**What is BTU?**

British Thermal Unit (BTU) is the basic measure of heat energy in the Imperial system. One BTU is defined as the amount of heat necessary to raise 1 pound (0.454 kg) of water 1 degree Fahrenheit (0.55 degrees Celsius). In HVAC industry, Btu’s measure the quantity of heat a conditioning unit can remove from a room per hours. One BTU per hour is equal to 0293 watts. As the Btu rating increases, so does the weight, size and cost of the air conditioner.

**Choosing the right BTU**

Picking the right home air conditioner is very easy with the right information. Before buying an air conditioner, find out how many BTUs you need. This means think of the size of the room, and what the room will be used for. Matching Btu requirements to room size is very important when you buying an air conditioner system. This air conditioner sizing tutorial will explain you how to choose the right air conditioner size. Choosing an air conditioner size that is too small (low BTU rating) for the intended room will leave you feeling hot and flustered! And buying an air conditioner that is too big for the room can have its problems too.

The following tables are estimates based on average room conditions in a moderate climate. Exceptions are Kitchens – add 4,000 BTU cooling capacity to the recommended size. ** **High Ceilings/Unusual Floor Plans – increase BTU by 10%. Warmer Climates – may require 10-20% additional BTUs per room over regular amount. **BTU size too large:** A conditioner that has a bigger BTU rating than the room size needs will cycle off too quickly. The unit will not remove the humidity from the air properly. **Air Conditioner size too small:** A conditioner that has a smaller BTU rating than the room requires will run continuously. The unit will not cool the room adequately.

**How to calculate BTU per square foot**

The British Thermal Unit (BTU) is the basic measure of heat energy in the Imperial system. One BTU is defined as the amount of heat necessary to raise 1 pound (0.454 kg) of water 1 degree Fahrenheit (0.55 degrees Celsius). Knowing the number of BTUs required per square foot of space is important in choosing the right size furnace and air conditioning system to heat and cool your house efficiently and effectively. Here’s how to determine the amount of space to be heated or cooled and how to choose the right or air conditioner unit for the job.furnace

Room Size | Square Footage | Recommended BTU Range |

10’x15′ | 150 | Up to 5400 |

10’x20′ | 200 | 6000 |

17’x20′ | 340 | 8000 |

20’x20′ | 400 | 9000 |

18’x25′ | 450 | 10000 |

22’x25′ | 550 | 12000 |

25’x30′ | 750 | 14500 |

25’x34′ | 850 | 16000 |

25’x40′ | 1000 | 18000 |

35’x40′ | 1400 | 24000 |

38’x40′ | 1520 | 28500 |

40’x40′ | 1600 | 32000 |

**Determining how much to heat or cool** ** **

(1)** Determine the square footage of each room to be heated or cooled**. Most rooms are either rectangular or triangular in shape. Rooms that aren’t either of these shapes can be broken down into spaces composed of these shapes. An octagonal room can be broken down into a room with a rectangular center area and four ends each composed of a small rectangle flanked by a right triangle on either side. You would figure the square footage for each component area and then add them together to get the total square footage of the room.

For a rectangular room or area, multiply the length by the width. (Remember that a square is a rectangle whose length and width are the same distance.) For a triangular room or space, multiply the length by the width and divide by 2 (the geometric formula of 1/2 the base times the height).

For a triangular room or space, multiply the length by the width and divide by 2 (the geometric formula of 1/2 the base times the height).

If the room is circular, multiply the distance from the center to a wall by itself and then by 3.14 (pi rounded to 2 decimal places). In the case of an octagonal or hexagonal room, you can also approximate the square footage by treating it as a circular room and measuring from the center to the edge of one of the walls.

(2) **Find the total square footage of the house to be heated or cooled**. If buying a furnace or central air conditioning system, you’ll need to calculate the total square footage of the house. Simply add the square footage of each room. Include basement, upstairs, and attic rooms if you have ducts running to those floors.

(3) **Determine the necessary heating or cooling capacity**. The capacity of furnaces and air conditioners is given in BTUs per hour. You need to establish the correct number of BTUs per square foot for your house needed to either heat or cool it. The methods differ for heating and cooling and are described below.

At a bare minimum, you can use a figure of 20 BTUs per square foot and multiply this by the total square footage to be heated or cooled, but this does not take into account climate, the amount of insulation you have, or the way you live.

If the room to be cooled is a kitchen, increase the necessary capacity by 4,000 BTUs.

http://www.wikihow.com/Calculate-BTU-Per-Square-Foot