Large, multi-storey buildings pose a particular challenge for natural ventilation design due to the interaction between heat and air flows through different building zones. We develop a demand-based preliminary design strategy for sizing ventilation openings in multi-storey buildings with heated atriums. This approach enables ventilation openings on each storey, and in the atrium, to be rapidly sized so that equal temperatures and per-person flow rates can be achieved on all storeys, regardless of the occupancy or usage. To achieve this, two key dimensionless parameters are identified which quantify the performance of the ventilation system and the effectiveness of the atrium in assisting ventilating flows. Simple analytical expressions for sizing vents are developed and an example design chart presented to provide quick and intuitive first-order design guidance.