Converting offset to backspacing for wheels?

I'm looking at these wheels:



They don't show backspacing only offset. The 15x7 is -9 offset and the 15x8 is -19 offset. What's the backspacing on each or how do I find out? Also which of these would give the best fitment if these two are my only options?

off set will be from the center of the rim. are your numbers for offset mm?

Sponge Bob said:

are your numbers for offset in mm?

Click to expand...

Yes.