Every circle's hidden number
What Is Pi and Why Does It Never End?
Pi is how many diameters fit around a circle, and it is the same for every circle there is.
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Step 1 of 5
Every circle has two natural measurements.
The distance around the outside, and the distance straight across through the centre. Those two are all pi needs.
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Step 2 of 5
Dividing one by the other always gives the same number.
Try it on a mug, a coin or a stadium and the answer never changes. About 3.14 diameters fit around any circle at all.
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Step 3 of 5
So a circle is a bit over three times around.
Three diameters nearly wraps the circle, leaving a small gap. That leftover fourteen hundredths is what makes pi awkward.
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Step 4 of 5
The digits never repeat and never stop.
Pi cannot be written exactly as a fraction, so its decimals run forever without settling into any repeating pattern.
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Step 5 of 5
A few digits are plenty in practice.
Fifteen decimal places would place a circle the size of the solar system to within the width of a finger. Nobody needs more.
NASA uses 15 decimal places of pi, enough to calculate a circle the size of Earth's orbit to within a couple of centimetres.
The short version
Pi is how many diameters fit around a circle, the same for every circle, and its decimals run forever without repeating.
Try it yourself
Wrap string around a mug, then measure across the top. The string is always a bit over three times the width.
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