Measuring pi using a pie

Follow these instructions to measure the constant pi using a pie and then you can eat the leftovers.

You will need

  • a round pie
  • drinking glasses
  • a tape measure
  • a calculator
  • pen and paper.

What to do

  1. Settle in around the pie and ask everyone to measure the circumference (distance around the outside) of the top of the pie with the tape measure - write down the value in millimetres.
  2. Measure the diameter (distance across the top) and write that down in millimetres too.
  3. Now calculate pi. Divide the circumference by the diameter and see what you all get. How close did you get to 3.14? Who was the closest?
  4. Try it with a glass, measuring the circumference and diameter of the top and calculating pi the same way. Did you get the same answer? How close was everyone this time?
  5. Any difference from 3.14 is probably due to measurement error, for example, misreading the tape measure or measuring the circumference and diameter of two different circles like the top and bottom of your pie. Another inaccuracy comes from the fact that even the most pedantic baker is unlikely to have baked a perfect circle.
  6. Now grab your piece of the pie and raise your glasses and say 'Cheers' to pi.

What's happening

There is something amazing about pi - it goes on forever. Pi is approximately equal to 3.14159 but it has an infinite number of decimal places.

No matter how big or how small a circle is, the value of pi is always the same. It is called a 'constant'.


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