How many BTUs per square foot do you actually need?
Quick answerA common rule of thumb is roughly 20 BTU per square foot of room area, adjusted for climate zone, ceiling height, sun exposure, floor level, and room type. As a rough guide: 350-450 sq ft → 9,000 BTU, 450-600 sq ft → 12,000 BTU (1 ton), 600-900 sq ft → 18,000 BTU (1.5 tons), 900-1,200 sq ft → 24,000 BTU (2 tons).
That 20-BTU figure is a starting point, not a precise answer. It assumes an 8-foot ceiling, average sun exposure, and average insulation — three assumptions that rarely all hold at once. A sunny attic bedroom and a shaded basement office of the identical square footage can land 30-40% apart in real BTU needs.
| Room area | Approx. BTU/h | Standard capacity | Tons |
|---|---|---|---|
| 250 sq ft | ~7,900 | 9,000 BTU | 0.75 |
| 400 sq ft | ~12,600 | 12,000-18,000 BTU | 1-1.5 |
| 650 sq ft | ~20,500 | 24,000 BTU | 2 |
| 1,000 sq ft | ~31,500 | 36,000 BTU | 3 |
These are illustrative example figures from a rule-of-thumb calculation, not a guarantee for your specific room — see the methodology note below.
The full formula: what actually changes the number
Quick answerBasic formula: BTU/h = Room area (sq ft) × climate-zone coefficient × (ceiling height in feet ÷ 8). That baseline is then adjusted for sun exposure, floor level, room type, insulation, window size, occupants, and heavy electronics.
Here's what each factor does, roughly, once you go past the basic square-footage rule:
- Climate zone — Hot-Dry desert and Hot-Humid Gulf Coast regions need more BTU per square foot than Marine or Very Cold/Mountain regions.
- Sun exposure — a room with south- or west-facing windows in sun most of the day needs roughly 12% more BTU; a shaded, north-facing room needs roughly 8% less.
- Floor level — a top-floor room or one directly under the roof needs roughly 15% more BTU from heat radiating down; a basement needs roughly 10% less.
- Room type — kitchens add extra load for cooking heat; home offices add a smaller bump for equipment heat.
- Insulation and windows — good insulation trims the number, poor insulation and large glass areas add to it.
- Occupants and electronics — each person beyond two, and heavy TV/gaming/computer use, both add a flat amount of extra BTU.
Why this mattersTwo rooms with identical square footage can need meaningfully different AC capacity once climate, sun, floor level, and room type are factored in — which is why a flat "20 BTU per sq ft" rule is only a starting estimate.
Ceiling height changes the number too
Quick answerStandard US ceiling height is 8 feet. Taller ceilings increase the volume of air that has to be cooled, so BTU need scales up with height ÷ 8 — a 10-foot ceiling needs about 25% more BTU than an 8-foot ceiling at the same floor area.
This trips people up because most online rules only talk about square footage, not cubic footage. A converted loft or a great room with a vaulted or 10-12 foot ceiling can easily need one capacity size larger than its floor plan alone would suggest.
Why "bigger is safer" is wrong: undersized vs. oversized AC
Quick answerAn undersized AC runs constantly at full power and still can't hit the target temperature, wasting energy and wearing out the compressor. An oversized AC cools too fast and shuts off before removing humidity — called short-cycling — leaving the room cold but clammy. The best fit is the standard capacity closest to your calculated number, not the next size up "just in case."
Example scenario: say a home office calculates to roughly 8,600 BTU/h. Jumping straight to an 18,000 BTU unit "to be safe" isn't actually safer — it's more likely to short-cycle and leave the room damp. A 9,000 BTU unit, the closest standard size above the calculated figure, is the better match.
1 ton = 12,000 BTU/h — this is the unit HVAC contractors quote central air and mini-split capacity in. An 18,000 BTU unit is 1.5 tons; a 36,000 BTU unit is 3 tons.
Get your exact BTU and tonnage number
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Frequently asked questions
How many BTUs per square foot do I need for an air conditioner?
A common rule of thumb is roughly 20 BTU per square foot of room area, though this varies by climate zone, ceiling height, sun exposure, and room type. As a general reference: 350-450 sq ft needs about 9,000 BTU, 450-600 sq ft needs about 12,000 BTU (1 ton), 600-900 sq ft needs about 18,000 BTU (1.5 tons), and 900-1,200 sq ft needs about 24,000 BTU (2 tons). These are baseline figures for an 8-foot ceiling and average sun exposure; your exact need depends on climate zone, ceiling height, floor level, and room type.
How many square feet does a 12,000 BTU (1 ton) air conditioner cool?
A 12,000 BTU (1 ton) air conditioner typically cools about 450-600 square feet under standard conditions (average climate zone, 8-foot ceiling, medium sun exposure, average insulation). A top-floor room, one with heavy sun exposure, or poor insulation needs a smaller area for the same 12,000 BTU unit; a shaded, well-insulated room in a mild climate zone can handle a somewhat larger space.
How does ceiling height affect air conditioner sizing?
Standard ceiling height in the US is assumed to be 8 feet. Taller ceilings increase the room's volume (cubic feet) of air that needs to be cooled, so the required BTU scales up proportionally with height divided by 8. A room with a 10-foot ceiling needs about 25% more BTU than the same floor area with an 8-foot ceiling.
What happens if an air conditioner is undersized or oversized?
An undersized AC runs continuously at full power and still can't reach the target temperature on hot days, wasting energy and wearing out the compressor faster. An oversized AC cools the room too quickly and shuts off before it can properly remove humidity — a problem called short-cycling — leaving the room feeling cold but clammy. Choosing the standard capacity closest to your calculated BTU figure, rather than rounding up multiple sizes, gives the best comfort and efficiency.
Should I get a window unit, portable AC, mini-split, or central air?
It depends mainly on your calculated BTU/h and how many rooms you need to cool. A window unit is the lowest-cost option for a single room up to about 12,000 BTU. A portable unit suits renters who can't mount anything in a window frame permanently. A ductless mini-split is the most efficient choice for 1-4 rooms without existing ductwork, generally 9,000-36,000 BTU. Central air is a whole-house, ducted system, typically sized 18,000 BTU and up.
Related guides
Methodology note: This guide reflects commonly used HVAC rule-of-thumb coefficients (roughly 14-25 BTU per square foot depending on climate zone, adjusted for sun, floor level, room type, insulation, windows, and occupants). It is provided for general information only and is not a substitute for a licensed HVAC contractor's Manual J load calculation, which accounts for local design temperatures, duct losses, and whole-house infiltration for precise, code-compliant sizing.