Room BTU Cooling Load Calculator
The U.S. Department of Energy publishes a simple rule of thumb for sizing a room air conditioner: start from roughly 20 BTU per hour per square foot, then adjust for sun, shade, and the number of occupants. This calculator applies that method. It multiplies your floor area by a cooling factor you can set, adjusts up 10% for a very sunny room or down 10% for a heavily shaded one, and adds 600 BTU per hour for each occupant beyond two. Treat the result as a quick estimate for a window or portable unit, not a precise whole-home design.
Cooling load estimate formula
Base load (BTU/hr) = floor area * cooling factor
Sun-adjusted load = base load * (1 + sun exposure / 100)
Occupant addition = max(0, occupants - 2) * 600
Adjusted load = sun-adjusted load + occupant addition
Tons = adjusted load / 12,000
The Department of Energy rule of thumb adds 600 BTU per hour for each occupant beyond two and adjusts the base by roughly plus or minus 10% for sun or shade. One ton of cooling equals 12,000 BTU per hour by definition.
Notes on cooling load
- DOE's published starting point is about 20 BTU per hour per square foot; set the factor to match your room.
- Use a positive sun adjustment for a very sunny room, negative for a heavily shaded one.
- For a kitchen, DOE suggests adding about 4,000 BTU per hour, which you can include via a higher factor.
- Do not oversize: an oversized unit cools fast but fails to remove humidity.
- For central systems, use an ACCA Manual J load calculation instead of this rule of thumb.
Cooling load: frequently asked questions
How many BTU do I need to cool a room?
The U.S. Department of Energy guidance for room air conditioners starts from about 20 BTU per hour per square foot of living space, then adjusts for sun, shade, and occupancy. This calculator uses that approach with a cooling factor you can set, plus standard adjustments DOE describes.
What adjustments does DOE recommend?
The Department of Energy suggests reducing the estimate by 10% for a heavily shaded room, increasing it by 10% for a very sunny room, and adding 600 BTU per hour for each occupant beyond two. For a kitchen, DOE suggests adding 4,000 BTU per hour. This tool applies the sun, shade, and occupancy adjustments.
Is 20 BTU per square foot exact?
It is a published starting point, not a precise design figure. Actual cooling load depends on insulation, window area and orientation, climate, and internal heat gains. The cooling factor here is a user-editable input so you can match DOE's base of 20 or adjust it for your room.
Should I oversize the air conditioner?
No. An oversized unit cools the air quickly but does not run long enough to remove humidity, leaving the room cool but clammy and cycling on and off. DOE advises matching capacity to the load. A correctly sized unit gives better comfort and efficiency than an oversized one.
Does this replace a full load calculation?
No. For central systems and precise sizing, an ACCA Manual J load calculation is the standard. This calculator follows DOE's room air conditioner rule of thumb for a quick estimate and is best for sizing a window or portable unit, not a whole-home system.
Official sources
- U.S. Department of Energy, room air conditioners: energy.gov.
- ENERGY STAR, room air conditioners: energystar.gov.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.