Method of calculating an exposure time for disinfecting a room using an ultraviolet disinfecting apparatus comprises the steps of measuring distances by: using a spatial sensor on the ultraviolet disinfecting apparatus, to perform a first scan of a boundary surface of the room, to measure a first set of distances from the sensor to the boundary surface; adjusting the height or orientation of the spatial sensor; using the spatial sensor to perform a second scan of the boundary surface of the room, to measure a second set of distances from the adjusted sensor to the boundary surface; and subsequently, calculating the exposure time in dependence the sets of distances.