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Add every room the cooler will serve. CFM = total volume ÷ 2 (30 air changes/hour), the standard published rule.
Room breakdown
| Room | Dimensions | Volume | CFM share |
|---|
Suggested unit
- Buy this unit size
- —
- Open this much window exhaust
- —
You already have (or are eyeing) a unit — how many square feet can it cover? Max sq ft = CFM ÷ 4 (hot, dry) or ÷ 3 (average), per PG&E.
Example: a 6,000 CFM unit covers up to 1,500 sq ft hot-dry or 2,000 sq ft average — exactly the PG&E table row.
One room, one number. CFM = room volume ÷ 2.
A swamp cooler pushes air in — the same amount must get out. Open windows/vents on the leeward side: 1–2 sq ft of opening per 1,000 CFM (energy.gov).
Windows open too far → hot air floods in. Not open far enough → humidity builds up inside (energy.gov). Where open windows are a security issue, install ceiling up-ducts to exhaust warm air into the attic.
Know the CFM you need? Pick the unit. PG&E rule: between sizes → select the next larger cooler capacity.
Evaporative cooling only works in dry air. Check your summer climate before buying.
Why humidity matters: a swamp cooler cools by evaporating water into the air — and it adds moisture. In humid air there is little evaporation, so you get damp air with almost no cooling. Above ~60% RH, a refrigerated air conditioner is the honest answer.
The published PG&E sizing table for ducted evaporative coolers, with metric equivalents.
| Cooler capacity | Average climate | Hot, dry climate |
|---|
Table values: PG&E "Technical Sheet: Energy-Efficient Ducted Evaporative Coolers". Rule behind it: hot, dry = floor area × 4 CFM/sq ft (30 air changes/hour, 8-ft ceilings); average = floor area × 3 CFM/sq ft (22.5 air changes/hour).
How the math works
Two published rules that say the same thing:
At 8-ft ceilings both agree: 1,500 sq ft × 8 ft = 12,000 ft³ ÷ 2 = 6,000 CFM = 1,500 × 4. The ×4 shortcut only holds at 8-ft ceilings — with vaulted ceilings, use the volume formula.
Adjustments (+20%)
If any of these apply — ceilings above 8 ft, heavy sun exposure, or extra heat load (kitchen, servers, workshop) — add ~20% CFM (retailer sizing guidance, e.g. Sylvane). Example: 2,000 sq ft × 9-ft ceilings = 18,000 ft³ → 9,000 CFM base → 10,800 CFM adjusted.
Exhaust opening
A swamp cooler is a supply device: it pushes cooled air in, so the same volume must escape. energy.gov: open windows or vents on the leeward side to provide 1–2 sq ft of opening per 1,000 CFM. A 6,000 CFM unit needs 6–12 sq ft — for example three 2×2 ft windows (12 sq ft total).
Between sizes → go larger
PG&E: "When the floor area of your home is between sizes shown in the table, select the next larger cooler capacity." A slightly oversized evaporative cooler just moves more air; an undersized one never cools the house. Need 5,200 CFM → buy 5,500 CFM.
A wrong rule to ignore
Some AI-generated content claims "20–40 CFM per square foot." The math disproves it: 20 CFM/sq ft at 8-ft ceilings = 150 air changes/hour — five times what the largest residential units (~8,000 CFM) can deliver. The published numbers are 3–4 CFM per sq ft, not 20–40.
Worked examples
| Space | Rule | Result |
|---|---|---|
| 1,500 sq ft home, 8-ft ceilings | Hot, dry: 12,000 ft³ ÷ 2 | 6,000 CFM |
| 1,500 sq ft home, 8-ft ceilings | Average climate: × 3 | 4,500 CFM |
| Bedroom 12 × 12 × 8 ft | 1,152 ft³ ÷ 2 (also 144 sq ft × 4) | 576 CFM |
| 2,000 sq ft home, 9-ft ceilings | 18,000 ft³ ÷ 2, then +20% | 10,800 CFM |
| 6,000 CFM unit | Exhaust: 1–2 sq ft per 1,000 CFM | 6–12 sq ft opening |
| Need 5,200 CFM | Next-larger rule | 5,500 CFM unit |
| Need 10,800 CFM | Above the 6,500 max single unit | 2 × 5,500 CFM units |
Examples computed with the same engine as the calculators above — change any input and re-run to verify.
Frequently asked questions
What size swamp cooler do I need for a 1,500 sq ft house?
How many CFM per square foot does a swamp cooler need?
What size swamp cooler for a 12×12 bedroom?
Do high ceilings change the swamp cooler size?
How much window opening does a swamp cooler need?
My calculation falls between unit sizes — which do I pick?
Will a swamp cooler work in my climate?
How is a swamp cooler different from a bathroom fan, AC, furnace, or dehumidifier?
Can I use two smaller swamp coolers instead of one big one?
Is the '20–40 CFM per square foot' rule true?
Related tools
- Bathroom fan CFM (#2532) sizes an exhaust fan that pushes stale air out of a bathroom — opposite airflow direction from a swamp cooler.
- Air conditioner room size (#785) sizes refrigerant cooling in BTU/hr — different physics, not comparable to CFM.
- Furnace size (#2530) is heating (BTU/hr), the opposite season.
- Dehumidifier size (#2531) removes moisture from air; a swamp cooler adds moisture — opposite humidity direction.