Our silicone hoses are sold by internal diameter (ID) not external diameter. We measure this internal diameter with measuring calipers from internal edge to internal edge.
In order to select the correct hose, measure the outer diameter (OD) of the pipe you want to connect it to. This will be the same as the Internal Diameter of the silicone hose. For example if the outer diameter of the pipe is 76mm then you need to choose a silicone hose with a 76mm internal diameter (ID).