Tube size and Circumference
There seems to be a fair amount of doubt and confusion as to what tube is the right size tube to buy. If you think of tubes by diameter starting at 1.5 inches and increasing by quarter inches and you know what your circumference is then it is easy to calculate the most appropriate tube for you to buy. If you are going for length then make the tube as close or slightly smaller than your circumference. If you are going for girth get a large diameter tube , bearing in mind the danger of sucking in a testicle( ouch) if it is too big.
Use this formula: C=pi xDiamater
Example: a 5inch circumference divided by 3.1416(Pi) =1.59 inches so for length buy a 1.75 inch tube, for girth 2.25 inch would be enough.