## finding the diameter of a tube

How can you find the diameter of a tube without measuring it? Is there a equation or something or any method other than measuring it

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 If you know the cross sectional area, A of the tube, you can use $$A = \pi \left(\frac{D}{2}\right)^2$$ and solve for D. Other than that, no. Edit: if you know the circumference of the circle, C: $C = \pi D$.
 Recognitions: Gold Member Science Advisor Kind of, although strictly speaking, it is 'measuring'. Just put a stick of known length inside the tube, and turn it through an angle (so it goes from one chord to another chord, with one point remaining the same). Measure the angle between the two chords, and then get trigg-ing!