Like the ones from Home Depot? Just measure the shorter length inside with a string, then the longer side, and average the two. Remember not all of it will be counted as "port length" tho. Some of the length is a flared joint which is used to connect the bend to another piece of PVC pipe. Don't count that in the length. If its the one from home depot i believe its around 7 inches of port length if I remember correctly.