Last updated October 12, 2026 · Report an error
Choosing the right size air conditioner or heater starts with estimating how many BTUs (British Thermal Units) per hour a room actually needs.
This calculator estimates that BTU load from the room's area, ceiling height, climate zone, number of occupants, and number of windows.
How it works
Enter the room's square footage, ceiling height, climate zone, occupants, and window count. The calculator applies a base BTU-per-square-foot rate for the climate zone, scales it for ceiling height, and adds adjustments for extra occupants and windows.
- Enter room/area size (sq ft).
- Enter ceiling height (ft).
- Enter climate zone.
- Enter number of occupants.
- Enter number of windows.
- Click Calculate to see your results.
Examples
300 sq ft, moderate climate
A 300 sq ft room with an 8 ft ceiling in a moderate climate, 2 occupants, and 2 windows needs about 9,500 BTU/hr — roughly a 0.8-ton AC unit.
Who should use it
- Sizing a portable or window air conditioner for a single room.
- Getting a rough estimate before consulting an HVAC professional for a full system.
Industry applications
- HVAC and home improvement
- Real estate and property management
Advantages
- Accounts for climate zone, ceiling height, occupants, and windows in one estimate.
- Gives both a BTU/hr figure and an approximate AC tonnage.
Limitations
- A rule-of-thumb estimate, not a substitute for a professional Manual J load calculation.
Common mistakes to avoid
- Undersizing based on square footage alone without accounting for a hot climate or many windows.
- Using this rough estimate as a final decision for an expensive HVAC installation instead of getting a professional load calculation.
Best practices
- Treat this as a starting estimate for window AC units or a sanity check — get a Manual J calculation for whole-house system sizing.
Tips
- An oversized AC unit cools too quickly without properly removing humidity — right-sizing matters as much as raw capacity.