A system for measuring the diameter of a human hair (50), the system comprising: a mobile device (20); and a reference card (10) including a hair attachment mechanism (11a, 11b); the mobile device configured to convert an image of a human hair attached to the reference card by the attachment mechanism into a measurement of the diameter of the human hair, using the size of the reference card itself and/or the size of a calibration marker (12) on a surface of the reference card as a size reference.