The first number is the length and the second number is the height. To measure the length, take a measuring tape and start halfway around the elbow and go all the way out to the test valve. Add a few inches so the pouch can sit comfortably. For the height, it's important to cover all of the PVB. Start your measurements from the top of the bell to the bottom of the device. Note, the measurements listed online are that of the pouch, not the backflow device.