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