but these length facts help you convert and compare.
How to measure a length in millimetres
- Line up the zero. Put the left end of the object exactly on the \(0\) mark of the ruler.
- Read the whole centimetres. Find the last numbered mark the object passes, and change it to millimetres by multiplying by \(10\).
- Add the extra millimetres. Count the small ticks past that number and add them on to get the total in millimetres.
How to work with lengths in mm and cm
- Make the units match. Change every length to millimetres (\(\text{cm} \times 10\)) before you compare, add or subtract.
- Choose the operation. Joined end to end: add. How much shorter or longer, or how many more to reach a length: subtract. Several equal lengths in a row: multiply.
- Write the answer. Give the unit. If you need centimetres and millimetres, split the tens off: \(54\text{ mm} = 5\text{ cm }4\text{ mm}\).
| \(4\text{ cm}\) | \(=\) | \(40\text{ mm}\) |
| \(40 + 6\) | \(=\) | \(46\) |
The leaf is \(46\text{ mm}\) long.
| \(9 \times 10\) | \(=\) | \(90\) |
| \(9\text{ cm}\) | \(=\) | \(90\text{ mm}\) |
Multiply the centimetres by \(10\).
| \(40\text{ mm}\) | \(=\) | \(4\text{ cm}\) |
| \(43\text{ mm}\) | \(=\) | \(4\text{ cm }3\text{ mm}\) |
Every \(10\text{ mm}\) is \(1\text{ cm}\), with \(3\text{ mm}\) left over.
| \(9\text{ cm}\) | \(=\) | \(90\text{ mm}\) |
| \(90\) | \(>\) | \(86\) |
Change both to millimetres first, so \(9\text{ cm}\) is longer.
| \(7\text{ cm}\) | \(=\) | \(70\text{ mm}\) |
| \(70 + 4\) | \(=\) | \(74\) |
| \(7\text{ cm }4\text{ mm}\) | \(=\) | \(74\text{ mm}\) |
Change the centimetres to millimetres, then add the extra millimetres.
| \(5\text{ cm }6\text{ mm}\) | \(=\) | \(56\text{ mm}\) |
| \(2\text{ cm}\) | \(=\) | \(20\text{ mm}\) |
| \(56 + 20\) | \(=\) | \(76\text{ mm}\) |
Write both lengths in millimetres, then add. The total is \(76\text{ mm}\).
| \(6\text{ cm}\) | \(=\) | \(60\text{ mm}\) |
| \(60 - 52\) | \(=\) | \(8\) |
| \(\text{shorter by}\) | \(=\) | \(8\text{ mm}\) |
Use the same unit for both, then subtract the smaller from the larger. It is \(8\text{ mm}\) shorter.
| \(10\text{ cm}\) | \(=\) | \(100\text{ mm}\) |
| \(100 - 87\) | \(=\) | \(13\) |
| \(\text{more}\) | \(=\) | \(13\text{ mm}\) |
\(10\text{ cm}\) is \(100\text{ mm}\). The crayon needs \(13\text{ mm}\) more.
| \(4 \times 35\) | \(=\) | \(140\text{ mm}\) |
| \(140 \div 10\) | \(=\) | \(14\) |
| \(140\text{ mm}\) | \(=\) | \(14\text{ cm}\) |
Multiply one width by the number of tiles. The row is \(140\text{ mm}\), which is \(14\text{ cm}\).
| \(7\text{ cm }1\text{ mm}\) | \(=\) | \(71\text{ mm}\) |
| \(7\text{ cm}\) | \(=\) | \(70\text{ mm}\) |
| \(68 < 70\) | \(<\) | \(71\) |
In millimetres they are \(71\), \(68\) and \(70\). Shortest to longest: \(68\text{ mm}\), \(7\text{ cm}\), \(7\text{ cm }1\text{ mm}\).
Common pitfalls
Frequently asked questions
How many millimetres are in a centimetre?
There are \(10\) millimetres in \(1\) centimetre. On a ruler these are the 10 small ticks between two numbered centimetre marks.
How do you change centimetres into millimetres?
Multiply the number of centimetres by \(10\). For example, \(11\text{ cm} = 110\text{ mm}\).
How do you read a length in millimetres on a ruler?
Line the left end of the object up with the \(0\) mark. Read the last whole centimetre, change it to millimetres, then count the extra small ticks and add them on.
Why measure in millimetres instead of centimetres?
A millimetre is a smaller unit, so it can measure small things more exactly. Whole centimetres would round \(37\text{ mm}\) to \(4\text{ cm}\), which is \(3\text{ mm}\) too long.
What is bigger, a millimetre or a centimetre?
A centimetre is bigger. One centimetre is the same as \(10\) millimetres, so it takes 10 millimetres to make a single centimetre.
How do you write 83 mm in centimetres and millimetres?
Every \(10\text{ mm}\) is \(1\text{ cm}\). \(83\text{ mm}\) is 8 tens and 3 ones, so it is \(8\text{ cm }3\text{ mm}\).