How to measure:
 Caps size is  determined by the external diameter of the tube    

For example: external diameter (outside size) of tube 50mm then size of plugs=50mm

The most common pipes have the following section - Rectangle​. All plastic plugs are marked exactly like pipes. The outside dimension of the pipe corresponds to the marking of the plug. This is done so that the plugs can be easily selected according to the same size.

Please if you not sure which size you need ask for help before you buy.