Whole-house evaporative coolers are sized by airflow in CFM, using (house volume in cubic feet × air changes per hour) ÷ 60 — not by BTU or tonnage.
The Building America Solution Center, a Pacific Northwest National Laboratory resource, lays out the industry method, starting with air changes per hour rather than refrigerant-style tonnage. Below is that method in order, plus a worked example.
Why CFM Beats Tonnage
Evaporative coolers move air, so airflow is the sizing unit; the same source defines 1 air change per hour (ACH) as the airflow needed to replace the home’s air volume once per hour. Refrigerated air conditioners are rated in tons; swamp coolers in cubic feet per minute. Treating the two as interchangeable is the most common sizing error.
The practical sequence: confirm the climate suits evaporative cooling, pick an ACH target for your region, calculate conditioned volume, then solve for CFM.
Is Evaporative Cooling Right for Your Climate?
Evaporative cooling adds moisture to air, so it performs in hot, dry climates and struggles in humid ones — suitability ties to local wet-bulb conditions, not just summer heat. Check your area’s cooling design wet-bulb temperature first. If it’s high, a whole-house cooler adds humidity without dropping temperature much, and no CFM math fixes that.
The cited guidance covers US residential whole-house applications where ducted or central direct-supply configurations are feasible. Both move cooled air through the house rather than dumping it at one window; whole-house jobs generally need ductwork and relief air openings so hot indoor air escapes as cool air enters.
Figure Your Target Air Changes per Hour
Most manufacturers suggest roughly 20 to 40 ACH, depending on location and outdoor conditions. The source’s ACH zone map sets your starting point, not a single nationwide number.
- Drier, hotter summers: lean toward the lower end.
- More humid or milder summers: lean higher when the manufacturer’s guidance for your area says so.
- Always defer to the rule that applies to you: your cooler maker’s sizing table for your climate zone overrides a generic number.
Do the CFM Math With Your Real Numbers
House volume is conditioned floor area multiplied by average wall height; multiply by ACH and divide by 60 to get required CFM. Worked example, 1,800 sq ft single-story home:
| Sizing Input | Example Value | How You Get It |
|---|---|---|
| Conditioned floor area | 1,800 sq ft | Measure heated/cooled rooms only |
| Average wall height | 8 ft | Floor to ceiling, averaged |
| House volume | 14,400 cu ft | 1,800 × 8 |
| ACH (hot, dry region) | 30 | Climate zone target |
| Volume × ACH | 432,000 | 14,400 × 30 |
| Required CFM | 7,200 CFM | 432,000 ÷ 60 |
| Unit to buy | 7,200 CFM rated or higher | Match the deliverable airflow |
Choose a cooler that can deliver that CFM and fits your layout (ducted or central direct-supply). Rated airflow at the unit is not the same as airflow at the far register, so confirm the figure describes the intended configuration. If you’re comparing models that meet your CFM target, our tested whole-house evaporative cooler roundup covers capacity and install fit side by side.
What Sizing Cannot Fix
Installation constraints decide whether a correctly sized unit works. Plan for accessibility so pads and pumps can be serviced, roof leakage risk if mounted on top, and required electricity, plumbing, and control wiring. Aesthetics matter too, since these units sit outside. After installation, commission the system to confirm it operates as designed.
Classic traps: sizing by tonnage, ignoring local wet-bulb conditions, using floor area alone instead of volume, assuming one ACH value applies everywhere, and forgetting relief air and ducting.
FAQs
Can I size a swamp cooler by square footage alone?
No. Square footage is only half of the volume figure. The Building America method multiplies conditioned floor area by average wall height for cubic feet, then applies ACH and divides by 60. A 1,800 sq ft home with tall ceilings needs more airflow than the same footprint with standard ceilings.
What if my area is humid in summer?
Check the local cooling design wet-bulb temperature first. Evaporative cooling depends on dry air absorbing moisture, so humid regions get little temperature drop and added indoor humidity. There, a refrigeration-based system is usually the honest answer regardless of sizing.
Which number should I trust, the ACH zone map or the manufacturer?
Both, in order. Use the regional ACH guidance to set your starting target, then check the cooler manufacturer’s sizing table for your climate zone, which typically refines the same 20 to 40 ACH range for local conditions. When they disagree, the maker’s figure for your area wins.
References & Sources
- Building America Solution Center (PNNL). “Evaporative Cooling Systems” Source of the CFM sizing formula, the 20–40 ACH range, and the climate and installation guidance.
- ASHRAE Handbook. “Chapter 53: Evaporative Cooling” Reference for evaporative cooling design principles and wet-bulb-based performance.
- PDH Online. “Evaporative Cooling Design Manual” Background on sizing and system types for evaporative coolers.