Air Conditioning BTU Calculator
Estimate the cooling BTU/h needed for a room from its area, occupancy, sun exposure, insulation, and climate, following ENERGY STAR sizing guidance.
Result
- Base (area ร 20 BTU/sq ft)6,000 BTU/h
- Occupant adjustment+0 BTU/h
- Kitchen adjustment+0 BTU/h
- Sun / insulation / climate factorร 1.000
- In tons of refrigeration0.50 tons
Step-by-step
- Base load: 300 sq ft ร 20 BTU/sq ft = 6,000 BTU/h.
- Occupants over two: max(0, 2 โ 2) ร 600 = 0 BTU/h.
- Subtotal = 6,000 BTU/h.
- Apply factors (sun ร insulation ร climate = 1.000): total = 6,000 BTU/h โ 0.50 tons.
How to use this calculator
- Enter the room floor area in square feet.
- Set the number of regular occupants โ only people beyond the first two add load.
- Choose whether it is a kitchen and pick the sun exposure (shaded / normal / sunny).
- Select insulation quality and climate zone, then read the BTU/h and tons needed.
About this calculator
This calculator estimates the cooling capacity, in BTU per hour, needed to cool a room with a window or portable air conditioner. It starts from the ENERGY STAR rule of thumb of 20 BTU/h per square foot of floor area, then applies the published ENERGY STAR adjustments: add 600 BTU for each occupant beyond two, add 4000 BTU if the unit cools a kitchen, and increase or decrease capacity by 10% for very sunny or heavily shaded rooms. It layers on additional planning multipliers for insulation quality and climate zone so the estimate better reflects your building. The result is given both in BTU/h and in tons of refrigeration (12,000 BTU/h = 1 ton). Correct sizing matters: an oversized unit cools fast but short-cycles and fails to remove humidity, while an undersized one runs constantly without reaching setpoint.
How it works โ the formula
Subtotal = Area ร 20 + max(0, Occupantsโ2) ร 600 + (Kitchen ? 4000 : 0)
Total BTU/h = Subtotal ร SunFactor ร InsulationFactor ร ClimateFactor
Tons = Total รท 12000The base load scales with floor area at 20 BTU/h per square foot. Discrete additions cover extra occupants and kitchen heat, then percentage multipliers tune for solar gain, envelope quality, and climate.
Worked examples
- Inputs:
- area=250, occupants=2, kitchen=no, sun=normal
- Output:
- 250 ร 20 = 5000 BTU/h
- Inputs:
- area=400, occupants=4, sun=sunny
- Output:
- (8000 + 1200) ร 1.10 = 10,120 BTU/h
- Inputs:
- area=200, occupants=2, kitchen=yes
- Output:
- 4000 + 4000 = 8000 BTU/h
Limitations
- Assumes standard ~8 ft ceilings; raise capacity for tall ceilings.
- Insulation and climate multipliers are planning heuristics, not from ENERGY STAR.
- Not a substitute for a Manual J calculation on ducted/central systems.
Sizing estimate for room units. Consult an HVAC professional for whole-home design.