I have a pipe 500mm long and it contains 8.836 litres of water
How do I calculate the internal diameter of the pipe

The cross section of the pipe is a circle. The volume in the pipe is the cross sectional area times the length of the pipe.



A = area of a circle, r = radius



V = Volume, A = cross-sectional area (circle), L = length

You're going to have to change liters into cubic meters and you'll have to change millimeters (mm) intometers. When you do that, you'll be able to solve for the radius, r, in meters. The diameter is, of course, twice the radius