Can anyone tell me how to find the offset of wheels.

I have just been looking at wheels for my Ssat and the dealer told me I had an offset of 47.

What does this allow me to do as far as wheel/tyre size, and when does diameter become an issue. One wheel i looked at, he told me would increase size by 18mm(no good) but another was 10mm(acceptable) hmmm

Please help,

Thanks