How this instrument works
A room air conditioner's cooling power is rated in BTU per hour (BTU/hr): the amount of heat it can remove from a space every hour. Buy a unit that's too small and it will run constantly without ever quite catching up on a hot day; buy one that's too large and it will cool the air fast but shut off before it removes enough humidity, leaving the room feeling cold and clammy at the same time.
ENERGY STAR's published room-size chart is a step table rather than a flat rate: it works out to roughly 40-50 BTU per square foot for a small room and closer to 15-20 BTU per square foot for a large one, since a small room loses relatively more heat around its perimeter than a large one does. This calculator simplifies that into a flat 20 BTU per square foot, a widely used round-number rule of thumb that tracks ENERGY STAR's own figures fairly closely for larger rooms (roughly 1,000 sq ft and up) but runs lower than the official chart for smaller and mid-size rooms — for anything under about 700 sq ft, check ENERGY STAR's exact table (linked below) if you want the precise published number rather than this simplified estimate.
On top of that base figure, this calculator applies the same real-world adjustments ENERGY STAR's guidance uses: heavy sun exposure adds load, shade reduces it, a kitchen's own heat sources (stove, oven, appliances) add a flat 4,000 BTU, and every occupant beyond the first two adds about 600 BTU, since bodies generate heat. It covers a single room only — for whole-house central cooling sized in tons, use a central AC tonnage estimate instead.
- Enter the room's floor area in square feet.
- Enter how many people beyond the first two regularly occupy the room.
- Select whether the room is a kitchen (adds a fixed 4,000 BTU for cooking heat).
- Select the room's sun exposure: heavily shaded, average, or very sunny.
- Read the estimated BTU per hour needed, then shop for a unit rated close to that figure.
A sunny 450 sq ft kitchen with four regular occupants
A family kitchen measures 450 sq ft, gets strong afternoon sun, and regularly has four people in it during meal prep — two more than the baseline of two the ENERGY STAR chart assumes. Base cooling load: 450 x 20 = 9,000 BTU. Add 600 BTU for each of the two extra occupants: +1,200 BTU. Add the flat kitchen allowance for cooking heat: +4,000 BTU. That subtotal, 14,200 BTU, then gets bumped up 10% for the sunny exposure: 14,200 x 1.10 = 15,620 BTU.
Compare that to a small, heavily shaded 150 sq ft bedroom with no extra occupants and no kitchen adjustment: base load is 150 x 20 = 3,000 BTU, reduced 10% for the shade to 2,700 BTU — roughly a sixth of the kitchen's requirement, despite being only a third of its floor area, because sun, cooking heat, and occupancy all stack the cooling load higher.
Questions
Why does a kitchen need so much more cooling than a similar-size bedroom?
Kitchens generate their own heat from stoves, ovens, and appliances on top of the room's baseline cooling load, which is why ENERGY STAR's guidance adds a flat 4,000 BTU for any room used as a kitchen — a significant addition for a small or medium-sized room.
What happens if I buy an oversized room air conditioner?
An oversized unit cools the air quickly and cycles off before it has run long enough to pull enough humidity out of the room, so you end up with air that feels cold but still damp and clammy — plus more frequent on/off cycling that wastes energy and shortens the compressor's life.
Do I really need to adjust for sun exposure and occupants?
Yes if you want an accurate estimate. Sun exposure can shift the load by 20 percentage points between a heavily shaded and very sunny room, and each additional occupant beyond two adds a real, if modest, 600 BTU of body heat — small adjustments individually but meaningful when combined with a small base room size.
Is this the same sizing method used for central air conditioning?
No. Central systems are sized in tons (1 ton = 12,000 BTU/hr) using a broader square-feet-per-ton rule that accounts for whole-house climate zone rather than a single room's sun and occupancy. Use the dedicated AC tonnage calculator for whole-home central systems.
How exact is the 20 BTU/sqft baseline compared to ENERGY STAR's own chart?
It's a simplified round-number approximation, not a literal reproduction of ENERGY STAR's chart. That chart is a step table which recommends notably more than 20 BTU per square foot for smaller and mid-size rooms (up to roughly 40-50 BTU/sqft for a small room) and converges toward this calculator's flat rate only for larger rooms, around 1,000 sq ft and up. It also assumes a typical 8-foot ceiling and average insulation; unusually high ceilings, poor insulation, or large uninsulated glass areas will need more capacity than either figure suggests. For a room under about 700 sq ft, check ENERGY STAR's published table directly if you want its exact recommended number.