BTU Calculator
Calculate BTUs needed to heat or cool a room. Enter room size, insulation, and temperature difference. Free, instant, no signup required.
How the BTU calculator works
This calculator estimates the heating or cooling capacity needed for a room based on its volume, insulation quality, sun exposure, number of occupants, and the temperature difference between your desired indoor temperature and the outdoor temperature. The base formula uses 4 BTU per cubic foot, adjusted by insulation and sun exposure multipliers, then adds 600 BTU per additional occupant beyond two people.
Understanding the results
- BTU/hr is the standard rating for HVAC equipment. Match this number to an AC unit or heater's rated output.
- AC Tonnage is BTU divided by 12,000. Residential central air units typically range from 1.5 to 5 tons.
- Kilowatts (kW) is useful for comparing electric heaters or heat pumps rated in metric units.
Tips for accurate sizing
- Measure the actual room dimensions rather than guessing. Even a few feet off can change the result by thousands of BTUs.
- For the temperature difference, use the hottest (or coldest) day you expect. If your area hits 100F and you want 72F indoors, enter 28.
- Oversizing an AC unit wastes energy and causes short-cycling, which raises humidity. Aim for a unit within 10% of the calculated BTU.
- This calculator sizes a single room. For whole-house estimates, calculate each room separately or consult a Manual J load calculation.
Frequently Asked Questions
Is my data sent to a server? No. All calculations run entirely in your browser. Nothing is uploaded.
Does this replace a professional HVAC assessment? This provides a good estimate, but a professional Manual J calculation accounts for additional factors like ductwork, window area, and local climate data.