How many BTU do I need for a 2 car garage?

Measure yours: we found no published standard size for a two-car garage that we could quote. At 20 x 20 ft with an 8 ft ceiling and a 60°F rise, the formula gives 17,300-21,100 BTU/h well insulated and 28,800-36,500 BTU/h poorly insulated; at 24 x 24 ft, 24,900-30,400 and 41,500-52,500. Table 1 also has 25 x 25, 30 x 30 and 30 x 40 ft.

How many BTU to heat a 30x40 garage?

9,600 cu ft at an 8 ft ceiling: 51,800-63,400 BTU/h well insulated, 69,100-80,600 moderately insulated and 86,400-109,400 poorly insulated, at a 60°F rise. With a 10 ft ceiling every figure is 25 percent higher.

How many BTU for an uninsulated garage?

Use the poorly insulated factor, 0.15-0.19, which the heater maker describes as "uninsulated walls, single-pane windows, gaps around doors". A 24 x 24 ft garage then needs 41,500-52,500 BTU/h at a 60°F rise, against 24,900-30,400 well insulated. The same maker adds: "Sealing and insulating a space before sizing often lets you install a smaller, less expensive heater."

How many watts to heat a garage?

Divide BTU/h by 3,412 for kW (1,000 W is about 3,412 BTU/h). A 24 x 24 ft garage at a 60°F rise is 7.3-8.9 kW well insulated and 12.2-15.4 kW poorly insulated.

What size mini split for a garage?

For cooling, the chart figure for the floor area, then the next size in the lineup: a 24 x 24 ft garage is 14,000 BTU/h, so the next size is 18,000 BTU/h (1.5 tons). Heating needs more here: 24,900-30,400 BTU/h well insulated at a 60°F rise; at the midpoint of that range the next size is 30,000 BTU/h (2.5 tons). Check the heating capacity at your coldest temperature.

How many BTU AC for a garage?

On the ENERGY STAR room chart, read as a room in a home: 20 x 20 ft is 10,000 BTU/h, 24 x 24 ft is 14,000 and 30 x 40 ft is 23,000; very sunny adds 10 percent. The chart has no adjustment for an uninsulated garage, so we do not add one.

How many BTU do I need for a garage? (heater, AC and mini split, 20x20 to 30x40)

A heater maker's sizing formula multiplies the garage's volume in cubic feet by an insulation factor and by the temperature rise you want. For a 24 x 24 ft garage with an 8 ft ceiling and a 60°F rise, that is 24,900-30,400 BTU/h well insulated and 41,500-52,500 BTU/h poorly insulated or uninsulated. To cool the same 576 sq ft, the ENERGY STAR room chart gives 14,000 BTU/h if you treat the garage like a room in a home. The tables cover 20 x 20 to 30 x 40 ft, with the mini split size for each.

Open the AC BTU calculator

1. How many BTU to heat a garage: the formula

Combustion Research Corporation, a maker of gas-fired radiant heaters, publishes this method: "BTUs Needed = Cubic Feet × Insulation Factor × Temperature Rise". Cubic feet are length x width x ceiling height. The insulation factor comes in three ranges:

Poorly insulated ("uninsulated walls, single-pane windows, gaps around doors"): 0.15-0.19.
Moderately insulated ("some wall/ceiling insulation, standard doors"): 0.12-0.14.
Well insulated ("fully insulated envelope, sealed doors, tight construction"): 0.09-0.11.

"Temperature rise is your target indoor temperature minus the coldest average outdoor low for your region." The maker's worked example uses "65°F indoor target minus 5°F outdoor low", a 60°F rise, and its 24 x 24 example has an 8-foot ceiling, so table 1 uses both. The same example, a 30 x 40 ft workshop with a 12 ft ceiling at a factor of 0.12, comes to 103,680 BTU in our code, the figure the maker prints. The maker calls the method "a widely used quick-estimate approach, not a substitute for a full ASHRAE-style heat-loss calculation".

Table 1: BTU/h to heat a garage by size and insulation (8 ft ceiling, 60°F rise; low and high end of each factor range)
Garage	Floor (sq ft)	Volume at 8 ft (cu ft)	Well insulated	Moderately insulated	Poorly insulated or uninsulated
20 x 20 ft	400	3,200	17,300-21,100	23,000-26,900	28,800-36,500
24 x 24 ft	576	4,608	24,900-30,400	33,200-38,700	41,500-52,500
25 x 25 ft	625	5,000	27,000-33,000	36,000-42,000	45,000-57,000
30 x 30 ft	900	7,200	38,900-47,500	51,800-60,500	64,800-82,100
30 x 40 ft	1,200	9,600	51,800-63,400	69,100-80,600	86,400-109,400

Every figure is proportional to the rise and to the ceiling height: at a 30°F rise, halve them; with a 10 ft ceiling, add 25 percent. For a 24 x 24 service bay with an 8-foot ceiling the maker's own answer is "25,000-40,000 BTU", "depending on insulation quality and your regional climate"; the formula gives 24,900 to 38,700 BTU/h for the well and moderately insulated columns. For an electric heater, divide by 3,412 for kW: the 24 x 24 garage is 7.3-8.9 kW well insulated and 12.2-15.4 kW poorly insulated. The maker's advice is to "round up to the nearest available heater rating", not to double it.

2. How many BTU to cool a garage

The ENERGY STAR chart sizes a room air conditioner for a room in a home, and "the capacities are calculated based on an 8-foot ceiling". A garage is not a room in a home, so read table 2 as what the chart gives for the same floor area, nothing more. The only adjustments here are the chart's own: very sunny adds 10 percent (heavily shaded takes 10 percent off). We found no published adjustment for an uninsulated garage, so we do not invent one; an installer's load calculation is the way to size cooling for one.

Table 2: the ENERGY STAR room chart for a garage's floor area (8 ft ceiling, average sun, two people), with the next single-zone mini split size
Garage	Floor (sq ft)	Chart band (sq ft)	BTU/h	Very sunny	Next mini split size
20 x 20 ft	400	400 up to 450	10,000	11,000	12,000 BTU/h (1 ton)
24 x 24 ft	576	550 up to 700	14,000	15,400	18,000 BTU/h (1.5 tons)
25 x 25 ft	625	550 up to 700	14,000	15,400	18,000 BTU/h (1.5 tons)
30 x 30 ft	900	700 up to 1,000	18,000	19,800	18,000 BTU/h (1.5 tons)
30 x 40 ft	1,200	1,200 up to 1,400	23,000	25,300	24,000 BTU/h (2 tons)

The AC BTU calculator reads the chart for any size and adds people and a taller ceiling; the BTU to tons converter turns any of these figures into tons and kW.

3. A mini split for a garage

A mini split can be a heat pump or cooling-only: Mitsubishi describes its MUZ-GS heat pump as "a single-zone heating and cooling solution" and its MUY-GS as a cooling-only system for "climates where heating is unnecessary". A heat pump can cool and heat the garage, but the two needs differ: a 24 x 24 garage is 14,000 BTU/h to cool on the chart and 24,900-30,400 BTU/h to heat well insulated at a 60°F rise. The last column of table 2 is the next size in one current lineup (9, 12, 18, 24, 30, 36 thousand BTU/h, Fujitsu's Altair 500) for the cooling figure; for heating, the midpoint of that range would take 30,000 BTU/h (2.5 tons).

Check the heating capacity at your coldest temperature, not only the nameplate. To earn ENERGY STAR's cold climate designation, a heat pump must keep at least 70 percent of its 47°F heating capacity at 5°F. Makers also state a heating operation range: 5°F to 75°F for the Altair 500, and "-15°F to 75°F" for Fujitsu's Orion XLTH line. The mini split size guide has 300 to 1,500 sq ft, one zone or several, and the heating side.

Frequently asked questions

How many BTU do I need for a 2 car garage?: Measure yours: we found no published standard size for a two-car garage that we could quote. At 20 x 20 ft with an 8 ft ceiling and a 60°F rise, the formula gives 17,300-21,100 BTU/h well insulated and 28,800-36,500 BTU/h poorly insulated; at 24 x 24 ft, 24,900-30,400 and 41,500-52,500. Table 1 also has 25 x 25, 30 x 30 and 30 x 40 ft.
How many BTU to heat a 30x40 garage?: 9,600 cu ft at an 8 ft ceiling: 51,800-63,400 BTU/h well insulated, 69,100-80,600 moderately insulated and 86,400-109,400 poorly insulated, at a 60°F rise. With a 10 ft ceiling every figure is 25 percent higher.
How many BTU for an uninsulated garage?: Use the poorly insulated factor, 0.15-0.19, which the heater maker describes as "uninsulated walls, single-pane windows, gaps around doors". A 24 x 24 ft garage then needs 41,500-52,500 BTU/h at a 60°F rise, against 24,900-30,400 well insulated. The same maker adds: "Sealing and insulating a space before sizing often lets you install a smaller, less expensive heater."
How many watts to heat a garage?: Divide BTU/h by 3,412 for kW (1,000 W is about 3,412 BTU/h). A 24 x 24 ft garage at a 60°F rise is 7.3-8.9 kW well insulated and 12.2-15.4 kW poorly insulated.
What size mini split for a garage?: For cooling, the chart figure for the floor area, then the next size in the lineup: a 24 x 24 ft garage is 14,000 BTU/h, so the next size is 18,000 BTU/h (1.5 tons). Heating needs more here: 24,900-30,400 BTU/h well insulated at a 60°F rise; at the midpoint of that range the next size is 30,000 BTU/h (2.5 tons). Check the heating capacity at your coldest temperature.
How many BTU AC for a garage?: On the ENERGY STAR room chart, read as a room in a home: 20 x 20 ft is 10,000 BTU/h, 24 x 24 ft is 14,000 and 30 x 40 ft is 23,000; very sunny adds 10 percent. The chart has no adjustment for an uninsulated garage, so we do not add one.

Sources

Combustion Research Corporation, How to Determine Heater Size in BTU (Jul 22, 2026), read 2026-10-19.
ENERGY STAR (U.S. EPA), Room Air Conditioners: the room size to capacity chart (undated), read 2026-10-19.
Fujitsu General (AIRSTAGE), AIRSTAGE Altair 500, single-room mini-split system (undated), read 2026-10-19.
Fujitsu General (AIRSTAGE), AIRSTAGE Orion XLTH, single-room mini-split system (undated), read 2026-10-19.
Mitsubishi Electric HVAC US, Premier Wall-Mounted Single-zone Heat Pump, MUZ-GS (undated), read 2026-10-19.
ENERGY STAR (U.S. EPA), Program Requirements for Heat Pumps, Eligibility Requirements (cold climate criteria) (Rev. February 2026), read 2026-10-19.

The heating figures are a heater maker's quick estimate, not a heat loss calculation; the cooling figures are the ENERGY STAR room chart applied to a garage's floor area. Brand names are named only as the source of a figure.

How many BTU do I need per square foot? (room size chart)
The ENERGY STAR room size to BTU chart with its adjustments, heating BTU and tons by home size and climate zone, and m² to kW at three insulation levels.
How many BTU do I need for a bedroom? (12x12 and other room sizes)
A 12 x 12 bedroom is 5,000 BTU/h on the ENERGY STAR chart. Eight bedroom sizes with the sun, people and ceiling adjustments, m² to kW, and the unit class for each.
How many BTU do I need for 1,000 sq ft? (200 to 1,200 sq ft, cooling and heating)
1,000 sq ft is 21,000 BTU/h on the ENERGY STAR chart (1.75 tons) and 32,500-55,000 BTU/h to heat by zone. 200 to 1,200 sq ft, with the adjustments.
What size air conditioner do I need for a 1,500 sq ft house? (tons by home size)
The chart reads 1,500 sq ft as 30,000 BTU/h (2.5 tons); room by room the units add to 44,000. Both readings, 1,000-3,000 sq ft, and a seven-room example.

Last updated 2026-10-19. Sizing estimates from published charts and rules of thumb (ENERGY STAR, manufacturer guides), each with its source and date: a Manual J load calculation by an installer is the final word.