The absolute (photometric) calibration of the Halley Multicolour Camera (HMC) is described. HMC, prior to its observations of comet Halley on 13-14 Mar. 1986, imaged several stars and planets. The fluxes from the objects and the observed signals in digital units (DU) have been computed. A spectrally independent measure of the response of the instrument has been determined. This value has been used to derive conversion factors from DU to mW m(-2) sr(-1) and reflectivities for all HMC configurations.