Size the right AC for your room.
Enter your room area, ceiling height, sun exposure and occupants to see the exact BTU per hour you need, in watts and tons of cooling, plus a recommended unit size.
Your room
What this means
BTU for common room sizes
| Room size | BTU/h | Watts | Tons |
|---|
How BTU sizing works
What Is a BTU?
A BTU (British Thermal Unit) is the amount of energy needed to raise the temperature of one pound of water by one degree Fahrenheit. In HVAC, BTU ratings indicate the heating or cooling capacity of equipment. Higher BTU means more powerful heating or cooling.
How BTU Requirements Are Calculated
Base BTU is calculated from room volume (length x width x height) multiplied by a factor of about 5 BTU per cubic foot for cooling. Adjustments are made for insulation quality, sun exposure, number of occupants, and heat-generating appliances.
AC Tonnage Explained
One ton of AC capacity equals 12,000 BTU per hour. A 2-ton unit provides 24,000 BTU/hr. Residential central AC systems typically range from 1.5 to 5 tons. The tonnage needed depends on your home square footage, climate zone, and insulation.
Oversizing vs Undersizing
An oversized AC unit short-cycles, failing to properly dehumidify and wasting energy. An undersized unit runs continuously and cannot maintain the desired temperature. Proper sizing within 10% of the calculated BTU need is ideal.
Common questions
How many BTU do I need per square foot?
A rough estimate is 20 BTU per square foot for cooling in a moderate climate. Hot climates may need 25-30 BTU per square foot. Well-insulated rooms need less, while poorly insulated rooms with large windows need more.
What size AC unit do I need?
Divide the total BTU by 12,000 to get the tonnage. A 300 sq ft room typically needs 7,000 BTU (a small window unit). A 1,500 sq ft home typically needs 30,000-36,000 BTU (2.5-3 ton central AC).
Does ceiling height affect BTU requirements?
Yes. Higher ceilings mean more air volume to heat or cool. Standard 8-foot ceilings use the base calculation. For 10-foot ceilings, increase BTU by about 25%. For 12-foot ceilings, increase by about 50%.
How does insulation affect BTU needs?
Poor insulation can increase BTU needs by 30% or more compared to well-insulated rooms. Good insulation with double-pane windows and sealed air gaps can reduce BTU needs by 20-30%, saving significantly on energy costs.
Estimates only. Actual sizing depends on climate zone, insulation, window area and local conditions. Note: 1 watt equals 3.412 BTU/h, so you can convert any wattage to BTU by multiplying by 3.412.