Apparatus for measuring bone depth for determining the length of fastener to be employed in medical treatment of the bone includes an elongated inner bone probe having a bone-engaging portion and a plurality of first manually-engageable elements secured thereto. An outer sleeve overlies at least a portion of the inner bone probe and is movable with respect thereto. A second manually-engageable element is secured to the outer sleeve. The first and second manually-engageable elements may be generally ring-shaped, so as to facilitate insertion of a user's fingers. The apparatus is structured to have the bone-engaging portion of the inner bone probe pass through an opening in the bone and engage the distal exterior surface in intimate contact as a result of the first manually-engageable elements being engaged and urged away from the bone. The outer sleeve is moved generally toward the bone so as to achieve contact between the distal end of the outer sleeve and the proximal exterior surface of the bone. A reading of desired fastener length may then be made. Other embodiments of the gauge employing different configurations of the two main components and additional functional features, as well as adaptors, which facilitate retro-fitting structural features, of the present invention into prior art devices. Associated methods are provided.