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If you were to draw a circle of diameter 1, the length of its circumference (if you were able to measure it exactly!) would be equal to the number above. If you were to draw a circle of diameter 2, the circumference would double and be twice that number. We call that number ‘pi’, or π.

No matter how big or small you make a circle, the diameter and circumference will always be in the same ratio.

pi diagrams

This means that if you have the diameter or the circumference of a circle, you can always work out the other using the equation below. All you need is an accurate estimate of π!


Try it for yourself!

  1. Find a circular plate.
  2. Measure around the outside edge with a tape measure.
  3. Measure the diameter of the plate.
  4. Divide the first measurement by the second. This gives you an estimate of π.
  5. Tell us what you got!

pi circpi diameter

Want to know what a really accurate estimate of π looks like? Here’s one million digits of π!

π is an irrational number, meaning that it never ends and it never starts repeating itself over, so we can only ever have an estimate for π – no one can ever known all of the digits of pi! Suresh Kumar Sharma from India currently holds the world record for number of digits of π memorised, at 70030 digits, a feat which took him 17 hours and 14 minutes. You can check out the rest of the leader-board here.

Why is march 14th 'pi day'?

Click to reveal answer

March is the third month of the year, so in a sense March 14th is 3.14, which are the first three digits of π! Hence, 'Pi Day'!

People have been aware of π as far back as ancient Babylonia in 1900 BCE! The ancient Egyptians estimated π to be 3.1605, which was a pretty good effort for the times… However the use of the Greek letter π to denote the number was not introduced until the 1700s. Below you can see a section of the A’h-mosè or Rhind Mathematical Papyrus, from c. 1550 BCE, which estimates π. Looks a little bit like your school exercise book doesn’t it?

Rhind Papyrus

π‘s been around so long, that people have got rather imaginative with it over the years! It’s even been the basis for a crop circle – the photo below was taken near Banbury Castle in 2008. If you divide the crop circle into tenths, the number of tenths each successive step covers will give you the digits of pi! In case you were wondering, the three blobs at the end are the ellipsis, like so…

Crop circleCrop Circle diagram

Interesting fact!

Because the digits of π never end, or start repeating themselves (like 3.1415141514151415… does), then every number in existence must be found within the digits of π! You can search for your birthday (or any other number) in the first 2 billion digits of π here!

Don't forget...

Tell us your best estimate of pi in the comments section below. All you need is a plate, a tape measure, and our instructions above!