area of a cross section

[ktpr2]

I'm going through my intro to mechanical engineering material and there was one thing I was never clear on:

If you have a cross section of a rod, why isn't its' area at a cross section pi * r^2 as a opposed to piD^2/4, where D is the diameter?

[dextercioby]

Becasue people are actually measuring the diameter...?

Daniel.

[ktpr2]

whoops. That makes sense :D