🏜️What size evaporative cooler does this space need?

✓ Tested & verified Updated: Aug 9, 2026 How we calculate this

Evaporative cooler size calculator

Room length (ft)

Room width (ft)

Ceiling height (ft)

1,300 CFM

The room works out at 1,200 CFM — 1,300 is the nearest size actually sold.

Floor area: 300 sq ft
Room volume: 2,400 cu ft · 68 m³
Working rate: 0.5 CFM per cu ft

Evaporative cooling only works when the air can leave. Open a window or door on the far side of the space, roughly one square foot of opening for every 1,000 CFM.

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What size evaporative cooler does a room need?

Take the room volume in cubic feet and halve it. That is the CFM rating to look for: a 20 × 15 ft room with an 8 ft ceiling is 2,400 cubic feet, so 1,200 CFM. The halving is the standard two-air-changes-per-minute rule — an evaporative cooler works by pushing a continuous river of cooled outdoor air through the space, so it is sized on how fast it can replace the air, not on how much heat it can remove.

That is also why the rule that matters most is not a size at all: the air has to have somewhere to go. Run a swamp cooler in a sealed room and you get a humid room with the same temperature. Open a window on the opposite side and the same machine drops the air 15 to 20 °F.

Evaporative cooling only works in dry air. At 20% relative humidity it is a genuine air conditioner at a tenth of the running cost; at 60% it is an expensive fan that makes the room clammy. The rough cut-off is around 50% — above that, buy a compressor.

Volume in, CFM out, and the exhaust opening that has to come with it.

Space	Volume	CFM needed	Exhaust opening
12 × 12 ft bedroom, 8 ft ceiling	1,150 cu ft	600	half a window
20 × 15 ft living room, 8 ft ceiling	2,400 cu ft	1,200	one window
24 × 20 ft garage, 10 ft ceiling	4,800 cu ft	2,400	side door ajar
1,000 sq ft house, 8 ft ceilings	8,000 cu ft	4,000	four windows cracked
1,800 sq ft house, 9 ft ceilings	16,200 cu ft	8,100	windows on the far side

The exhaust column is not advice, it is part of the specification. A cooler with nowhere to push the air is a humidifier with a big motor.

Frequently asked questions

How many CFM do I need for a 1,000 square foot house?

With 8 ft ceilings that is 8,000 cubic feet, so about 4,000 CFM. Whole-house evaporative coolers are sold in that range, and it is worth going to the next size up rather than down: an undersized unit runs flat out permanently, which is both noisier and wetter than a larger one on a low speed.

Does a swamp cooler work in humid weather?

Barely. Evaporative cooling comes from water changing state, and humid air cannot absorb much more water — at 60% relative humidity you might get 5 °F and a damp room. Below about 40% the same machine gives 15 to 20 °F. If your summers are humid, this is the wrong machine, not the wrong size.

Do I have to leave a window open?

Yes, and this is the single most common mistake. The cooler pushes outdoor air in, so an equal amount has to leave. Allow roughly one square foot of open window or door per 1,000 CFM, on the far side of the space from the cooler, so the cool air crosses the room on its way out.

How much does an evaporative cooler cost to run against an air conditioner?

Roughly a tenth to a quarter. A 4,000 CFM cooler draws around 400 to 700 watts — a fan motor and a small pump — where a central air conditioner covering the same house draws 3,000 watts or more. The trade-off is that it only performs in dry air and it uses water, typically 3 to 15 gallons a day.

What is the difference between a swamp cooler and an air conditioner?

An air conditioner is a closed loop that removes heat and moisture from the air already in the room. An evaporative cooler is an open loop: it takes outdoor air, drags it through a wet pad, and blows it in. That is why one wants the house sealed and the other wants a window open — and why sizing is in CFM here and BTU there.

Should I size on floor area or room volume?

Volume. Two rooms with the same floor plan and different ceiling heights hold different amounts of air, and the machine’s job is to replace all of it twice a minute. A 300 sq ft room with a 12 ft ceiling needs half again as much CFM as the same room with an 8 ft ceiling.

How we calculate this

CFM is calculated as room volume ÷ 2, which is the industry standard sizing rule of two air changes per minute (volume × 30 air changes per hour ÷ 60 minutes gives the same figure). Volume is floor area × ceiling height, so ceiling height is a direct multiplier rather than a correction. The result is then rounded up to the nearest size evaporative coolers are actually sold in. The 15–20 °F temperature drop quoted applies in air below about 40% relative humidity; at higher humidity the achievable drop falls sharply, because the cooling comes entirely from water evaporating into air that still has room for it. The one square foot of exhaust opening per 1,000 CFM is the figure published by evaporative cooler manufacturers for relief openings.

Worked example

With Room length (ft) 20, Room width (ft) 15, Ceiling height (ft) 8, this page works out 1,300 CFM. The room works out at 1,200 CFM — 1,300 is the nearest size actually sold.

Floor area300 sq ft
Room volume2,400 cu ft · 68 m³
Working rate0.5 CFM per cu ft

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