Abstract: A core barrel capacity gauge (30) for use on a core barrel assembly (10) having a barrel (16) for receiving a core sample. The core barrel capacity gauge comprises a core sample marker (32) located within the barrel (16) such that the core sample marker (32) rests against the top of the drilled core sample. A wheel (42) is provided on the core sample marker (32) that engages with the inside of the barrel (16) such that movement of the core sample marker (32) along the length of the barrel (16) rotates the wheel. A rotation sensor (44) is provided in the core sample marker (32) that senses rotation of the wheel (42) and a transmitter (48) is provided in the core sample marker (32) in communication with the rotation sensor (44). The transmitter (48) transmits information regarding the rotation of the wheel (42) is received from the rotation sensor (44).