Now that we know how to work out diameter and measure the curved arches of our rainbows, we were able to work out the perimeter.


Some of our rainbows:


First we needed to finish off measuring the curved arches of our rainbows. They remembered how to use the string to measure the arch.

It was quite fiddly to hold the string in place, so Ioan and Finn decided to help each other.

They remembered to record their units as centimetres (cm).

There was good teamwork measuring Cian’s water colour rainbow.

The largest rainbow was Finn’s footprint rainbow. The arch was even longer than out tape measure, so they had to put a ruler next to the end of the tape. The tape measure was 160cm and there was 9cm more string measured against the ruler. They worked out that the total length was 169cm.

Measuring perimeter

The perimeter of any 2-D shape is the total length of all its sides. I asked Ioan to explain to Finn what perimeter is:

Next Ioan worked out that he needed to add the red arch to the diameter to find the perimeter. Since a circle has no sides, its perimeter (also known as its circumference) is the length of the circle’s exterior. Our rainbows are semicircles so we decided to call the perimeter, our ‘circumference’.

Ioan added a diameter of 23cm to the red arch of 50cm. He used the base ten to get the answer of 76cm.

Ioan looked at the diameter of 20cm and length of the red arch of 35cm and worked out that the circumference would be 55cm, but I asked him to check his answer using the base ten.

Finn added 28cm + 52cm. He started with the units and knew that 8+2=10, so swapped his 10 yellow units for one green ten rod. He then added 20+50+10 to get 80.

They used addition to work out the circumference of the remaining rainbows.

DfES Outcomes for EYFS and National Curriculum (2013)

Numeracy Year 1 programme of study

Number – measurements

  • compare, describe and solve practical problems for lengths and heights
  • measure and begin to record lengths and heights

Pupils move from using and comparing different types of quantities and measures using non-standard units, including discrete (for example, counting) measurement, to using manageable common standard units. In order to become familiar with standard measures, pupils begin to use measuring tools such as a ruler.

Numeracy Year 3 programme of study

Number – measurements

  • measure, compare, add and subtract: lengths (m/cm/mm)
  • measure the perimeter of simple 2-D shapes