BTU Calculator
Enter the length and width of a room in feet or meters and pick its sun exposure. The calculator multiplies the floor area by 30 BTU per square foot for a sunny or hot room, 25 for an average room or 20 for a shaded or cool one. Add the ceiling height if it is above 8 feet, and the estimate scales up with it. The result is in BTU per hour and in tons. Insulation, windows and the number of people do not change the estimate.
Estimate BTU needed to heat/cool a room
How to use the BTU Calculator
- Enter your numbers in the BTU Calculator above.
- Press the Calculate BTU button to see the result.
- To try other numbers, change any value and press the button again.
The BTU estimate
BTU = Floor area (ft²) × 30, 25 or 20 · · · Above 8 ft: BTU × Ceiling height / 8 · · · Tons = BTU / 12,000
The factor comes from the Sun Exposure menu: 30 for Sunny / Hot climate, 25 for Average and 20 for Shaded / Cool climate. In metric mode the calculator turns square meters into square feet with 1 ft² = 0.09290304 m² and meters into feet with 1 ft = 0.3048 m. A ceiling at or below 8 feet, or an empty field, changes nothing. The BTU figure is rounded to a whole number and the tons to 2 decimal places. These factors and the ceiling scaling are the calculator's own rule of thumb. They are not a load calculation.
Worked example
A 20 ft × 15 ft room is 300 square feet. With Average sun exposure the calculator shows 300 × 25 = 7,500 BTU, or 0.63 tons. Marked Sunny / Hot climate, the same room gets 9,000 BTU (0.75 tons). Shaded / Cool climate gives 6,000 BTU (0.50 tons). With a 10-foot ceiling at Average, 7,500 × 10 / 8 = 9,375 BTU, or 0.78 tons.
Frequently asked questions
How does ceiling height change the BTU?
ENERGY STAR's room air conditioner guide bases its chart on an 8-foot ceiling and says a higher ceiling may call for more capacity, without giving an amount. This calculator scales the BTU by height / 8, so it follows the volume of the room: a 12-foot ceiling means 1.5 times the BTU. Other guides differ. A flat 1,000 BTU per foot above 8 feet is another rule of thumb in use, and it gives a smaller increase for a big room and a larger one for a small room. A ceiling at or below 8 feet does not lower the estimate.
Does it account for windows or people?
No. It uses the floor area, the sun exposure and the ceiling height only. ENERGY STAR's guide adds capacity when more than two people regularly use the room or when the unit cools a kitchen. Those adjustments are not in this calculator.
How does it compare with the ENERGY STAR chart?
It can come out lower or higher. For a 200 square foot room, the ENERGY STAR chart lists 6,000 BTU per hour. The calculator gives 5,000 BTU with Average sun exposure, 6,000 with Sunny / Hot climate and 4,000 with Shaded / Cool climate.
What's the difference between BTU and tons?
A ton of cooling is 12,000 BTU per hour, so the calculator divides the BTU figure by 12,000. A 36,000 BTU system is 3 tons.
Does it work in metric?
Yes. Switch to Metric and enter the sizes in meters. The calculator turns the square meters into square feet before it multiplies. A 5 m × 4 m room is 20 m², about 215.28 square feet, and at Average it gives 5,382 BTU.