The National Institute of Standards and Technology has developed a contact micrometer for accurate measurement of the outer diameter of optical fibres. The contact micrometer is used to measure reference fibres that are artefacts used by the telecommunications industry for calibrating their own measurement systems. We present a model for diameters measured by the contact micrometer. Based on this model, the probability distribution of the diameters is derived and two diameter estimates are presented. We illustrate and compare the diameter estimates using simulated data.