BTU Calculator
sq ft
8 ft
Insulation
Sun Exposure
BTU Required4,000
AC Tonnage0.33
This BTU calculator estimates the heating or cooling capacity a room needs so you can size an air conditioner, heat pump, or heater correctly. It starts from a base rule of 20 BTU per square foot at an 8-foot ceiling, scales for taller or shorter ceilings, and then adjusts for insulation quality and sun exposure. The result is given both in BTU per hour and in cooling tons.
Formula
BTU = 20 × area × (ceiling/8) × insulationFactor × sunFactor; tons = BTU / 12000
- area
- Room floor area in square feet
- ceiling
- Ceiling height in feet (8 ft is the baseline)
- insulationFactor
- 1.1 poor, 1.0 average, 0.9 good
- sunFactor
- 1.1 high sun, 1.0 medium, 0.9 low sun
How it works
- Enter the room area in square feet and the ceiling height in feet. The base load is 20 BTU/sq ft, multiplied by ceiling height ÷ 8 so taller rooms need proportionally more.
- Choose an insulation level: poor adds 10% (×1.1), good cuts 10% (×0.9), and average leaves the base unchanged.
- Choose sun exposure: high adds 10% (×1.1), low cuts 10% (×0.9), medium is unchanged. The final BTU is divided by 12,000 to express the load in cooling tons.
Worked example
A 300 sq ft room with an 8 ft ceiling, average insulation, and medium sun exposure.
- Base load: 20 × 300 × (8/8) = 6,000 BTU.
- Average insulation (×1.0) and medium sun (×1.0) leave it unchanged.
- Tons: 6,000 ÷ 12,000 = 0.5 tons.
About 6,000 BTU/hr, which is 0.5 cooling tons.
Frequently asked questions
- What is a BTU and what does a "ton" of cooling mean?
- A BTU (British Thermal Unit) measures heat energy; air conditioners are rated in BTU per hour. One ton of cooling equals 12,000 BTU/hr, a unit carried over from the cooling power of melting a ton of ice in a day.
- Why does ceiling height matter?
- Taller ceilings mean more air volume to condition, so the calculator scales the base load by ceiling height divided by 8 feet. A 10-foot ceiling needs 25% more capacity than the 8-foot baseline.
- How do insulation and sun exposure change the result?
- Poor insulation or strong sun each add 10% to the load, while good insulation or a shaded, low-sun room each subtract 10%. They are applied as multipliers on top of the area-and-ceiling base figure.
- Is this enough to choose an exact HVAC unit?
- It is a solid first estimate for a single room. Whole-home sizing also weighs windows, climate zone, occupancy, and air leakage, so for a full system a professional Manual J load calculation is recommended.