Evaporative cooler size calculator

Evaporative coolers are sized by airflow (CFM or m³/h), not by tons or BTU like air conditioners. This gives a preliminary estimate, not professional sizing.

Assumptions (disclosed)

• Airflow = room volume × air changes per hour (÷ 60 for per-minute units). The 20–40 ACH range and method come from the US DOE Building America Solution Center, which notes that sizing "is not an exact science."^[1]
• Relief area: 2 ft² of unrestricted opening per 1,000 CFM (≈ 0.11 m² per 1,000 m³/h) per DOE/PG&E guidance. Screens and louvres need more. An older DOE fact sheet gives 1–2 ft² per 1,000 CFM.^[1][2]
• Cooling capacity is the sensible heat the supply air can absorb as it warms from outlet to indoor temperature: mass flow × specific heat × (indoor − outlet). It is a derived estimate shown for comparison with heat-load estimates. It is not an equipment rating.
• Manufacturer airflow ratings may be measured with less restriction than your installation (pads, ducts, grilles), so delivered airflow can be lower.

Worked example (from DOE)

A 2,000 ft² (186 m²) home with 8 ft (2.44 m) ceilings in a 20-ACH zone: 16,000 ft³ × 20 ÷ 60 = 5,333 CFM (≈ 9,060 m³/h).^[1] Relief needed: about 10.7 ft² (1.0 m²). Both values are checked in our tests.

When to get professional help

Have a qualified installer verify the design for ducted systems, homes with large west-facing glass, poorly insulated roofs, multi-storey layouts, or any climate where the suitability check shows reduced performance. Duct design should follow a recognised method (DOE references ACCA Manual D).^[1]