The size of a gas bellows is usually described using "cents" as a unit, which here is actually a common name for the imperial unit "inch". In gas bellows, "min" refers to the nominal diameter of the pipe, specifically, the inner diameter of the pipe or the size of the joint to match it.

For gas bellows, the common sizes are:

• 3 cents (3/8 inch): Approximately equal to an inner diameter of 9.525 mm (0.375 in).

• 4 points (1/2 inch): Approximately equal to an inner diameter of 12.7 mm (0.5 inches).

In practical applications, 4-point gas bellows are the most common. The 4-point gas bellows typically have an outside diameter of about 20 mm, which allows it to be suitable for the connection needs of most domestic gas installations. The 3-point gas bellows may be used in some specific situations, such as space constrained places or specific equipment connections.

When purchasing and installing gas bellows, ensure that the size of the selected pipe matches the interface size of gas equipment to ensure the tightness and safety of the connection. In addition, the quality certificate of the product should be checked, such as the production license number, the date of manufacture, etc., to ensure that the purchased product meets the safety standards.