Air Conditioner BTU Calculator
Calculate the BTU rating needed to cool a room. Based on room area, ceiling height, and sun exposure.
How to use this tool
- Enter the room length and width in feet.
- Select the sun exposure level for your room.
- The calculator recommends the minimum BTU/hr rating for your AC unit.
Find the right BTU rating for your room before buying an air conditioner.
Formula
Room area (ft²) = length (ft) × width (ft)
BTU/hr = room area × 20 × sun exposure factor
The base rate of 20 BTU/hr per square foot is adjusted by a sun exposure multiplier (e.g. 1.0 for average, higher for sunny rooms).
How it works
This calculator estimates the cooling capacity (in BTU/hr) an air conditioner needs to maintain comfortable temperatures in a rectangular room. It applies a base rate of 20 BTU/hr per square foot — a common rule of thumb derived from ASHRAE guidance for average-height ceilings — then scales the result by a sun exposure factor to reflect solar heat gain through windows. A factor of 1.0 represents normal exposure; sunnier rooms require higher factors. The result does not account for occupancy loads, kitchen heat, or climate zone, so it is a starting-point estimate rather than a full Manual J calculation.
Worked example
Worked example
- Room area = 15 ft × 12 ft = 180 ft².
- Apply base rate: 180 × 20 = 3,600 BTU/hr.
- Sun exposure factor = 1.0 (average), so BTU/hr = 3,600 × 1.0 = 3,600 BTU/hr.
A room of 180 ft² with average sun exposure needs approximately 3,600 BTU/hr of cooling capacity.
Key terms
- BTU/hr (British Thermal Unit per hour)
- The standard US unit for air conditioner cooling capacity; 1 BTU is the energy needed to raise 1 lb of water by 1°F.
- Sun exposure factor
- A multiplier applied to the base BTU rate to account for solar heat gain; rooms with heavy direct sun require a higher factor than shaded rooms.
- Cooling load
- The total rate of heat gain a space experiences that an air conditioner must remove; influenced by area, insulation, windows, occupants, and climate.
- Manual J
- The industry-standard ACCA method for calculating residential cooling and heating loads; more accurate than the BTU-per-square-foot rule of thumb.
Frequently asked questions
- How many BTU do I need for a room?
- A general rule is 20 BTU per square foot for average rooms. Add 10% for sunny rooms and subtract 10% for heavily shaded rooms.
- What does BTU stand for?
- BTU stands for British Thermal Unit. It measures the amount of energy needed to raise one pound of water by one degree Fahrenheit. Air conditioners are rated in BTU/hr.