A calculation device that calculates a length of a calculation target region of a human body on the basis of results of measuring a measurement target region of the human body, the calculation device comprising: a state information acquisition unit configured to acquire state information that indicates a position or the position and a state of a posture of the measurement target region of the human body on the basis of measured results measured by a wearable device comprising inertial sensors, wherein the wearable device is worn on the measurement target region of the human body, the state information is calculated by the wearable device or the state information acquisition unit, and the wearable device provides the state information or the measured results to the state information acquisition unit; and a calculation unit configured to calculate a length using a region from a rotation center in a motion of the human body to the measurement target region as the calculation target region on the basis of the state information acquired by the state information acquisition unit, wherein the rotation center is a joint, the calculation unit calculates the rotation center and the length of the calculation target region on the basis of the state information by approximately obtaining a circle or a sphere having a length from the rotation center to the calculation target region as a radius, and the rotation center comprising a first rotation center (p 0) and a second rotation center (p 1); the calculation device being characterized in that the calculation unit comprises: wherein the calculation unit comprises: a unit configured to calculate a length (l 1 +l 2) using a region from the first rotation center (p 0) to the measurement target region (p s) as a first calculation target region; a unit configured to calculate a length (l 2) using a region from the second rotation center (p 1) between the first rotation center (p 0) and the measurement target region (p s) to the measurement target region (p s) as a second calculation target region; and a unit configured to calculate a length (l 1) using a region from the first rotation center (p 0) to the second rotation center (p 1) as a third calculation target region on the basis of a difference between the length (l 1 +l 2) of the first calculation target region and the length (l 2) of the second calculation target region. a state information acquisition unit configured to acquire state information that indicates a position or the position and a state of a posture of the measurement target region of the human body on the basis of measured results measured by a wearable device comprising inertial sensors, wherein the wearable device is worn on the measurement target region of the human body, the state information is calculated by the wearable device or the state information acquisition unit, and the wearable device provides the state information or the measured results to the state information acquisition unit; and a calculation unit configured to calculate a length using a region from a rotation center in a motion of the human body to the measurement target region as the calculation target region on the basis of the state information acquired by the state information acquisition unit, wherein the rotation center is a joint, the calculation unit calculates the rotation center and the length of the calculation target region on the basis of the state information by approximately obtaining a circle or a sphere having a length from the rotation center to the calculation target region as a radius, and the rotation center comprising a first rotation center (p 0) and a second rotation center (p 1); the calculation device being characterized in that the calculation unit comprises: wherein the calculation unit comprises: a unit configured to calculate a length (l 1 +l 2) using a region from the first rotation center (p 0) to the measurement target region (p s) as a first calculation target region; a unit configured to calculate a length (l 2) using a region from the second rotation center (p 1) between the first rotation center (p 0) and the measurement target region (p s) to the measurement target region (p s) as a second calculation target region; and a unit configured to calculate a length (l 1) using a region from the first rotation center (p 0) to the second rotation center (p 1) as a third calculation target region on the basis of a difference between the length (l 1 +l 2) of the first calculation target region and the length (l 2) of the second calculation target region. a unit configured to calculate a length (l 1 +l 2) using a region from the first rotation center (p 0) to the measurement target region (p s) as a first calculation target region; a unit configured to calculate a length (l 2) using a region from the second rotation center (p 1) between the first rotation center (p 0) and the measurement target region (p s) to the measurement target region (p s) as a second calculation target region; and a unit configured to calculate a length (l 1) using a region from the first rotation center (p 0) to the second rotation center (p 1) as a third calculation target region on the basis of a difference between the length (l 1 +l 2) of the first calculation target region and the length (l 2) of the second calculation target region.