A depth gauge for measuring the depth of blind holes in bones and/or nails inserted into bones. The measurement tool comprises: a probe with an abutment at its end to engage against the distal edge of the blind hole; a tubular body; and a grooved insert. After the probe, lying into the tubular body, is positioned through the blind hole to be measured, the grooved insert is slid forward inside the tubular body, surrounding the probe, and pushing the probe sideward, towards the blind hole wall. When probe and groove insert are retracted together, they firmly engage against the distal bone wall. The tubular housing is then pushed forward until it abuts against the proximal bone wall, allowing even in small diameter holes, an accurate reading of hole depth on the scale located at the rear end of the probe.