odvelasc wrote on 01/28/09 at 10:49:35:by a cheap caliper and measure it. Or simple math. Measure the diameter and multiply by Pi (3.14). Tada
C=dxpi
err, you left out a couple of... important words, like circumference.
So if you use a tape rule to measure the circumference...
divide by pi and you'll have the diameter of the pipe.