BTU Calculator

The AC form starts from a common rule of thumb, about 20 BTU per hour for each square foot of floor area with an 8-foot ceiling, and scales it for taller ceilings. It then adjusts for insulation, sun and climate, adds 600 BTU/h for each regular occupant beyond two and 4,000 BTU/h for a kitchen, following ENERGY STAR's room air conditioner guidance. A 150-square-foot bedroom with 9-foot ceilings works out to about 3,375 BTU/h by this rule, less than the smallest common window units, so in practice you would pick the smallest model that matches ENERGY STAR's chart for the floor area, shown in the results.

What is a BTU?

A British thermal unit (BTU) is the amount of heat needed to raise the temperature of one pound of water by one degree Fahrenheit. It equals about 1,055 joules, roughly the energy released by burning a kitchen match. Furnaces, boilers, air conditioners and heat pumps are rated in BTU per hour (BTU/h), often shortened to just BTU, which describes how much heat they can add or remove each hour. One watt is about 3.412 BTU/h.

Cooling equipment is also sized in tons. One ton of cooling is 12,000 BTU/h, the heat absorbed by melting a ton of ice over 24 hours, a unit left over from the days of ice-based cooling. A typical central air conditioner for a home is 1.5 to 5 tons. Natural gas is sold in therms, where one therm equals 100,000 BTU.

Size and Ceiling Height

The amount of air to cool or heat grows with both floor area and ceiling height, so a room with 10-foot ceilings needs about 25% more capacity than the same room with 8-foot ceilings. ENERGY STAR's sizing chart for room air conditioners, based on 8-foot ceilings, is a useful reference:

Insulation Condition

Insulation slows the flow of heat through walls, ceilings and floors, and its effectiveness is measured by R-value: the higher the number, the better it resists heat flow. A well-insulated, tightly sealed room holds its temperature, so it needs a smaller unit than a drafty room with many windows. The calculator lowers the estimate by about 10% for good insulation and raises it by about 20% for poor insulation.

Air leaks are often as important as insulation itself. Gaps around doors, windows, attic hatches, plumbing and wiring let conditioned air escape. The US Department of Energy recommends sealing these leaks with caulk and weatherstripping before adding insulation, and many older homes have far less attic insulation than current codes call for. Windows are the weakest part of most walls; single-pane glass loses heat several times faster than an insulated wall. Improving the building envelope lets you buy a smaller, cheaper system and cuts energy bills every year.

Desired Temperature Increase or Decrease

The general-purpose form sizes equipment from the room's volume and the temperature difference you need to maintain between inside and outside. For heating, use the difference between the indoor temperature you want and a typical cold outdoor temperature for your area; for cooling, the difference between a hot day and your thermostat setting. It multiplies cubic feet by the temperature difference and an insulation factor of about 0.133 BTU/h per cubic foot per °F for average construction. A 12 × 12 ft room with 9-foot ceilings and a 50°F rise needs about 8,600 BTU/h.

Other Factors

Many other details affect how much heating or cooling a space needs. Professional contractors account for them with a load calculation, such as ACCA Manual J in the US, which looks at each wall, window and duct run.

Oversizing is a common mistake. An air conditioner that is too large cools the room quickly and shuts off before it has removed much moisture, leaving the space cold and clammy, and frequent short cycles waste energy and wear out the compressor. An undersized unit runs constantly and may never reach the set temperature on the hottest days. Aim for a unit close to the estimate rather than a much bigger one.

Heat pumps can both heat and cool, and modern cold-climate models keep working well below freezing, though their heating output drops as the outdoor temperature falls. Check the capacity at your design temperature, not just the rated capacity.

Frequently asked questions

Sources

• ENERGY STAR – Room Air Conditioners
• US Department of Energy – Insulation