A blood volume monitor includes a carrier mountable on a subject's body part. A measuring arrangement is mounted on the carrier. The measuring arrangement provides data relating to a change in volume of the body part, and outputs signals representative of the change. A control unit is in communication with the measuring arrangement to process data output by the measuring arrangement to determine, using the volume data, the volume of the body part and determines blood volume in the body part. The control unit uses the response signals received from the measuring arrangement and the determined volume of the part of the body to determine the blood volume.