Definition
Pi
Divide any circle’s distance around by its distance across the centre; the result is pi, π.
What it means
Measure around any circle, then divide by the distance straight across through its centre. The answer is always pi. Using 3.14 gives a useful approximation, but keeping π in an expression keeps the value exact.
Remember it
Around the circle ÷ straight across the circle = π.
Example
A circle has diameter 10 cm. Its circumference is exactly 10π cm, which is approximately 31.42 cm.
Watch for
Pi is not exactly 3.14; 3.14 is a rounded approximation of π.
Common mix-up
Do not multiply a diameter by 3.14 and call the result exact. Write diameter × π for an exact circumference.
Exact meaning
Pi, written π, is the same constant for every circle: circumference divided by diameter. It is an irrational number with decimal digits that never end or repeat in a fixed pattern; π is approximately 3.14159.