The pipe has a slightly larger id measuring in at almost 1 5/8-inces. This rule applies to steel and PVC pipe. The OD of 1/2-inch galvanized pipe is about 7/8-inch and so is 1/2-inch PVC. 3/4-inch pipe measures 1-1/16 inches, 1-inch measures 1 5/16 inches and 1 1/4 inches measures 1 5/8 inches. These same rules apply to rigid copper plumbing.