- 1-car garage (12 × 22, 264 sq ft): 8,300 to 9,200 BTU, so a 10,000 BTU unit.
- 2-car garage (20 × 20 to 24 × 24): 12,600 to 20,000 BTU, so a 14,000 to 24,000 BTU unit.
- 3-car garage (30 × 24, 720 sq ft): 22,800 to 25,000 BTU, so a 24,000 to 30,000 BTU unit.
That is for a typical garage: no insulation in the door or walls, afternoon sun on the door and a 10 ft ceiling, in a mild to hot climate. An insulated garage often needs one size less. Run your own garage in the calculator.
A garage needs a lot more cooling than a bedroom with the same floor area. A 24 × 24 bedroom needs about 11,500 BTU. A typical 24 × 24 garage in a hot climate needs 20,000, almost twice as much. That is why so many people buy a 12,000 BTUunit for a 2-car garage and find it can't keep up. This guide shows where the extra comes from, gives real numbers for 1, 2 and 3-car garages, and helps you pick the type of unit that actually works out there.
Why a garage needs more than a bedroom
The usual rule is 20 BTU per square foot. That rule assumes a normal room in a house. A garage breaks it in four ways:
- The big door. A 16 ft steel door with no insulation is a giant radiator. If it faces west, it soaks up the afternoon sun and gets hot to the touch. The seals around it also leak hot air in.
- Bare walls and a hot roof. Many garages have no insulation in the walls or ceiling, and the attic or roof above can get very hot.
- Tall ceilings. Garages with door openers are often 9 or 10 ft tall. More air means more to cool: a 10 ft ceiling adds 25%.
- Car heat. A car you just drove in keeps giving off heat from the engine and tires for a while. The calculator does not count this, so if you park a hot car inside and want the garage cool right away, aim for the top of the range.
In the calculator, a typical garage means poor insulation (+15%), very sunny (+10%) and a 10 ft ceiling (+25%). In a hot climate, add another 10%. Those stack on top of each other, which is how a garage ends up needing nearly double.
Garage AC size chart
| Garage | Floor area | Mild climate | Hot climate | Unit to buy |
|---|---|---|---|---|
| 1-car, 12 × 22 | 264 sq ft | 8,300 BTU | 9,200 BTU | 10,000 BTU, window unit or mini-split |
| 2-car, 20 × 20 | 400 sq ft | 12,600 BTU | 13,900 BTU | 14,000 BTU, window unit or mini-split |
| 2-car, 24 × 24 | 576 sq ft | 18,200 BTU | 20,000 BTU | 24,000 BTU, mini-split |
| 3-car, 30 × 24 | 720 sq ft | 22,800 BTU | 25,000 BTU | 24,000 to 30,000 BTU, mini-split |
Compare those with the same sizes as normal rooms on the 250 sq ft, 400 sq ft, 600 sq ft and 700 sq ft pages, and you can see the garage penalty clearly.
A lower ceiling helps. A 20 × 20 garage with an 8 ft ceiling in a hot climate needs 11,100 BTU instead of 13,900. If your garage is shaded most of the day or the door faces north, set sun to average in the calculator and the number comes down again.
Worked example: a 24 × 24 garage in a hot climate
Here is how the calculator gets to its answer for a 24 × 24 garage with a bare door, afternoon sun and a 10 ft ceiling in a hot climate:
- 576 sq ft × 20 BTU = 11,520 BTU to start
- 10 ft ceiling, × 1.25 = 14,400
- Very sunny, × 1.10 = 15,840
- Poor insulation, × 1.15 = 18,216
- Hot climate, × 1.10 = 20,038
The calculator rounds that to 20,000 BTU (range 18,000 to 22,000) and recommends a 24,000 BTU unit. At that size you are shopping for a 24,000 BTU mini-split, not a window unit. Open this example in the calculator and change any input to match your own garage.
Insulate the door before you buy a bigger AC
The cheapest BTU is the one you don't need. A garage door insulation kit (foam or foil panels cut to fit each section of the door) costs far less than the next size of AC and takes an afternoon. Add a new rubber seal along the bottom and weatherstrip the sides while you are at it.
With the door and walls insulated, the calculator moves from poor to average insulation. For the 24 × 24 garage above, that takes the load from 20,000 to 17,400 BTU, and the unit from 24,000 down to 18,000 BTU. A 1-car garage goes from 9,200 to 8,000 BTU, which is an 8,000 BTU unit instead of a 10,000.
The door alone won't get you all the way there if the walls and ceiling are bare, but it is the biggest single surface, so start there. Our insulation guide explains how to judge your walls and ceiling.
Which type of AC works in a garage
Mini-split: best if you use the garage often
A ductless mini-split is the best choice for a garage gym, workshop or hangout you use most days. It is quiet, cheap to run, and the inverter slows down on mild days instead of switching on and off. Most also heat in winter, which covers the job on our garage heater page. Mini-splits come in 9,000, 12,000, 18,000 and 24,000 BTU sizes, so round up: a 20 × 20 garage that needs 13,900 BTU gets an 18,000. Because it can slow down, that extra room will not cause the problems an oversized window unit would. The mini-split sizing guide has more.
Window unit: needs a window or a wall sleeve
A window unit gives you the most cooling for the money, but many garages have no window. The fix is a through-the-wall unit in a wall sleeve, which means cutting a hole in the wall. Window units make sense up to about 14,000 BTU, which covers a 1-car garage (10,000 BTU) or a 20 × 20 2-car garage (14,000 BTU). Most units in that range plug into a normal outlet. Bigger garages are mini-split territory.
Portable: the weakest option
A portable is the easiest to set up and the worst at cooling a garage. The number on the box overstates what it really does, because the hot parts of the machine sit inside with you. A single-hose portable also blows garage air out the hose, which pulls hot outside air in through every gap around the big door. A dual-hose model is much better in a garage, since it takes outside air in through one hose and sends it back out the other. Even so, a 14,000 BTU dual-hose portable really delivers about 9,500 to 11,000 BTU. That covers a 1-car garage, not a 2-car one. Our window vs portable guide explains the real ratings.