1" pvc is just under 1 3/8" diameter. 3/4" pvc is just under 1 1/8" diameter. 1/2" pvc is just under 7/8" diameter (all measurements OD). These measurements hold true for pvc, abs, black iron pipe (gas), and galvanized pipe, as well as some others. Thickening the wall of the pipe (ie. sch. 40, etc.) just reduces the inside diameter.
I just installed Eibach 43kg springs for XR400. I'm 230lbs. It took 38mm spacers to make 5mm of preload. I haven't riden it yet since I also shipped my forks and shock off to FRP for Gold valves and shock revalve. Ride report to follow (soon, I hope).