Is there any way I can work out what size skate I may need in different makes of boots?

and how do you work out the millimetre measurement of a boot as I have noticed some like the Risports are coming up with millimetre measurements and I was wondering how you measure the skate to get the millimetre measurement.

Typically if you ask around about the fit of the boot in comparison with shoe size you'll know. Like my Graf's are one size smaller then my shoes, and that's a pretty standard way to buy them. Other skates I really wouldn't know, sorry!