Abstract: A measuring device for setting the correct length or penetration depth of root canal instruments comprising relatively movable lower and upper portions and scale means cooperating with said two portions to enable accurately setting the intended depth of penetration of a root canal instrument used by the dentist. The upper portion has a substantially plate-like member having a top face whose distance from a top face or surface of the lower portion, defining a base portion, can be accurately set with the aid of the scale means. The root canal instrument then is inserted through a proper size hole in the plate-like member, onto the top face of which there has been previously placed a stopper, typically in the form of a disc, whereafter the root canal instrument is inserted through the stopper and the hole until the tip of the root canal instrument abuts against the top face or surface of the lower base portion.