BTU Calculator

The BTU Calculator estimates the amount of BTUs required to heat or cool homes based on the dimensions, insulation conditions, and the desired temperature changes. The latter is the proper increase/decrease from unaltered outdoor temperatures to the desired temperatures. As an example, an unheated Boston home during winter could be as low as 0°F. A comfort temperature of 75°F within the home would require a desired temperature increase of 75°F. This calculator can only gauge rough estimates.

Room/House Width
Room/House Length
Ceiling Height
Insulation Condition
Desired Temperature
Increase or Decrease

e.g. 75°F for Boston winter, 45°F for Atlanta winter.


The British Thermal Unit, or BTU, is an energy unit. It is approximately the energy needed to heat one pound of water by 1 degree Fahrenheit. 1 BTU = 1,055 joules, 252 calories, 0.293 watt-hour or the energy released by burning one match. 1 watt is approximately 3.412 BTU per hour.

BTU is often used as a point of reference for comparing different fuels. Even though they're physical commodities and quantified accordingly, such as by volume or barrels, they can be converted to BTUs depending on the energy or heat content inherent in each quantity. BTU as a unit of measurement is more useful than physical quantity because of fuel's intrinsic value as an energy source. This allows many different commodities with intrinsic energy properties to be compared and contrasted; for instance, one of the most popular is natural gas to oil.

BTU can also be used pragmatically as a point of reference for the amount of heat that an appliance generates; the higher the BTU rating of an appliance, the greater the heating capacity. As for air conditioning in homes, even though ACs are meant to cool homes, BTUs on the technical label refer to how much heat the air conditioner can remove from their respective surrounding air.

Room/House Width, Length, and Ceiling Height

Obviously, a smaller area room or house with shorter lengths and widths require fewer BTUs to cool/heat. However, volume is a more accurate measurement than area for determining BTU usage because ceiling height is factored into the equation; each three-dimensional cubic square foot of space will require a certain amount of BTU usage to cool/heat accordingly. The smaller the volume, the fewer BTUs are required to cool or heat.

Insulation Condition

Thermal insulation is defined as the reduction of heat transfer between objects in thermal contact or in range of radiative influence. The importance of insulation lies in its ability to lower BTU usage by managing as much as possible the inefficient wasting of it due to the entropic nature of heat – it tends to flow from warmer to cooler until there are no longer temperature differences.

Generally, newer homes have better insulating ability than older homes due to technological advances as well as more strict building code. Owners of older homes with dated insulation that decide to upgrade will not only improve on the ability for the home to insulate (resulting in friendlier utility bills and warmer winters), but also have the value appreciation of their homes.

The R-value is the commonly used measure of thermal resistance, or ability of heat to transfer from hot to cold through materials and their assembly. The higher the R-value of a certain material, the more it is resistant to the heat transfer. In other words, when shopping for home insulation, higher R-value products are better at insulating, though they're usually more expensive.

When deciding on the proper input of insulation condition into the calculator, use generalized assumptions. A beach bungalow built in the 1800s with no renovations should probably be classified as poor. A 3-year-old home inside a newly developed community most likely deserves a good rating. Windows normally has poor thermal resistance than walls. Therefore, a room with lots of windows normally means poor insulation. When possible try to install double glazed windows to improve insulation.

Desired Temperature Increase or Decrease

To find the desired change in temperature to input into the calculator, find the difference between the unaltered outdoor temperature and the desired temperature. As a general rule of thumb, a temperature between 70 and 80°F is a comfortable temperature for most people.

For example, a house in Atlanta might want to determine the BTU usage during winter. Atlanta winters tend to hover around 45°F with chances to reach 30°F occasionally. The desired temperature of the dwellers is 75°F. Therefore, the desired temperature increase would be 75°F - 30°F = 45°F.

Homes in more extreme climates will obviously require more radical changes in temperature, resulting in more BTU usage. For instance, heating a home in Alaskan winter or cooling a home during a Houston summer will require more BTUs than heating or cooling a home in Honolulu, where temperatures tend to stay around 80°F year-round.

Other Factors

Obviously, size and space of house or room, ceiling height, and insulation conditions are very important when determining the amount of BTUs required to heat or cool a house, but there are other factors to keep in mind: