The 20 BTU per Square Foot Rule
Standard sizing starts at 20 BTU/hr per square foot. A 300 sq ft bedroom needs about 6,000 BTU. Add 600 BTU for each occupant beyond two, add 10% for sun-facing rooms, and subtract 10% for heavily shaded rooms. Kitchens need an extra 4,000 BTU to offset cooking heat.
Ceiling Height and Climate
The base formula assumes 8-foot ceilings. For each extra foot of height, increase BTU by 12–15% to handle the additional air volume. Climate matters too: northern homes typically need 25–30 BTU/sq ft for heating, while hot southern climates require up to 45 BTU/sq ft for cooling.
Choosing the Right Unit Size
Undersized units run continuously without reaching target temperature. Oversized units cool too quickly without removing humidity, leaving rooms feeling clammy. The right unit runs in 15–20-minute cycles. Round up to the nearest half-ton when your BTU requirement falls between standard sizes.
Frequently Asked Questions
What is a BTU?
British Thermal Unit — the energy needed to raise 1 pound of water by 1°F. For HVAC, BTU/hr measures the heating or cooling capacity of a unit.
How many BTU per square foot?
The general rule is 20 BTU per square foot in moderate climates. Very hot or cold regions may need 30–45 BTU/sq ft. High ceilings, large windows, and poor insulation all push requirements higher.
Can one air conditioner cool multiple rooms?
A single window or portable unit works best in the room it occupies. Open floor plans can share a larger unit; closed rooms each need their own separate calculation.