Under The Palms

I was wondering how Vans shoe sizes compare?

For instance I normally wear an 8.5 in GR Sk8 Hi / Reissue and would like to get some (GR or otherwise) slip ons.
Should I go smaller or bigger or stay the same?
I can only order them online so can't actually try them on.

I wear the same size in Sk8-His as I do for Slip-on, but then again they don't make a 12.5 so I don't have too many options.. When comparing sizes, I usually pay attention to materials, ie. leather toe boxes tend to fit tighter than canvas or suede toe boxes.

Thanks for the info,so you would go slightly bigger in slip-ons than Sk8 Hi's if possible?