BTU Calculator
Find the right BTU capacity for an air conditioner or heater based on room size, climate, insulation and occupancy.
Estimate BTU needed to heat/cool a room
How to use the BTU Calculator
- Enter your inputs into the BTU Calculator above.
- Results update instantly as you type — no submit button needed.
- Adjust any value to see how the result changes in real time.
The BTU sizing formula
Base BTU = Floor area (ft²) × 20 · · · Adjusted for ceiling, insulation, windows, occupancy and climate
The 20 BTU/ft² baseline is for an average-insulation room with standard 8-foot ceilings. Adjustments add or subtract based on real-world factors that change the cooling load.
Worked example
A 350 ft² living room with 9-ft ceilings, two south-facing windows and typically 3 occupants: baseline 7,000 BTU + 10% ceiling + 15% windows + 1,200 BTU occupants ≈ 10,200 BTU. A 10,000 BTU unit is correct; 12,000 would short-cycle.
Frequently asked questions
What if I oversize the AC?
It short-cycles, fails to dehumidify properly, and wastes energy. Comfort suffers despite (and because of) the bigger unit. Correct sizing matters.
What's the difference between BTU and tons?
In HVAC, 1 ton = 12,000 BTU/hr cooling. A 3-ton system is 36,000 BTU/hr — typical for a 1,500–2,000 ft² home.
Should I always buy a bigger unit "just in case"?
No — oversizing is more common than undersizing and causes the comfort problems described above. Size to the load; modern inverter units handle variable loads gracefully.