A rule of thumb for direct evaporative coolers (desert coolers) has been evolved in terms of the size of the floor area to be cooled for two different climates, namely hot dry and composite, typified by Jodhpur and Delhi, respectively. Linear relations have been obtained to evaluate the cooler size for the space to be cooled, in order to reduce the discomfort index of the building by 90% of its value without a desert cooler.NOMENCLATUREA D door area, m 2 A F cross-sectional area of fan, m 2 A m area of mth component of building, m 2 A p pad area, m 2 A R cross-sectional area of room normal to the air flow from the coolerA w window area, m 2 C a specific heat of air, J/kg °CC R specific heat of room air, J/kg °CE m , n element of admittance matrix [7]F m , n element of admittance matrix [7]F p packing factor of pads (wet surface area per unit pad area)Q u amount of heat removed by cooler, J/sT a ambient temperature, °CT R room air temperature, °CT w wet bulb temperature of air coming from cooler, °CT R W room wet bulb temperature, °CT s m , n nth harmonic component of solair temperature on the mth component of the room at mth wall, roof, °CU w overall heat loss coefficient from window, W/m 2 °CU D overall heat loss coefficient for door, W/m 2 °CV F velocity of fan, m/sV R velocity of air in room, m/sGreek symbolsτ transmittivity of glassα absorptivityρ a density of air, kg/m 3