Hello Gerald, I believe a part of the confusion over this comes from perspective. Pipe sizes from 1/8" to 12" are not sized on the O.D. to match the size they are called out as. 14" and up they are called out on the O.D. according to the size they are called out at. Examples: 1/2" pipe actually measures .840 on the O.D., 3" pipe actually measures 3 1/2" on the O.D., 14" pipe measures 14" on the O.D., sizes including 14" and greater, measure the same on their O.D. as the diameter that they are called out as. In all instances however, the O.D. of the pipe remains the same as the schedule of the pipe increases(sch. 5 being the thinnest, sch. 10, sch. 40, progressing upwards, indicating thicker wall pipe), the wall thickness will increase, this is not to say that the sch. # is specific with regard to thickness, for different diameters of pipe sch. #'s there can be different thickness values. Thus different wall thicknesses for different sizes of pipe will indicate smaller I.D. dimensions as the schedule numbers increase, the O.D. however, will remain the same. Don't mean to beat a dead horse so to speak, but I guess I haven't been able to let this one rest. Best regards, Allan