simple the measurements are i believe for outside diameter so just get a cm ruler remember each little line is 1mm so in 1cm theres 10 little lines simple enough....most bongs female downstems are 12mm while the male slide is 9mm..but in your position just measure the inside of the rubber grommit side to side or up and down dont matter if its like 13 or 11 then u just need a 12mm downstem and 9mm male bowl.remember rubber will stretch to fit...simple really man hope i helped and good luck post pics if u can