Measuring an Accurate Circle

When the Solar Bulb group of students wanted to mount a coke bottle into a rigid piece of cardboard, they came up against a problem that they tried hard to solve by themselves. How can you make the hole the right size if you have a bottle and you cannot measure the it’s diameter. The idea they came up with to solve the problem was after much discussion…but was it accurate!


It looks simple to embed a coke bottle into a sheet of plastic ... but how do you measure the circle accurately?

Does the diameter equal half of the circumference?

How can you work it out? Is there a Mathematical formula that can help you?

Below are the calculations that Grade 5 came up with.

What we wanted to find out was - Does a = 2b? a is the circumference and b is the diameter.