Length and Perimeter — South Australian Year 3 Maths
Measuring in centimetres and metres, converting between them, the perimeter of a garden bed, then comparing and ordering lengths. Two subtopics and 40 practice questions, aligned to AC9M3M02.
Length and Perimeter in South Australian Year 3
Length is how long, tall or wide something is. In this topic students choose between centimetres and metres, measure with a ruler from the zero mark, change between the two units and add up the sides of a shape to find the distance all the way around, its perimeter. They then compare and order lengths written in different ways.
The two subtopics below are Measure length in centimetres and metres and Compare and order lengths. Length and Perimeter is one of the 18 topics in South Australian Year 3 Maths and belongs to the Measurement strand. Both subtopics come under Australian Curriculum code AC9M3M02. These skills lead on to reading scales between the labelled marks and to the perimeter of more shapes in later years.
Subtopics
Length and Perimeter: common questions
How do you measure length with a ruler correctly?
Line up one end of the object with the 0 mark, not the edge of the ruler, then read the number at the other end. If a crayon starts at 0 and ends at the 9, it is 9 cm long. Starting at the 1 by mistake makes every answer 1 cm too long.
How do you change metres to centimetres?
Each metre is 100 centimetres, so multiply the metres by 100: \(5\text{ m} = 500\text{ cm}\). For a mixed length, change the metres and add the rest: 8 m 15 cm is \(800 + 15 = 815\) cm. To go back, split off the hundreds: 410 cm is 4 m 10 cm.
What is perimeter?
Perimeter is the distance all the way around a shape. Add every side: a rectangle 5 m long and 3 m wide has a perimeter of \(5 + 3 + 5 + 3 = 16\) m. A common mistake is adding only the two sides you can see labelled.
How can children practise length at home?
Give your child a tape measure and a list: the bed, a door, a spoon, a shoe. First guess whether each is better measured in centimetres or metres, then measure and order the results from shortest to longest. Comparing 1 m 20 cm with 115 cm is a good test: change both to centimetres first.
Which curriculum codes cover Length and Perimeter in South Australian Year 3?
Both subtopics come under Australian Curriculum v9 code AC9M3M02, which covers measuring and comparing in metric units such as centimetres and metres, read from rulers and tape measures with numbered marks.