According to various, but not necessarily all, embodiments there is provided a respiratory volume measurement system. The respiratory volume measurement system comprises: at least one motion sensor and a machine learning system. The at least one motion sensor is configured to be placed on a chest wall of a subject to produce at least one sensor output signal dependent upon respiratory motion of the chest wall of the subject. The machine learning system is configured to receive at least the sensor output signal as an input and to produce at least a respiration measurement output, different to the sensor output signal, that provides at least a measure of respiration volume of the subject.
