Good question. As for the hubs, it's neither. PVC pipe is measured based on the inside diameter, because they use it to measure the air or water volume that is passed through. The size of the PVC pipe at your local hardware store goes by that inside diameter measurement. So when we say the hubs are ½”, it means that ½” is the inside diameter of the pipe that is used with the hubs. The outside diameter of the pipe is the critical size - and it's .84", almost an inch, and that's what the inside diameter of the hubs are expecting. This diameter of pipe is commonly available at hardware stores and is sold as "Schedule 40", and it's usually found in the plumbing or irrigation sections. Always try to buy the longest length they sell - ie. 20' to minimize spoilage.