Circumference of a circle divided by pi is equal to the circle's diameter.
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pi = circumference/diameter
The diameter of a circle is twice its radius and spans the circle passing through its centre.
Use a caliper that's made for inside measurement.
pi was not made - it is not a human artefact. The ratio of the circumference of a circle to its diameter exists independent of human existence.
It depends on the thickness of the material that the pipe is made from.