It isn't usually talked about in centimeters or inches. When you insert the probe, mark how far it goes in (generally by holding that place with your thumb or something along those lines). Then, when you remove it, just lay it on top of the snake, and count how many scales it went in. I am at the moment drawing a blank as to how many it is supposed to be, though, so hopefully someone else will chime in with the numbers. The # of scales thing works very well because it is constant regardless of the age/size of the snake.