Compare & order lengths
This Stage 2 subtopic covers outcome MA2-GM-02 (measures and estimates lengths in metres, centimetres and millimetres). Students decide which length is longer or shorter and put lengths in order, changing metres and millimetres into centimetres first so that every length uses the same unit.
Students also use >, < and =, spot lengths that are written differently but are equal, and subtract to find how much longer one length is, using heights, ropes, ribbons and long-jump results.
Theory
Comparing and ordering lengths is a Year 3 measurement skill: deciding which length is longer or shorter and putting lengths in order. The trick is to convert to the same unit first — millimetres, centimetres or metres — then compare the numbers, use greater than and less than, and work out how much longer one length is.
Length is measured in millimetres (mm), centimetres (cm), metres (m) and kilometres (km). To compare two lengths means to decide which is longer, which is shorter, or if they are equal.
You can only compare lengths that are in the same unit. If two lengths use different units, you must convert one of them first. For example, \(1\) m is the same as \(100\) cm, so to compare \(1\) m with \(85\) cm you change \(1\) m into \(100\) cm and see that \(100 > 85\).
To order lengths means to line them up from shortest to longest, or from longest to shortest. The difference between two lengths is how much longer one is than the other, which you find by subtracting.
Comparing lengths has no formula, but these conversions let you put every length in the same unit before you compare.
How to compare or order lengths
- Match the units. Change every length to the same unit, usually the smallest one in the list, such as centimetres or millimetres.
- Compare the numbers. With the same unit, the larger number is the longer length. Use \(>\), \(<\) or \(=\).
- Order or subtract. To order, write them smallest to largest (or largest to smallest if the question asks for longest first). To find the difference, take the smaller length from the longer one.
- Answer in the units asked. Change back if needed: \(170\) cm \(=1\) m \(70\) cm. If something is higher or longer by an amount, add that amount on.
| \(\text{same unit (cm)}\) | ||
| \(63\) | \(>\) | \(58 > 39\) |
The longest length is \(63\) cm.
| \(1\text{ m }45\text{ cm}\) | \(=\) | \(145\text{ cm}\) |
| \(160\) | \(>\) | \(145\) |
So \(160\) cm is longer than \(1\) m \(45\) cm.
| \(8\text{ cm}\) | \(=\) | \(80\text{ mm}\) |
| \(5\text{ cm}\) | \(=\) | \(50\text{ mm}\) |
| \(50\) | \(<\) | \(62 < 80\) |
In order: \(5\) cm, \(62\) mm, \(8\) cm.
| \(2\text{ m}\) | \(=\) | \(200\text{ cm}\) |
| \(200 - 165\) | \(=\) | \(35\) |
Rope A is \(35\) cm longer than rope B.
| \(1\text{ m }6\text{ cm}\) | \(=\) | \(106\text{ cm}\) |
| \(1\text{ m }60\text{ cm}\) | \(=\) | \(160\text{ cm}\) |
| \(106\) | \(<\) | \(160\) |
\(160\) cm and \(1\) m \(60\) cm are the same length. The shortest is \(1\) m \(6\) cm, which is only \(106\) cm.
| \(2\text{ m }5\text{ cm}\) | \(=\) | \(205\text{ cm}\) |
| \(2\text{ m}\) | \(=\) | \(200\text{ cm}\) |
| \(215 > 205 > 200\) | \(>\) | \(192\) |
| \(215 - 192\) | \(=\) | \(23\text{ cm}\) |
Longest to shortest: \(215\) cm, \(2\) m \(5\) cm, \(2\) m, \(192\) cm. The longest is \(23\) cm longer than the shortest.
| \(1\text{ m }35\text{ cm}\) | \(=\) | \(135\text{ cm}\) |
| \(135 + 25\) | \(=\) | \(160\text{ cm}\) |
| \(160\text{ cm}\) | \(=\) | \(1\text{ m }60\text{ cm}\) |
Going higher means adding. The top of the box is \(1\) m \(60\) cm from the floor.
| \(2\text{ m }30\text{ cm}\) | \(=\) | \(230\text{ cm}\) |
| \(230 - 185\) | \(=\) | \(45\) |
| \(\text{longer by}\) | \(=\) | \(45\text{ cm}\) |
Write both in centimetres, then subtract. Tom's rope is \(45\) cm longer.
Common pitfalls
Frequently asked questions
How do you compare two lengths in different units?
Change both lengths to the same unit first, then compare the numbers. For example, \(3\) m \(=300\) cm, so \(3\) m is longer than \(270\) cm.
How do you put lengths in order from shortest to longest?
Put every length in the same unit, then write the smallest number first and the largest number last. That gives the order from shortest to longest.
How many centimetres are in a metre?
There are \(100\) cm in \(1\) m. So \(2\) m \(=200\) cm, and \(1\) m \(45\) cm \(=145\) cm.
Is 2 m longer than 190 cm?
Yes. \(2\) m \(=200\) cm, and \(200 > 190\), so \(2\) m is longer than \(190\) cm.
How do you find how much longer one length is than another?
Put both lengths in the same unit, then subtract the smaller length from the longer length. The answer is how much longer one is.
How many millimetres are in a centimetre?
There are \(10\) mm in \(1\) cm. So \(5\) cm \(=50\) mm, and \(7\) cm \(3\) mm \(=73\) mm.