Why a Garage Cannot Be Sized Like a Room
Run a garage through a normal room calculator and the answer comes out too small, sometimes by half. The reason is that every assumption a room calculator makes is wrong for a garage.
A bedroom is surrounded by other heated rooms, so most of its surfaces barely lose anything. A detached garage loses heat on all four walls, the ceiling, and the slab, with nothing warm on the other side of any of them. Its ceiling is often open to the roof deck. Its largest single surface is a steel door on rollers that does not seal, and unless someone paid extra, that door is about R-2.
So this calculator works the way a load calculation actually works: it takes each surface, applies a real U-value, adds the air leaking through a door that was never built to seal, and multiplies by your climate's design temperature difference. That is why two identical 450 sq ft garages on this page can differ by a factor of 2.6 in what they need.
Every U-value, air-change rate and design temperature difference behind that is published rather than hidden. You can read the whole model, including the door and insulation tables and the equipment ladders it rounds onto, in our garage sizing methodology.
Garage Heater Size by BTU
These are the sizes garage heaters are sold in, and the garage each one realistically serves. Electric equivalents are listed because below about 30,000 BTU the market is mostly electric.
30000 BTU Garage Heater
A 30,000 BTU heater is the entry point for gas unit heaters and covers an insulated 2-car garage in a mixed climate with margin, or an insulated 4-car in a mild one. If your garage is uninsulated and you are in a cold climate, this size will run continuously and still lose ground on the worst days. It is the right answer far more often than people expect, though, because most buyers overestimate what a garage needs.
45000 BTU Garage Heater
45,000 BTU suits an uninsulated 2-car garage in a cool climate, which calculates to about 36,400 BTU, or a well-insulated 3-car in a cold one. This is the size where you should stop and ask whether insulating first would be cheaper than the extra heater capacity, because the answer is usually yes and it lasts the life of the building.
50000 BTU Garage Heater
A 50,000 BTU heater covers the case this calculator sees most often at the top end: an uninsulated 2-car garage in a cold climate, which comes out around 46,700 BTU. Worth knowing that the same garage with walls, ceiling, and door insulated needs about 17,800 BTU, so this heater size is very often a symptom of the envelope rather than a requirement of the space.
80000 BTU Garage Heater
80,000 BTU is a large-garage or workshop size, suiting uninsulated 3 and 4-car garages in cold climates. At this output a single heater struggles to move air across the whole space, so two smaller units at opposite ends, or one unit plus a circulating fan, holds a more even temperature than one large heater blowing down the middle.
Mini Split for a Garage: What Size Head
Ductless is the usual way to cool a garage, because most garages have no ductwork and no window worth using. The capacities work out lower than people expect.
An insulated 2-car garage in a mixed climate calculates to about 6,100 BTU of cooling. The smallest ductless head commonly sold is 9,000 BTU, so that is the answer, and it is oversized on paper. An uninsulated 2-car in a hot climate reaches about 15,100 BTU and genuinely needs an 18,000 head. Between those two extremes almost every garage lands on 9,000 or 12,000.
If your garage calculates under 9,000 BTU, buy the 9,000 and expect it to cycle rather than hunting for something smaller. The same market-floor problem exists here as with the smallest window units. For sizing a ductless head in a normal room rather than a garage, use the mini split size calculator, and to price the install, the mini split installation cost calculator.
What Actually Happens When a Garage Mini Split Is Undersized
The numbers above are the calculation. This is what it looks like when someone gets it wrong, taken from owners posting their own results.
One owner in Arizona put a 12,000 BTU head into a garage just under 400 sq ft. He had done the work most people skip first: new garage door seals, new side weather stripping, and insulation added to the door itself. He ran a stress test for four to five hours with no cars inside and the main door shut the entire time. The coldest it reached was 84.4°F, with the unit at full blast, set to 61°F, fan on turbo.
Nothing was wrong with the equipment. The replies identified what the calculator above would have: the ceiling was not properly insulated and the wall the unit was mounted on was uninsulated, on a west-facing elevation taking full afternoon sun. He had insulated the one surface that is easiest to see and left the two that were losing the most.
Run that garage through this page as hot climate with no ceiling insulation and it calls for roughly 15,000 BTU, which rounds to an 18,000 head. He bought 12,000. On a separate thread about a 400 sq ft garage in South Florida, the advice landed in the same place: insulate walls as well as ceiling and door, then “the 18K should be able to cool the space easily”, with one commenter noting that 400 sq ft is “possibly pushing the limit for 12,000 BTU for an uninsulated garage”.
Two more things from those threads worth carrying into your own decision. A commenter's rule that no air conditioner should be expected to hold more than about 25°F below outdoor temperature explains a lot of garage disappointment on 110°F days regardless of size. And opening the main door resets everything: as one put it, you lose every bit of cooling and start from zero, which is hours of pull-down rather than minutes. A car pulled in adds to that, since it arrives as roughly a ton of metal sitting well above ambient.
The Garage Door Is the Surface Worth Fixing
There is a widely repeated claim that the overhead door is the biggest source of heat loss in a garage. That is true, but only under a condition nobody mentions.
In a completely uninsulated garage the door accounts for only about 12% of the heat loss, because the bare walls and open ceiling are losing so much more. Insulate the walls and ceiling and the picture inverts: the door becomes roughly 25% of what remains, and the largest single surface in the building.
The practical consequence is a sequence rather than a single fix. Insulate the ceiling first, since heat rises and an open roof deck is the easiest escape. Then the walls. Then the door, which on an otherwise-insulated 2-car garage in a cold climate takes the requirement from about 22,100 BTU down to 17,800, a cut of roughly 20% from one component.
Attached vs Detached Garages
A wall shared with a heated house loses almost nothing compared with a wall facing outside, so attachment changes the number materially. A finished, detached 2-car garage needing about 17,800 BTU in a cold climate drops to roughly 12,800 BTU when attached on two sides, about 28% less for the same floor area. It also means an attached garage is quietly stealing heat from the house next to it, which is part of why the room on the other side of a garage wall is so often the coldest in the house.
Window AC in a Garage
If your garage has a window that takes one, a window unit is the cheapest way to cool it, and the sizing carries straight across from the cooling figure above. Two practical catches. Most garages have no suitable window, and those that do often have small hopper or slider openings that standard units do not fit. And a garage runs far dirtier than a house, so the filter needs cleaning much more often than the manual suggests. Size one with the window AC size calculator.
Mini Split for a Shed or Detached Workshop
A shed or a detached workshop is a garage without the car, and it sizes the same way: every surface faces outdoors, so the load is driven by the envelope rather than the floor area. The difference is usually in your favour. Sheds are smaller, they have no large uninsulated door, and nobody parks a hot engine in one.
The constraint is normally electrical rather than thermal. Plenty of sheds run off a single circuit or an extension lead, and a mini split needs its own supply. Work out the load with the calculator above, then check what the building actually has before choosing a size. If the run from the panel is long or the shed is fed by a lead across the garden, sort the power out first, because that decides the job more often than the BTU number does.