A blood volume monitor which includes a carrier mountable about a part of a body of a subject, the carrier being in the form of a sleeve of a resiliently flexible material; a measuring arrangement carried by the sleeve , the measuring arrangement including a volume determining mechanism with at least a part of the volume determining mechanism being configured to expand and contract in unison with the sleeve , the volume determining mechanism comprising at least one element of an electrically resistive material configured to approximate a truncated cone, the electrically resistive material being resiliently flexible and configured so that resistivity increases with extension, the at least one element providing volume data relating to a volume of the part of the body of the subject underlying the carrier and circumscribed by the at least one element for measuring change in volume of the part of the body of the subject underlying the carrier and for outputting response signals representative of the change in volume, the volume determining mechanism being calibrated against known cylinders and/or known cones to provide volume estimation within a tolerance of approximately 10% of specific calibration values; and a control unit in communication with the measuring arrangement to process data output by the measuring arrangement to determine, using the volume data, the volume of the part of the body of the subject underlying the carrier and to determine blood volume in the part of the body of the subject, the control unit using the response signals received from the measuring arrangement and the determined volume of the part of the body to determine the blood volume, the control unit containing the specific calibration values as parameters to perform volume measurements, the calibration values being derived from a linear approximation of at least two known values of known cylinders and/or known cones. a carrier mountable about a part of a body of a subject, the carrier being in the form of a sleeve of a resiliently flexible material; a measuring arrangement carried by the sleeve , the measuring arrangement including a volume determining mechanism with at least a part of the volume determining mechanism being configured to expand and contract in unison with the sleeve , the volume determining mechanism comprising at least one element of an electrically resistive material configured to approximate a truncated cone, the electrically resistive material being resiliently flexible and configured so that resistivity increases with extension, the at least one element providing volume data relating to a volume of the part of the body of the subject underlying the carrier and circumscribed by the at least one element for measuring change in volume of the part of the body of the subject underlying the carrier and for outputting response signals representative of the change in volume, the volume determining mechanism being calibrated against known cylinders and/or known cones to provide volume estimation within a tolerance of approximately 10% of specific calibration values; and a control unit in communication with the measuring arrangement to process data output by the measuring arrangement to determine, using the volume data, the volume of the part of the body of the subject underlying the carrier and to determine blood volume in the part of the body of the subject, the control unit using the response signals received from the measuring arrangement and the determined volume of the part of the body to determine the blood volume, the control unit containing the specific calibration values as parameters to perform volume measurements, the calibration values being derived from a linear approximation of at least two known values of known cylinders and/or known cones. <EOS>