BTU Calculator

BTU Converter tab: Type any value into any energy field and all other units update instantly. Edit the Watts field to convert from watts to BTU, or edit the Joules field to convert from joules. The active field is highlighted with a blue border.

AC Sizing Calculator tab: Enter your room's dimensions, then adjust the climate and sunlight settings to match your situation. The calculator applies Energy Star and ASHRAE sizing guidelines to recommend the right BTU/hr capacity. Here is how each input affects the result:

• Room size: Larger rooms need more BTU. Base rule is 20 BTU per square foot.
• Ceiling height: Higher ceilings increase the volume of air to cool. An 8-foot ceiling is the baseline. A 10-foot ceiling adds about 25% more load.
• Climate zone: Hot, humid climates (Texas, Florida, Arizona) add 15% to the base. Cool northern climates subtract 15%.
• Sunlight: Heavily sun-exposed rooms need 10% more cooling capacity. Shaded rooms need 10% less.
• Occupants: Each person beyond 2 generates about 600 BTU/hr of heat load.

One BTU (British Thermal Unit) is the energy needed to raise 1 pound of water by 1 degree Fahrenheit. All other units derive from this definition:

1 BTU = 0.29307 watts
1 BTU = 0.000293071 kWh
1 BTU = 252.164 calories (kcal)
1 BTU = 1,055.06 joules
1 BTU = 0.00001 therms
1 ton (AC) = 12,000 BTU/hr

AC Sizing Formula:

Base BTU = Room Area (sq ft) × 20
Adjusted BTU = Base × (Ceiling Height / 8) × Climate Factor × Sunlight Factor
Final BTU = Adjusted BTU + (Extra Occupants × 600)
Tons = Final BTU / 12,000

Climate factors: hot = 1.15, moderate = 1.00, cool = 0.85. Sunlight factors: high = 1.10, moderate = 1.00, low = 0.90. The result is rounded to the nearest 500 BTU to match standard unit sizes available in stores.