I’m looking at buying a new stem online while at work today, my bike is at home but I don’t know the correct diameter to buy (I know 31.8, but I don’t know if to get 1.8″ or 1.5″, or different again without checking.

I have a Ghost ASX 5500 (2011), spec list on below link. Does anyone know which one I am most likely to have please?

http://www.idealo.co.uk/compare/2429758/ghost-asx-5500-2011.html

It says tapered headset so I’d go with 1 1/8″. Only Giant use an odd upper steerer size, iirc.