First, ground joints are used in a set: the male inner piece with a conical end and a female piece ground on the inside to the corresponding dimensions of the male piece. All measurements are in millimeters, so get a ruler that reads in those measurements. You will also need a pipe cleaner or new pencil. Measure the inside diameter of the FEMALE piece and write that measurement down, this is your downstem size. Now slide the pipe cleaner or pencil into the same hole and mark the depth. Now measure that and subtract the length of the ground joint (about one inch). What remains is your downstem length.