Air Conditioner (BTU) Sizing Calculator
This calculator starts from about 20 BTU per square foot, then adjusts: 10% less for a heavily shaded room, 10% more for a very sunny one, 600 BTU per occupant beyond two and 4,000 BTU for a kitchen, rounded up to a standard size. A 300 ft² room used by four people needs 7,200 BTU — an 8,000 BTU unit.
How it's calculated: Load = area × 20 BTU/ft² × sun factor (0.9 shaded, 1.0 average, 1.1 sunny) + 600 BTU per occupant beyond two + 4,000 BTU for a kitchen; recommended = next standard size at or above the load.
Result appears here
Fill in the fields and press Calculate.
Formula, calculation, assumptions and sources
Base load = Room area (ft²) × 20 BTU/ft² × sun-exposure factor (0.9 shaded / 1.0 average / 1.1 sunny)
+ 600 BTU per occupant beyond 2, + 4,000 BTU if the room includes a kitchen
Recommended size = smallest standard AC capacity (5,000–24,000 BTU) at or above the calculated load
Run the calculation to see each step with your own numbers.
A 300 ft² living room, average sun, 4 usual occupants, no kitchen.
Base = 300 × 20 × 1.0 = 6,000 BTU/hr
+ 2 extra occupants × 600 = 1,200 BTU/hr
Estimated load = 7,200 BTU/hr exact → nearest standard size: 8,000 BTU unit
- A single room with a standard ~8 ft ceiling. Very high ceilings, open-plan spaces spanning multiple rooms, or poor insulation increase the true load beyond this estimate.
- This is the DOE Energy Saver rule-of-thumb method (area-based with simple adjustments), not an ACCA Manual J load calculation — the professional standard for sizing a ducted or whole-house system.
- Assumes a room being cooled independently (a window, portable or single-zone mini-split unit), not a central ducted system sized for a whole house.
- Is this accurate enough to buy a unit?
- It's a solid rule-of-thumb for a single window or portable unit, and it's the same method DOE publishes for consumers. For a ducted, multi-zone or whole-house system, have a contractor run a full ACCA Manual J load calculation — it accounts for insulation, windows, orientation and climate in detail this estimate doesn't.
- Why does 'recommended' jump to a fixed number instead of adding a percentage?
- Air conditioners are sold in fixed capacity steps (5,000, 6,000, 8,000 BTU and so on) — there's no such thing as buying '10% more' BTU. The recommended figure is simply the next standard size at or above your calculated load, so you don't undersize.
- Should I round down to save money?
- No — an undersized AC runs constantly, struggles on the hottest days and cycles less efficiently (worse humidity control). An oversized unit short-cycles and also hurts humidity control, but modestly. Rounding UP to the next standard size is the safer choice.
- How many BTUs should I add for each extra person?
- 600 BTU for each person beyond two who regularly uses the room, following ENERGY STAR's sizing adjustments. A room used by four people gets 1,200 BTU added.
- Does a kitchen need a bigger air conditioner?
- Yes. ENERGY STAR's guidance adds 4,000 BTU when the unit is used in a kitchen, and the calculator does the same when you answer yes.
- Should I adjust AC size for a sunny or shaded room?
- Yes. ENERGY STAR suggests about 10% more capacity for a very sunny room and 10% less for a heavily shaded one; the calculator's sun-exposure setting applies those factors.
Sizing rule of thumb + adjustments
U.S. Department of Energy, Energy Saver — 'Room Air Conditioners' (energy.gov/energysaver/room-air-conditioners): ~20 BTU per square foot, +10%/-10% for a sunny/shaded room, +600 BTU per extra occupant beyond two, +4,000 BTU if used in a kitchen.
Standard AC capacity steps
Common window/portable air conditioner nameplate capacities (5,000–24,000 BTU/hr) as sold by major manufacturers.
Room adjustments (ENERGY STAR)
ENERGY STAR, Room Air Conditioners sizing guidance: reduce capacity by 10% for a heavily shaded room, increase it by 10% for a very sunny room, add 600 BTU for each regular occupant beyond two, and add 4,000 BTU if the unit is used in a kitchen.
Room adjustments (ENERGY STAR) — reference
About the Air Conditioner (BTU) Sizing Calculator
The Air Conditioner (BTU) Sizing Calculator gives a rule-of-thumb cooling capacity for a single room, using the U.S. Department of Energy's Energy Saver sizing guidance: roughly 20 BTU per square foot, adjusted for sun exposure, extra occupants and a kitchen nearby. It also rounds up to the nearest commonly sold window/portable AC size, since buying undersized is the more common — and less comfortable — mistake. This is a room-by-room rule of thumb, not a substitute for an ACCA Manual J load calculation, which a contractor should run for whole-house or ducted-system sizing.
What you enter
- Room area
- Sun exposure
- Usual occupants — The first 2 are included in the base figure; each extra person adds load.
- Room includes a kitchen?
- Extra safety margin — Default 0%. Add a margin (e.g. 10%) before rounding up to the nearest standard AC size, if you'd rather size generously.
What you get back
- Estimated cooling load — BTU/hr. Recommended = the nearest standard AC size at or above the calculated load.
The formula, a worked example, the assumptions behind it and the sources are shown in full below the calculator.
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Related articles
- How to Choose the Right Air Conditioner Size for a Room — Air conditioner sizing starts with room area and a BTU-per-square-foot estimate, adjusted for factors like ceiling height, sun exposure and occupancy.
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