I need to buy a smaller stem but I'm a bit confused on the stem clamp sizing. I have an 04 Allez. The steerer tube is 1 1/8" so I know I need a 28.6mm clamp on that end but I'm a little uncertain of what size clamp I need on the handlebar end.

I just have the generic Specialized comp handlebars that came on the bike but I don't know if they are 26 or 31.8mm. I measured around the handlebar clamp area and I got about 8.5cm so I know that can't be right.

How do I find out what size I need?