# Area of a cross section

1. Apr 17, 2005

### 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?

2. Apr 17, 2005

### dextercioby

Becasue people are actually measuring the diameter...?

Daniel.

3. Apr 17, 2005

### ktpr2

whoops. That makes sense :D