Length calibration for computer models of users to generate inputs for computer systems

A system including a plurality of sensor modules, each module having an inertial measurement unit (IMU) and being attached to a respective body portion of a user (e.g., upper arm, hand, and/or head) to measure the current orientation of the corresponding portion of the user. A computing device coupled to the sensor modules is configured to identify that a user is at a predefined pose, which the predefined pose of the user is representative of the forearms and the upper arms of the user lying in a horizontal plane. A head mount display (HMD) attached to a head of the user using a camera to generate camera data, the HMD calculating the first length using the camera data, and the first length is calculated as corresponds to a distance between hands of the user and shoulders of the user; the computing device determining one or more lengths of one or more bones of the user based on the first length and the plurality of orientations of arm bones of the user at the predefined pose.