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Evaporative cooler sizing calculator: how many CFM do you need?

Evaporative (“swamp”) coolers are sized by airflow — CFM — not by BTU. The US Department of Energy's rule is simple: take your room's volume, multiply by the air changes per hour your climate needs, and divide by 60. The tool below does exactly that, and — just as important — tells you honestly whether an evaporative cooler will work where you live at all.

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Work out your CFM

Work out your CFM — and the cooler that meets it

Evaporative (“swamp”) coolers are rated in CFM airflow, not BTU. This uses the US Department of Energy’s formula — CFM = (volume × air changes per hour) ÷ 60 — then points you to a unit that meets it, if your air is dry enough.

Formula, the 20–40 ACH range and the wet-bulb ≤70 °F effectiveness threshold are the US DOE Building America Solution Center (PNNL)’s own — validated against its Salt Lake City worked example (16,000 cu ft × 20 ACH ÷ 60 = 5,333 CFM).

How the number is built — no guesswork

This isn't a made-up rule of thumb. It's the formula the US Department of Energy publishes through its Building America Solution Center (run by Pacific Northwest National Laboratory):

Cooler size in CFM = (cubic feet × air changes per hour) ÷ 60US DOE Building America Solution Center (PNNL)

  • Cubic feet is your floor area times ceiling height — a 500 sq ft room with 8-ft ceilings is 4,000 cu ft.
  • Air changes per hour (ACH) is how many times the cooler should replace all the air in the room each hour. DOE says most manufacturers suggest 20 to 40 ACH, higher in hotter, drier desert conditions.
  • DOE's own worked example: a 2,000 sq ft home with 8-ft ceilings near Salt Lake City needs 20 ACH → 16,000 cu ft × 20 ÷ 60 = 5,333 CFM. Our calculator reproduces that number exactly — that's how we know the arithmetic is right.

Airflow is why these are rated in CFM, not BTU. A compressor air conditioner refrigerates the air, so it's measured by heat removed (BTU). An evaporative cooler just moves a lot of humidified air through the room, so what matters is how much air it moves. If you're shopping a real air conditioner instead, use the BTU sizing tool.

First, the honest part: will it even work where you live?

An evaporative cooler cools by evaporating water into the air, so it only helps when the air is dry. DOE rates them for climates with a design wet-bulb temperature at or below 70 °F. In dry air they can drop the incoming air 15–40 °F (Portacool cites up to ~30 °F when humidity is 30% or less); in humid air the drop is small and they mostly just add moisture — making a muggy room feel worse. This is not a small caveat: it's the whole difference between a great buy and a returned box. Check your actual humidity before you spend anything:

Is your air dry enough for evaporative cooling?

Enter your air temperature and humidity — or pull the latest reading from your nearest US National Weather Service station — to see the “feels-like” heat index, the NWS caution level, and whether an evaporative cooler will actually help at your humidity.

Heat index uses the National Weather Service Rothfusz regression. Live readings come from api.weather.gov (US only, public domain) and stay in your browser — we don’t store your location. Not medical advice.

Two evaporative coolers, honestly rated

Once you have a target CFM and you've confirmed your air is dry, match the airflow rating to your room. A couple we've verified on the manufacturer's own numbers:

Hessaire MC18M

$140–215 (varies by color variant and retailer) as of 2026-07-04

A genuine 1,300 CFM portable evaporative cooler that Hessaire rates for rooms, patios and workshops up to 500 sq ft — in dry air. By the DOE formula its 1,300 CFM matches a room of about 480 sq ft at 8-ft ceilings at the milder end of the range (roughly 20 ACH — a full 500 sq ft works out to about 1,350 CFM); in a hotter, drier climate that needs 30–40 ACH you would size down the room or step up the cooler.

Type
Evaporative — truly hose-free
Cooling Btu
1,300 CFM airflow
Hose
None
Coverage
Up to 500 sq ft (dry climates only)
Noise
53.4 dB
Tank
4.8 gal (3–4 h, or garden-hose fill)
Weight
16 lb

Where it earns its price

  • Real airflow number published by the manufacturer, not a vague “cools up to” claim
  • Garden-hose fill and a 4.8-gal tank mean less refilling than a personal unit
  • Works outdoors and in open garages/workshops where a compressor AC can’t vent

Where it doesn't

  • Useless in humid air — this is a dry-climate tool only
  • It adds humidity as it runs; in a closed room that eventually undercuts its own cooling — evaporative coolers want a cracked window or door
  • 53.4 dB is not quiet; it’s a workshop/patio machine, not a bedroom one

Skip it if: your summers are humid, or you need to cool a sealed room without any outdoor air path.

Check price at Home Depot

Sold as MC18M / MC18MT (Ice Blue) / MC18V (Green) — same 1,300 CFM platform, price differs by variant and retailer, so we quote a range.

Evapolar evaCHILL (EV-500)

$83.40 (sale) / $139 (list) as of 2026-07-04

A tiny personal evaporative cooler — think “cool air on your face at a desk,” not room cooling. At 340–1,190 BTU-equivalent it only conditions the 21–27 sq ft directly in front of it, so the CFM formula above doesn’t really apply; it’s a spot device.

Type
Evaporative — truly hose-free
Cooling Btu
340–1,190 BTU/hr
Hose
None
Coverage
21–27 sq ft, directly in front
Power
7.5 W
Tank
800 ml (3–8 h per fill)

Where it earns its price

  • Genuinely hose-free and sips power (7.5 W) — fine on a USB power bank
  • Cools the air right in front of you within seconds in dry conditions
  • Cheap enough to be a low-risk try in a dry climate

Where it doesn't

  • Not a room cooler in any sense — the coverage is a personal bubble
  • The 800 ml tank runs dry in a few hours
  • Same humidity ceiling as every evaporative device — dry air only

Skip it if: you want to cool a room rather than a desk, or your air is humid.

Check price at Evapolar

Evaporative cooling adds humidity and fades fast in humid air — see the humidity section before buying.

Questions people actually ask

How many CFM do I need for an evaporative cooler?
Take your room volume (floor area × ceiling height) and use the US Department of Energy formula: CFM = (cubic feet × air changes per hour) ÷ 60, with 20–40 air changes per hour depending on how hot and dry your climate is. A 500 sq ft room with 8-ft ceilings (4,000 cu ft) works out to about 1,350 CFM at 20 ACH and about 2,650 CFM at 40 ACH. The calculator on this page does the math for your room.

Is the “divide the room’s cubic feet by 2” rule correct?
It’s a rough shortcut, not the official method. Dividing cubic feet by 2 is the same as assuming 30 air changes per hour, which lands in the middle of DOE’s 20–40 range — fine as a ballpark, but the real guidance scales the air changes to your climate, which is why our tool asks. We build the number from DOE’s actual formula rather than the shortcut.

Do evaporative coolers work in humid weather?
No. They cool by evaporating water into the air, so they only work when the air is dry — DOE rates them for climates with a design wet-bulb temperature at or below 70°F. In humid conditions the temperature drop is small and the added moisture makes a muggy room feel worse. If your summers are sticky rather than dry, buy a compressor air conditioner instead.

How much cooler will an evaporative cooler make the air?
In dry air, DOE says the incoming air can be cooled by 15 to 40°F; Portacool cites drops of up to about 30°F when relative humidity is 30% or less. The drier the air, the bigger the drop. In humid air the effect shrinks to a few degrees.

What’s the difference between CFM and BTU for cooling?
BTU measures heat removed and is how refrigerant air conditioners (window, portable, mini-split) are rated. CFM measures airflow and is how evaporative coolers and fans are rated, because they move air rather than refrigerate it. You can’t convert one to the other directly — they describe different things. Use the BTU tool for air conditioners and the CFM tool here for evaporative coolers.

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