A square gauge apparatus configured for being selectively and removably mounted on a leg of a carpenter's square has a ledge having a ledge first surface and an opposite ledge second surface, the ledge first and second surfaces being parallel and offset, a first abutment extending from the ledge and having a first abutment front surface that is perpendicular to the ledge first surface, a second abutment extending from the ledge opposite the first abutment and having a second abutment front surface that is perpendicular to the ledge second surface and parallel to and coplanar with the first abutment front surface, and a slot formed adjacent to the ledge between the first and second abutments, the slot having a slot end surface that is parallel to and coplanar with the first and second abutment front surfaces.