The most common sizes are3/16″, 1/4″, and 5/16″ (4.8mm, 6.4mm, and 7.9mm). The size to be used depends on the size of the torch, and the length of hose needed. The 3/16″ (4.8mm) ID hose is very flexible and light, and is used for light duty welding.
Why are there different fittings on the oxygen and acetylene hoses?
Fittings on the oxygen hose have right-hand threads; while those on the acetylene hose have left-hand threads.