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Calculator · Garage AC
Garage AC size calculator
Enter the garage size, insulation, garage door, windows and the heat you have to cool against. The calculator adds up heat gain surface by surface, then shows the cooling load in BTU/h, the same load in tons of refrigeration and the sensible part. It does not pick a unit, so it works for a window unit, a mini split or a ducted system alike.
- Enter the inside size, wall height and which walls face outside.
- Choose wall and ceiling insulation, then enter the garage door, the windows and the direction they face, and any entry door.
- Enter the summer design temperature and climate zone for your location, the temperature you want inside, the people and the tools and lights that run at the hottest time.
- Compare the BTU/h line with the capacity of a unit at your design temperatures. The tons line is the same load in another unit.
Example result for the default inputs
24 x 24 ft garage, 9 ft walls, 3 exterior walls, a long side against the house, zone 3A, held at 78°F with 95°F outside.
Cooling load
3,955 BTU/h
Same load in tons of refrigeration, 12,000 BTU/h per ton
0.33 tons
Sensible part of the load
3,755 BTU/h
| Floor area | 576 sq ft |
|---|---|
| Volume | 5,184 cu ft |
| Exterior wall length | 72 ft |
| Temperature difference | 17°F |
| Walls, net of openings, wall factor 1.05 | 728 BTU/h |
| Garage door | 952 BTU/h |
| Window conduction | 50 BTU/h |
| Sun through windows, W factor 1.00, shading factor 1.00 | 335 BTU/h |
| Entry door | 0 BTU/h |
| Ceiling, roof factor 1.15 | 593 BTU/h |
| Slab | 392 BTU/h |
| Air leakage, 0.30 ACH, 26 CFM | 476 BTU/h |
| 1 person, sensible | 230 BTU/h |
| Tools and lights, 0 W | 0 BTU/h |
| People, latent | 200 BTU/h |
| Sensible part of the cooling load | 3,755 BTU/h |
| Cooling load per sq ft of floor | 6.9 BTU/h |
For estimation purposes only. Consult a licensed engineer for final designs.
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How the cooling load is calculated
Heat gets into a garage through its surfaces, through air leaking in, through the windows as sunlight, and from the people and equipment inside. Each surface gains heat in proportion to its area, how poorly it insulates and the gap between outside and the temperature you want. The total is the cooling load on a design-hot afternoon.
- Temperature difference
dT = summer outdoor design temperature - temperature you want in the garageIf outdoors is at or below the target, the wall, door, glass conduction, ceiling, slab and air leakage lines are zero. The calculator never reports a negative gain. Sun through the glass, people and equipment still count.- Exterior walls
Q = (exterior wall length x wall height - window, door and garage door area) x (1 / R) x wall factor x dTExterior wall length follows the walls you pick. A long side against the house removes one long side, a corner removes one long and one short side. The wall factor is a BuildSolver engine multiplier by climate zone, 1.15 in zones 1A to 2B, 1.05 in 3A to 5C and 1.00 in 6A to 8. The R-value you pick is treated as the whole wall. A wall with no insulation is taken as R-4.- Garage door
Q = door width x door height x U x dTAn uninsulated steel door is taken as U 1.2. A bare steel panel is little more than its two air films, which DASMA Technical Data Sheet 163 puts at R 0.85, so its U is close to 1 / 0.85 = 1.18 before joints and perimeter gaps. An insulated door is taken as U 0.5 unless you enter a tested U-factor. This line has no sun term, so a door that faces the afternoon sun is gaining more than shown.- Windows, conduction
Q = window area x U x dTWindow U by type is 1.04 single pane, 0.49 double clear, 0.32 double low-e and 0.22 triple. An opaque entry door uses U 0.40. These are BuildSolver engine defaults, used here as assumptions.- Windows, sun
Q = window area x SHGC x zone sun factor x direction factor x shading factorSHGC is 0.86 single pane, 0.62 double clear, 0.40 double low-e and 0.35 triple. The zone sun factor runs from 55 to 100 BTU/h per sq ft of glass, by climate zone. The direction factor is north 0.40, northeast and northwest 0.65, east and west 1.00, southeast and southwest 0.85, south 0.70. All the glass takes the factor of the direction you pick. Shading is the number you enter, 1 for bare glass. All of these are BuildSolver engine values.- Ceiling and slab
Q = floor area x (1 / R) x roof factor x dT, and Q = slab U x floor area x dTThe roof factor allows for sun on the roof and is 1.40 in zones 1A to 2B, 1.15 in 3A to 5C and 1.00 in 6A to 8. An open rafter ceiling with no insulation is taken as R-3, this calculator's assumption. With a conditioned room above, the ceiling line is zero. The slab uses U 0.04 on the full floor area, the engine value for a slab on grade.- Air leakage
CFM = ACH x garage volume / 60, Q = 1.08 x CFM x dTSummer air changes per hour are 0.10 tight, 0.18 average and 0.30 leaky, from the BuildSolver engine. The very leaky choice at 0.60 ACH is this calculator's assumption for a garage with visible gaps at the door. 1.08 is the standard-air sensible heat factor at sea level (60 min/h x 0.075 lb/cu ft x 0.24 BTU/lb-°F, ASHRAE Handbook of Fundamentals). Only the sensible heat of that air is counted.- People and equipment
Q = people x 230 + watts x 3.412, latent = people x 200Each person gives off 230 BTU/h of sensible heat and 200 BTU/h of latent heat, BuildSolver engine defaults. Lights and tools turn their electric power into heat. The U.S. Energy Information Administration lists 1 kilowatthour as 3,412 Btu, so 1 watt is 3.412 BTU/h. All of the watts you enter count as heat.- Total and tons
load = sum of the lines above, tons = load / 12,000The ton of refrigeration is defined as 12,000 BTU/h (NIST SP 811, Appendix B.8). Dividing by it only changes the unit. It is not a unit size. The latent line covers the people only, so the page shows the sensible part of the load beside the total.
Worked examples for five garages
Row one is the default on the form. The second keeps the same garage and strips the insulation, the door insulation and the weatherstripping. Row three puts people and tools in the default garage. In the last two, the size, the walls against the house, the glass and the climate change.
| Garage | Cooling load | Tons | Sensible part |
|---|---|---|---|
| 24 x 24 attached on a long side, R-13 walls, R-19 ceiling, insulated door, one person, 78°F with 95°F outside | 3,955 BTU/h | 0.33 tons | 3,755 BTU/h |
| Same garage, no insulation, uninsulated door, very leaky | 10,562 BTU/h | 0.88 tons | 10,362 BTU/h |
| Same garage as a workshop, two people and 1,500 W of tools and lights | 9,503 BTU/h | 0.79 tons | 9,103 BTU/h |
| 12 x 22 one-car at a corner of the house, R-13 walls, R-30 ceiling, no windows | 1,798 BTU/h | 0.15 tons | 1,598 BTU/h |
| 30 x 40 detached shop, R-19 walls, R-38 ceiling, 24 sq ft of south low-e glass, zone 2A, 80°F with 98°F outside, two people and 2,000 W | 14,502 BTU/h | 1.21 tons | 14,102 BTU/h |
Taking the insulation out of the same 24 x 24 ft garage multiplies the cooling load by 2.7. Adding 2 people and 1,500 W of tools and lights multiplies it by 2.4, and the tools and lights becomes the largest single line at 54% of the load. In the insulated default, the garage door carries the most, 24% of the load. The default comes to 0.33 tons, which may be less than the smallest unit sold. Check the line-by-line result for your own garage, then compare the BTU/h with the capacity of a unit at your design temperatures.
Standards and data sources
- NIST Special Publication 811, Appendix B.8, conversion factors - the ton of refrigeration as 12,000 BtuIT/h behind the tons line.
- U.S. Energy Information Administration, British thermal units (Btu) - the conversion 1 kilowatthour = 3,412 Btu behind the watts of tools and lights.
- DASMA Technical Data Sheet 163, U-factor and R-value for Residential and Commercial Garage Doors - the air film figure of R 0.85 and the note that steel facings add almost nothing, behind the uninsulated door assumption. DASMA also explains why a tested U-factor beats a calculated section R-value.
- Building America Solution Center, Ductless (Mini-Split) Heat Pumps - the guide from Pacific Northwest National Laboratory for the U.S. Department of Energy says that "Oversized or incorrectly located indoor units can result in short cycling, which wastes energy and does not provide proper temperature or humidity control" and that "even the smallest indoor heads may provide much more heating or cooling than is needed for the individual space." Both bear on a small garage load.
- BuildSolver load engine defaults - window and door U-values, SHGC, slab U, wall and roof sun factors, zone sun factors, direction factors, air change rates and people gains. The engine code names ACCA Manual J 8th edition and ASHRAE Fundamentals tables as their origin, and BuildSolver adjusted several values after comparing them against completed Manual J reports. This page uses them as assumptions, not as quoted ACCA figures. The summer design temperature and climate zone are entered by you, and the HVAC design temperatures page lists both by ZIP.
Limitations of this calculator
- This is a simplified heat gain estimate for one space. It is not a Manual J load calculation and not a room-by-room calculation of the house.
- It does not pick a unit and does not name a nominal capacity. The tons line is the load divided by the ton definition, not an equipment size. Capacity at your design temperatures comes from the manufacturer's performance data, and equipment selection is ACCA Manual S, on Pro and Team.
- The garage door gets no sun term. A dark door facing the afternoon sun is hotter than the air, so a garage with a big sunny door gains more than this page shows.
- The heat of a car that has just been driven, a hot water heater or a dryer vent is not modeled. Add those to the tools and lights line in watts if they run at the hottest time. The tools line counts every watt as heat.
- The estimate covers steady gains with the door closed. An open garage door lets in hot, humid air, and pulling the temperature back down afterward takes more cooling than the steady number.
- Moisture in the outside air leaking in is not counted. The only latent line is the people, a BuildSolver engine default, so there is no sensible heat ratio to check equipment against. In a humid climate the real latent load is higher.
- All the glass takes the direction factor of the one direction you pick, and the page does not know which compass side each wall of the rectangle faces.
- Nominal R-values are used as the whole wall or ceiling, with no allowance for studs, air films or insulation gaps. Block and uninsulated metal buildings can gain more than the assumptions here, and a cool roof or radiant barrier is not credited.
- Heat that moves through a shared wall from the house is not counted. If the garage is held below the house setting, that wall adds a little gain.
- Duct gains and altitude are left out. Air density is taken at sea level, so air leakage runs high at elevation, and a ducted system with ducts in a hot attic needs more than the number here.
Common garage cooling mistakes
- Sizing by floor area. The same 576 sq ft garage works out to 6.9 BTU/h per sq ft with insulation and an insulated door, and 18.3 BTU/h per sq ft without them. A rule of thumb in BTU per sq ft cannot tell those two apart.
- Leaving out the shop's own heat. A garage used as a workshop has people and tools in it. In the workshop example, 2 people and 1,500 W of tools and lights add 5,118 BTU/h from the equipment alone, 54% of the load, and the whole load is 2.4 times that of the empty garage.
- Rounding up to be safe. A unit that is bigger than the load reaches the temperature quickly, shuts off and does not run long enough to pull moisture out of the air. The Building America Solution Center says oversized units can short cycle, which wastes energy and does not control temperature or humidity properly. Compare the load with the capacity at your design conditions instead.
- Taking the tons line as the size to buy. Tons here is the load divided by 12,000 BTU/h and nothing more. The default example comes to 0.33 tons, and units are sold in fixed capacities. The smallest ductless indoor heads may provide much more cooling than a small insulated garage needs, so a small insulated garage can sit below the smallest unit on the market.
- Using an average July temperature. An average day runs below the afternoons the cooling has to cover. Use the 1% cooling design temperature for the location, or the garage will fall behind on the hottest days.
- Cooling an uninsulated garage as it is. Taking the insulation out of the example garage, along with the door insulation and the weatherstripping, multiplies the load by 2.7. Insulation, a better door and weatherstripping change the load a lot before any equipment is chosen.
Common questions about cooling a garage
What size AC do I need for a two-car garage?
It depends on the insulation, the door, the sun and the climate. In the default example on this page, a 24 x 24 ft attached garage with a long side against the house, R-13 walls, an R-19 ceiling and an insulated door, with one person in it, held at 78°F with 95°F outside in climate zone 3A, has a cooling load of about 4,000 BTU/h. That is 0.33 tons of refrigeration.
Is a load of 0.33 tons really all a garage needs?
Treat it as a floor, not a size. The example gives 3,955 BTU/h, 6.9 BTU/h per sq ft, but it leaves out sun on the overhead door and the heat of a car that just came in, and it counts the attic over the ceiling with a fixed allowance rather than its real summer temperature. A real garage often needs more. The same garage with no insulation, an uninsulated door and gaps around the door comes to about 10,600 BTU/h, or 0.88 tons. Equipment is sold in fixed sizes, and the smallest may be larger than the load. Compare the load with the capacity on the manufacturer's performance data.
What size mini split do I need for a garage workshop?
Start from the load, not the floor area. The workshop example on this page is the same 24 x 24 ft garage with 2 people and 1,500 W of tools and lights running at the hottest time. Its cooling load is about 9,500 BTU/h, or 0.79 tons. The unit that covers it at your design temperatures comes from the manufacturer's performance data, and this page does not name one.
Does the direction of the windows matter?
Yes. Sun through glass uses a direction factor of 1.00 for east or west, 0.70 for south and 0.40 for north. In the default example, 6 sq ft of double pane glass gives 335 BTU/h of sun gain facing west and 134 BTU/h facing north, which is 5% off the whole load. A garage with a lot of glass sees a much bigger difference. If the windows face several ways, pick the sunniest.
How much does the garage door add?
In the default example the insulated garage door carries 24% of the cooling load. This calculator uses U 1.2 for an uninsulated door and U 0.5 for an insulated one. If the door has a tested U-factor from the DASMA program, enter it for a closer number. The door gets no sun term here, so a door in the afternoon sun gains more than the page shows.
Which outside temperature and climate zone do I use?
Use the 1% summer cooling design temperature for your location, not the record high and not a monthly average. The IECC climate zone sets the sun and roof factors. The HVAC design temperatures page on BuildSolver lists the temperature and the zone by ZIP code.
Does this account for humidity?
Only for the people in the garage. Moisture in the air leaking in is not counted, so the page reports no sensible heat ratio and gives no dehumidification number. In a humid climate, check moisture removal against the equipment data. If the garage needs standalone dehumidification, the dehumidifier size calculator covers that.
Is this a Manual J calculation?
No. It is a simplified heat gain estimate for one space, using U times area times temperature difference, sun through glass, air leakage, people and equipment. A room-by-room Manual J for a whole house is a separate BuildSolver Pro and Team feature.
Used on the same job
Run the whole-house load in chat
The chat has no garage or single-space tool, so the garage number comes from this page. Describe the house in chat and the assistant runs the free whole-house Manual J heating and cooling load for the same job.
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