For an accurate diameter measurement:

1. 1.Lay the pipe flat on a level surface.
2. 2.Place a coin (ideally 1-€ or 2-€) or a credit/debit card flat right next to it – on the same plane as the pipe, not on top.
3. 3.Shoot straight from above, with the reference object and pipe opening in sharp focus.

The flatter the reference object and pipe lie in one plane, the more precise. Use a card for large pipes, a coin for small ones. Without a reference we can only roughly estimate the diameter.