Insulate and drywall the garage you are about to heat.
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BTU Calculator for Rooms & Garages
Heating & coolingQuick answer: most rooms need roughly 20 BTU per square foot to cool and a garage needs roughly 30–45 BTU per square foot to heat — before insulation and climate corrections, which is where sizing usually goes wrong. A well-insulated 20×20 ft garage in a moderate climate needs about 12,800 BTU of heat; the same garage with poor insulation and 10 ft ceilings needs 19,200.
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BTU you need
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Standard unit size to buy
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Equal to (tons, cooling)
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Floor area (ft²)
This is a rule-of-thumb sizing estimate (the same class of estimate retailers use for window units, mini-splits and garage heaters) — not a Manual J load calculation. For a whole-house system, a contractor's Manual J is the standard that protects your warranty and comfort.
How it works — the formula
Base BTU = Floor area (ft²) × BTU per ft² (mode & climate)| Mild climate | Moderate climate | Extreme climate | |
|---|---|---|---|
| Cooling, per ft² | 18 BTU | 20 BTU | 25 BTU |
| Heating, per ft² | 25 BTU | 32 BTU | 45 BTU |
Ceiling factor: +10% for every foot of ceiling above 8 ft. Insulation: good ×0.90, average ×1.00, poor ×1.25. Sun (cooling only): shady ×0.90, sunny ×1.10. People: +600 BTU for each person beyond two regular occupants. Kitchen (cooling): +4,000 BTU for cooking heat. The result is rounded up to the next standard unit size — 5,000–60,000 BTU steps for AC, and common garage-heater sizes for heating — because units are sold in those steps, not in exact BTUs.
Do not oversize 'to be safe': an oversized AC short-cycles, cools without dehumidifying, and wears out faster. An oversized garage heater is merely wasteful; an oversized AC is actively worse.
Worked examples
| Space | Calculation | BTU needed | Buy |
|---|---|---|---|
| Bedroom 12 × 10 ft, cooling, moderate, 8 ft ceiling, average everything, 2 people | 120 ft² × 20 | 2,400 | 5,000 BTU (smallest standard) |
| Kitchen-living 15 × 20 ft, cooling, moderate, 9 ft ceiling, poor insulation, sunny, 4 people | 300 × 20 × 1.10 × 1.25 × 1.10 + 1,200 + 4,000 | 14,275 | 15,000 BTU |
| Garage 20 × 20 ft, heating, moderate, 10 ft ceiling, poor insulation | 400 × 32 × 1.20 × 1.25 | 19,200 | 20,000 BTU heater |
Notice the garage: same room, and insulation plus ceiling height alone move the answer by 50%. That correction — not the base rate — is the part quick answers and AI summaries leave out.
Frequently asked questions
How many BTU do I need for a 20x20 garage?
Heating, moderate climate: ≈12,800 BTU average insulation at 8 ft ceilings (buy 15,000); ≈19,200 with poor insulation and 10 ft ceilings (buy 20,000).
How many BTU per square foot is normal?
≈20 BTU/ft² for cooling in a moderate climate; ≈25–45 BTU/ft² for heating depending on winter severity and insulation.
What size AC for a 12x12 room?
144 ft² × 20 = 2,880 BTU — buy the smallest standard unit, 5,000 BTU. A bigger unit short-cycles and leaves the room clammy.
Is a bigger BTU unit always better?
No. Oversized AC cools without dehumidifying and wears faster. Round up only to the next standard size.
How many BTU is one ton of AC?
12,000 BTU/hr per ton. Residential central systems run roughly 1.5–5 tons (18,000–60,000 BTU/hr).
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See the full list on the Home & DIY Calculators page.
Estimates only — a rule-of-thumb sizing guide, not a Manual J load calculation. Whole-house systems, high-humidity climates and spaces with large glass areas need a professional load calculation.