Originally Posted by Sprocket Man
There are 3 different sizes you need to know for a stem - 1. The length (usually from 8cm to 13cm) 2. The steering tube (either 1" or 1 1/8"), and 3. The handlebar clamp diameter (25.9mm or 26mm is standard road bike, while 31.8mm is oversized road bike) This is assuming that you have a road bike and a threadless stem.
There's more to diameter than diameter, though. A "one inch" quill stem is actually 22.2mm in diameter (an inch is 25.4). I can't remember what a 1-1/8 measures (I don't have any), but it isn't 1.125 inch. And there are still a few 1-1/4 inchers out there, one of them on my Cannondale mountain bike. If you can't tell by looking (no reason you should be able to, until you've looked at a bunch of them), easiest fix is to go to a bike shop and ask them what you need. If you don't get the part there after they help you, decency requires that you at least buy a tube or something.