BTU Calculator (AC Sizing)
BTU (British Thermal Unit) is the standard measure used to rate the cooling capacity of air conditioning units. Choosing the right BTU rating for your room is essential: an undersized unit will struggle to cool the space, while an oversized one will cycle on and off too frequently, wasting energy and failing to properly dehumidify the air.
This calculator estimates the cooling capacity you need based on room area, sun exposure, number of regular occupants, and ceiling height. The baseline is 20 BTU per square foot, with science-backed adjustments for each factor. Use the result to select a window AC, mini-split, or central air system of the appropriate size.
How it works
Base BTU = area (sq ft) × 20. Adjust ×0.9 for low sun exposure or ×1.1 for high sun exposure. Add 600 BTU for each occupant beyond two. Multiply by 1.1 for ceilings 9–11 ft or ×1.25 for ceilings above 11 ft. Final BTU is rounded to the nearest whole number.
Use cases
- Sizing a window air conditioner for a bedroom or living room
- Selecting a mini-split system for an open-plan office
- Estimating central AC capacity for a new home addition
- Comparing energy efficiency between different BTU ratings
- Planning cooling for a sunroom or south-facing studio
Frequently asked questions
How many BTU do I need per square foot?
A common baseline for cooling is 20 BTU per square foot of floor area. A 300 sq ft living room therefore starts at about 6,000 BTU before adjustments. This calculator then refines that figure for sun exposure, the number of regular occupants, and ceiling height to give a more realistic sizing.
What happens if my air conditioner has too many BTU?
An oversized unit cools the air quickly but short-cycles — it switches on and off too frequently. That wastes energy, increases wear on the compressor, and leaves the room clammy because the unit does not run long enough to dehumidify the air properly. Matching the BTU rating to the room is more comfortable and more efficient than simply buying the biggest unit.
Does sun exposure change how many BTU I need?
Yes. This calculator multiplies the base figure by 1.1 for rooms with high sun exposure and by 0.9 for shaded rooms — roughly a 10% adjustment either way. A sun-drenched room with large windows absorbs significant heat, so a south-facing living room needs more capacity than a shaded bedroom of the same size.
Do more people in a room increase the required BTU?
Yes. Each person at rest releases heat, so the standard sizing rule adds about 600 BTU for every regular occupant beyond two. For example, a home office where four people work would get an extra 1,200 BTU on top of the area-based figure.
How does ceiling height affect BTU sizing?
Higher ceilings mean more air volume to cool for the same floor area. This calculator multiplies the result by 1.1 for ceilings between 9 and 11 feet (about 2.7–3.3 m) and by 1.25 for ceilings above 11 feet. Standard 8-foot ceilings need no adjustment.