Helmet sizes

[leman]

I have an Arai helmet size 56-57 would it be a fair assumption that the BMW system 5 size 56-57 will be nearly the same or do i really need to try one on ?
I know all helmets are different but i was hoping someone on the site may have had an Arai then converted to a systen 5 & had the same size for each helmet

 

[Wanderer]

I have an Arai and a System 5. I find the fitting to be slightly different (60/61 Arai yet 62/63 System 5 - maintly due to forehead fitment) but a pal of mine has the same size in both. Better try before you buy, just to be on the safe side.

 

[leman]

Thanks for that will