Measure the OUTSIDE CIRCUMFERENCE of the pipe to install the unit on (measure in inches). 

Divide that measurement by 3.14

If the resulting number is between .5 and .75 then order a ½” unit

If the resulting number is between .75 and 1.0 then order a 3/4” unit

If the resulting number is between 1.0 and 1.25 then order a 1” unit

If the resulting number is between 1.25 and 1.5 then order a 1 1/4” unit

If the resulting number is between 1.5 and 1.75 then order a 1 1/2” unit

If the resulting number is between 2. and 2.25 then order a 2” unit