Evaporative (Swamp) Cooler Sizing Calculator

How it works

CFM = room volume (ft³) × ACH ÷ 60; T_out = T_db − ε × (T_db − T_wb), ε = media effectiveness; wet-bulb from Stull’s approximation; water ≈ air mass flow × 0.00041 kg/kg per °C of cooling

An evaporative cooler pulls hot outdoor air through wet pads: water evaporates, taking its latent heat from the air, so the air leaves cooler and more humid with the same total enthalpy. The lowest temperature it can reach is the outdoor wet-bulb, and good media get 75–90% of the way there — so dry desert air at 38 °C and 20% RH comes out around 24 °C, while at 60% humidity the drop is only a few degrees and the room feels clammy. Because the cooled air must be exhausted rather than recirculated, sizing is by air changes (20–40 an hour) rather than BTUs, and windows must be open downstream. The prize is energy: a fan and a small pump instead of a compressor, at the cost of water — roughly 10 L per hour per 1,000 CFM for a 14 °C drop, since every kilowatt of cooling evaporates about 1.5 L an hour.

Worked example

120 m² at 2.7 m (324 m³) at 30 air changes an hour needs about 5,700 CFM. At 38 °C and 20% RH the wet-bulb is 21.3 °C; 85% media deliver 23.8 °C air at roughly 80% RH — a 14.2 °C drop, about 42 kW of sensible cooling for around 62 L of water an hour.

Frequently asked questions