A device can include a multi-dimensional posture sensor that provides an electrical sensor output representative of alignment of first, second, and third non-parallel axes of the device with the gravitational field of the earth, and a processor that includes a calibration circuit and a posture circuit. The calibration circuit measures a first sensor output for the first device axis and a second sensor output for one of a second device axis while the subject is in a first specified posture, measures sensor outputs for the first, second, and third device axes while the subject is in a second specified posture, calculates one or more coordinate transformations, generates transformed sensor outputs using the coordinate transformations, and calibrates the posture sensor by calculating a calibration transformation using the first and second sensor outputs and the transformed sensor outputs. The posture circuit determines a subsequent posture of the subject using the posture sensor.
