BTU Calculator
Calculate BTU needed for heating and cooling any room.
This tool is for informational and educational purposes only. It is not a substitute for professional financial, medical, legal, or engineering advice. See Terms of Service.
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This calculator helps you determine the right size air conditioner, heater, or HVAC system for any room. Follow these steps for an accurate estimate:
- Enter room dimensions. Measure the length and width of your room in feet. If the room is L-shaped or irregular, estimate the total rectangular area that best represents the space. Enter the ceiling height as well, since taller ceilings increase the volume of air that needs conditioning.
- Select your insulation quality. Choose "Poor" for older homes without updated insulation, single-pane windows, or drafty rooms. Choose "Average" for standard construction with reasonable insulation. Choose "Good" for new construction with modern insulation, double-pane windows, and sealed air barriers.
- Select your climate zone. This adjusts the calculation based on how hard your system will need to work. Hot and very cold climates require significantly more capacity than moderate ones.
- Set sun exposure and occupants. South-facing rooms with lots of windows need more cooling. Each person in the room generates approximately 600 BTU of body heat, so occupancy matters for accurate sizing.
Results update instantly as you adjust any input. The calculator shows both cooling and heating BTU requirements, along with tonnage (used to size central AC units) and wattage equivalents. Use the Share button to send your results to an HVAC contractor, or Copy to save the numbers.
About BTU Calculations
A BTU (British Thermal Unit) is the amount of energy required to raise the temperature of one pound of water by one degree Fahrenheit. HVAC equipment is rated in BTU per hour, which indicates its heating or cooling capacity. The general rule of thumb is 20 BTU per square foot for cooling, but this varies significantly based on insulation, climate, ceiling height, sun exposure, and occupancy.
One ton of cooling capacity equals 12,000 BTU per hour. Residential central air conditioners typically range from 1.5 to 5 tons. Oversizing an AC unit causes short cycling (turning on and off frequently), which wastes energy and fails to dehumidify properly. Undersizing leads to the system running constantly without reaching the desired temperature. Accurate sizing saves money on both equipment and energy bills.
Frequently Asked Questions
What size AC do I need for my room?
For a standard room with average insulation, you need roughly 20 BTU per square foot for cooling. A 300 sq ft room needs about 6,000-7,000 BTU (a small window unit). A 500 sq ft room needs 10,000-12,000 BTU. A 1,000 sq ft space needs about 18,000-20,000 BTU (1.5 tons). These numbers increase with poor insulation, heavy sun exposure, or hot climates.
What is a BTU?
A BTU (British Thermal Unit) is a unit of heat energy. One BTU is the amount of energy needed to raise the temperature of one pound of water by one degree Fahrenheit. In HVAC, BTU per hour (BTU/h) measures the heating or cooling output of equipment. Higher BTU ratings mean more powerful heating or cooling capacity. Air conditioners, furnaces, and space heaters are all rated in BTU.
How many BTU per square foot do I need?
The baseline is about 20 BTU per square foot for cooling in a moderate climate with average insulation. For heating, the range is 30-60 BTU per square foot depending on climate. Cold climates require more BTU per square foot, as do rooms with poor insulation, high ceilings, or large windows. Well-insulated homes in mild climates may need as few as 15 BTU per square foot for cooling.
What is the difference between heating and cooling BTU?
Heating BTU measures the energy needed to warm a space, while cooling BTU measures the energy an air conditioner must remove. Heating BTU requirements are generally higher than cooling for the same space, because the temperature difference between indoor and outdoor air is typically larger in winter than in summer. A room that needs 12,000 BTU for cooling might need 20,000-30,000 BTU for heating in a cold climate.