How does one know exact pixel size / relationships between CSS elements?

Question

I'm diving into CSS in an effort to be more well-rounded. I'm currently reading "Designing with Web Standards" which is great so far.

My question is, when you get a mockup in PSDs, how do you know the exact pixel dimensions of backgrounds, images, various blocks/divs, font-sizes, etc?

I'll most likely be using Photoshop to work with files.

Answer 1

Photoshop has a grid tool and rulers. You can measure it there. It's pretty accurate. take a look at this img.

these are inches by the way, you can change it to pixels.