BTU Calculator

What this calculator does

The BTU Calculator estimates how much heating or cooling capacity a space needs. It offers two methods. The cooling method sizes an air conditioner for a room or house using the floor area, ceiling height, occupancy, room type, insulation, sun exposure, and climate. The heating method estimates a furnace or heater load from the room dimensions, the desired temperature difference, and the insulation condition.

Results are shown in BTUs per hour, watts, and, for cooling, tons of capacity. One ton of cooling is 12,000 BTU per hour. These estimates are useful for comparing appliance sizes, but they are not a substitute for a room-by-room Manual J load calculation.

How to use this calculator

1. Choose Cooling or Heating.
2. For cooling, enter the floor area in square feet, the ceiling height in feet, and the number of people who use the room regularly. Then choose the room type, insulation condition, sun exposure, and climate.
3. For heating, enter the room or house width and length in feet, the ceiling height in feet, and the expected temperature difference in degrees Fahrenheit. Choose the insulation condition.
4. Click Calculate and compare the BTU-per-hour result with the capacity on the appliance label.

The default cooling example is a 150-square-foot bedroom with a 9-foot ceiling, two occupants, average insulation, average sun, and an average climate. It returns 6,500 BTU per hour, or about 1,905 watts.

BTU formulas

Cooling base
  = area × 20 BTU/hr per sq ft
  + ceiling allowance
  + room-type allowance
  + 600 BTU/hr for each person above two

Cooling capacity
  = base × insulation factor × sun factor × climate factor

Heating capacity
  = width × length × ceiling
  × temperature difference
  × 0.1778
  × insulation factor

The cooling ceiling allowance is 2,500 BTU per hour plus 1,000 for each foot above 8 feet, with a smaller increment above 10 feet. Insulation factors are 0.8 for good, 1.0 for average, and 1.2 for poor in cooling mode. Sun factors are 0.9 for shaded, 1.0 for average, and 1.1 for sunny. Climate factors are 0.85 for cold, 1.0 for average, and 1.2 for hot. A second-floor room receives a 1.1 multiplier, and living rooms and kitchens add 1,000 and 4,000 BTU per hour respectively. In heating mode, good insulation uses 0.6 and poor insulation uses 2.1.

Assumptions and limitations

• Dimensions are in feet, floor area is in square feet, and temperature differences are in degrees Fahrenheit.
• The cooling method assumes a typical occupied room with two people included in the base load. Each additional person adds 600 BTU per hour.
• The heating method assumes a single open space. It does not account for separate rooms, duct losses, or air infiltration rates.
• Neither method accounts for window area, window glazing, skylights, appliance heat, or the shape and orientation of the building.
• Results are rough sizing estimates. A professional load calculation is the correct way to size HVAC equipment for a whole house.
• Very high ceilings, unusual construction, and extreme climates may need a larger safety margin.

Frequently asked questions

What is a BTU?

A British Thermal Unit is the amount of energy needed to raise the temperature of one pound of water by one degree Fahrenheit. In air conditioning, the BTU rating describes how much heat the unit can remove per hour.

How many BTUs do I need per square foot?

A rough cooling starting point is 20 BTU per hour per square foot, then adjusted for ceiling height, room type, people, insulation, sun, and climate. Heating loads depend more on volume and the outdoor-to-indoor temperature difference.

Is 6,500 BTU enough for a 150-square-foot bedroom?

For the default example, 6,500 BTU per hour is the estimated cooling load. Appliance sizes are sold in standard increments, so choose the nearest available size above the estimate. A room with large west-facing windows or poor insulation may need more.

Source

Reference: Calculator.net — BTU Calculator.

This calculator is a planning aid. Consult an HVAC professional before purchasing or installing equipment.