Gauges are used as the measurement of the diameter for hypodermic tube. Unlike other measurements, where the size increases proportionate to the number, gauges actually decrease in diameter as the number increases. It is important to remember this when you are ordering your tubing, to ensure you receive the right size. For example, 10 gauge tubing has an outer diameter of approximately 0.133 to 0.135 inches, whereas 15 gauge tubing has an outer diameter ranging from 0.0715 to 0.0725 inches.
In addition to remembering the information about gauge sizes, you also need to determine the correct wall size to order. Hypodermic tube is available in three different wall options including regular, thin, and extra-thin. While the gauge size and outer diameters of each wall remains the same, the difference is the size of the inner diameter. As the wall becomes thinner, the inner diameter increases in size. Looking at 10 gauge tubing, you will notice the inner diameter of regular wall is 0.104 to 0.108 inches, thin wall is 0.112 to 0.116 inches, and extra-thin wall is 0.1165 to 0.120 inches.
The reason there are three different wall sizes is because some applications require a specific gauge of tubing. However, the rate at which fluids are able to pass through the tubing could be an issue. In order to make fluids and viscous materials flow easier, you are able to switch from regular wall, to thin wall or extra-thin wall tubing. The added increase in the inner diameters does help to increase flow rates.