Sizing-
We recommend using a tube that is at least 1/4" but not more than 1/2" more than your natural erect diameter. Why? You need some room for expansion, but over sizing the tube tends to create two problems. One, the extra void space around the base provides the opportunity for your testicles to be suddenly sucked up into the tube.

This can be a painful experience. Two- the tissues along the penis are not all the same. Some engorge more quickly than others. If the tube is sized correctly, these "fast" tissues will reach the cylinder walls and then be restrained, while slower engorging tissues catch up. That results in even enlargement and stretching. If the tube is oversize, the appearance will be uneven, and the stretching unequal. Many people pump in stages, using step up tube sets. They pack the one they are in (fill it to the sides full shaft length) then move up to the next size tube, in the same session.

Measuring-
Penis tubes are sold based on their Inside Diameter, the width of the opening. The easiest way to find your diameter is to measure your circumference first. This is the distance around the penis shaft. Use a strip of paper, perhaps an inch wide. Wrap this around and back over itself, about midway on your shaft. Make a mark parallel to the penis shaft that appears on both ends of the paper at the point they cross. Then lay the paper out flat, and measure the distance between the marks. That will be your circumference. Now, divide the circumference by 3.14, or Pi. The answer will be your diameter. For example if your circumference is 5-1/2 inches (5.5") your diameter would be 1.75", and the proper tube would be a 2" or 2-1/4" model. Length matters too. There needs to be airspace remaining in the cylinder; it is like a cushion that moderates the vacuum effect. For example penis cylinders should be 1-3" longer than the length you expect to fill. Custom lengths are available in most cylinder types.

NOTE: The chart below is only to assist with conversion of circumference measurements into diameter. It provides exact conversion of measurements. It does not adjust for, calculate or show what diameter should be ordered for any given circumference.