Calculator
Swamp Cooler Size Calculator (CFM)
Calculate the CFM your evaporative cooler needs from floor area, ceiling height and climate, with an allowance for ductwork and a recommended unit type.
Evaporative coolers are sized by airflow, not tonnage. The goal is to replace all the air in the space every 1.5 to 3 minutes, depending on how hot and dry your summers are. Enter your space below.
The formula
For ducted installations the calculator adds 20% because published CFM ratings are usually taken at low static pressure, and real duct systems deliver less. See CFM explained for why.
Worked example: 1,800 sq ft house in Albuquerque
Volume: 1,800 × 8 = 14,400 cu ft. Albuquerque summers peak in the mid 90s, so use 30 air changes per hour: 14,400 × 30 ÷ 60 = 7,200 CFM delivered. The cooler feeds a duct system, so look for a rated airflow near 7,200 × 1.2 = 8,640, or roughly 8,500 to 9,000 CFM.
Quick sizing table
| Floor area | Mild (20 ACH) | Hot, dry (30 ACH) | Hot desert (40 ACH) |
|---|---|---|---|
| 300 sq ft | 800 | 1,200 | 1,600 |
| 800 sq ft | 2,133 | 3,200 | 4,267 |
| 1,200 sq ft | 3,200 | 4,800 | 6,400 |
| 1,600 sq ft | 4,267 | 6,400 | 8,533 |
| 2,000 sq ft | 5,333 | 8,000 | 10,667 |
| 2,500 sq ft | 6,667 | 10,000 | 13,333 |
Do not forget the exit. Every cubic foot the cooler pushes in has to leave. Plan roughly 1 to 2 sq ft of open window or relief vent per 1,000 CFM. Our venting guide explains how.
For the full method, including how insulation, sun exposure and open floor plans change the answer, read how to size a swamp cooler. For garages, patios and shops, see sizing portable and outdoor coolers.
Frequently asked questions
How many CFM do I need for a 1,500 square foot house?
With 8 ft ceilings, 1,500 sq ft is 12,000 cubic feet. At 30 air changes per hour that is 6,000 CFM delivered; in a hot low desert at 40 air changes it is 8,000 CFM.
Is it bad to oversize a swamp cooler?
Moderate oversizing is fine because you can run on low speed, which is quieter and uses less power. The real risks are undersizing and not providing enough relief air for the airflow you have.
Why add 20% for ducted systems?
Manufacturer CFM ratings are often measured with little or no external static pressure. Ducts, registers and grilles add resistance, so a ducted unit delivers noticeably less than its label figure.
Do I size by square feet or cubic feet?
Cubic feet. Air changes are based on volume, so a room with 12 ft ceilings needs 50% more airflow than the same floor area with 8 ft ceilings.