How to Calculate BTU

Written by joan whetzel
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How to Calculate BTU
The number of BTU's emitted into a room determines the air conditioning capacity. (impianto industria image by Fotograv from

Technically speaking, a British Thermal Unit (BTU) is a measure of the energy necessary to increase the temperature of one pound of water by one degree Fahrenheit. BTUs are created by everything from our bodies, to our kitchen ovens, to our computers. Choosing the best air conditioning unit to compensate for all these BTUs means first finding the amount of BTUs being generated, then selecting an air conditioner with a BTU-per-hour rating robust enough to equalise the heat.

Skill level:

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Things you need

  • Calculator
  • Graph paper
  • Measuring tape or laser distance measuring device
  • Pencil with eraser

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  1. 1

    Using the measuring tape or laser distance measuring device, find the room's floor area. Draw the floor area onto the graph paper, making each square equal to 1 square foot, 1/2 square foot or 1 square meter---whatever works best for you. Draw in locations for windows and lighting, desks and furniture, and equipment or oven/stove.

  2. 2

    Calculate the room's heat load:

    -Floor area in square meters: length (m) x width (m) x337

    -Window size and position, with or without blinds: South window: window length (m) x width (m) x 870 North window: windows length (m) x width (m) x 165 Windows with no blinds: multiply the result(s) by 1.5

    -Number of people: Number of occupants x 400

    -Heat produced by computer equipment or kitchen appliances: (Server wattages + Switch wattages + Routers wattage) x 3.5 or (Stove wattage + Oven wattage) x 3.5

    -Heat emitted by lighting: Sum of all lighting wattage x 4.25

  3. 3

    Record the BTUs for all the relevant factors (above) for the room. Add them together to find the best estimate of total heat (BTUs) generated. The air conditioner unit's BTUs/hour rating must be at least this number or higher to keep the room cool.

  4. 4

    Determining the necessary AC unit size is essential for two reasons. First, a unit that's too small won't cool effectively, which could lead to equipment failure in the long run. Second, a unit that's oversized will cycle on and off too quickly; meaning its run-time is too short to properly dehumidify the room, it won't cool the area evenly and it will eat up more electricity than it needs to.

Tips and warnings

  • You may need to get wattage information from the company that makes the computer equipment or kitchen appliances.
  • Even gas stoves and ovens put out wattage or BTUs, this needs to be taken into account.
  • For large spaces, especially with equipment or appliances that create a lot of heat, seek advice from a qualified contractor who can double-check your calculations and advise you on additional considerations when choosing an air conditioning unit.
  • Disclaimer: These calculations are meant only as ballpark estimates. Absolute accuracy can't be guaranteed. Before deciding on an air conditioner, consult a qualified air conditioning specialist, installer or a contractor.

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