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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 (0.45 kg) of water 1F. Furnaces are rated based upon how much thermal heat they can produce per hour (BTU/h), while air conditioners are rated based upon how much thermal heat they can remove per hour (BTU/h). Choosing a furnace or air conditioning system with the right number of BTUs per square foot of space can help you heat or cool your home efficiently and costeffectively.
Steps
Method 1
Method 1 of 2:Determining the Correct Heating Capacity

1Measure the square footage of your entire home. If you're installing a furnace, find the square footage of each room in your home and add them together. This will give you the total square footage of your space so you can choose a furnace that will adequately heat your home.^{[1] X Research source }
 For a rectangular room, multiply the length and width, measured in feet.
 For a triangular room, multiply the length and width, then divide by 2.
 For a circular room, measure the radius (“r” which is the distance from the center to the edge). Plug the radius into the following equation, using 3.14 for π: πr^{2}
 For rooms with odd shapes, divide them into regular shapes and measure each shape separately.

2Find out which climate zone you live in to determine the heating factor. Look for a climate zone map online and figure out which zone you live in to help you determine the heating factor, or the number of BTUs you need per square foot to adequately heat your home. In general, the further from the equator you live, the greater the number of BTUs you’ll need.^{[2] X Research source }
The heating factor in BTUs per square foot for each zone is as follows:
Zone 1: 3035
Zone 2: 3540
Zone 3: 4045
Zone 4: 4550
Zone 5: 5055Advertisement 
3Multiply your square footage by the heating factor for your zone. All you have to do to find out what capacity furnace is best for your home based on its location is to multiply the square footage of the space by the heating factor.^{[3] X Research source }
 For instance, if you live in Zone 2 and have a 1,200squarefoot home, multiply 1,200 by 3540 to get a BTU range of 42,00048,000.

4Use the lower end of the range if your home is well insulated or the higher end if it’s not. Better insulated homes need fewer BTUs per hour per square foot than older homes. If your home is new or well insulated, you can use the lower of the 2 numbers for your climate zone; if it is older or poorly insulated, use the higher number of the range.^{[4] X Research source }
 Say that you live in a brand new home in Zone 1. Multiply your square footage by 30 BTUs to find out what capacity furnace you need. Alternatively, if you live in an older home in Zone 6, multiply your square footage by 60 BTUs to ensure you purchase a furnace with enough capacity to heat the space.
 Note that newer homes tend to be better insulated than older homes because of revisions to building codes over the years.

5Take the efficiency rating of the furnace into account. Furnaces are rated not by the actual BTU output you receive but by the amount of heat they generate. How much of the heat a furnace generates (input heat) that actually reaches you (output heat) is a measure of how efficient the furnace is. The efficiency is expressed in percentage as a ratio of the output to input heat. Most modern furnaces are rated as either 80 or 90% efficient.^{[5] X Research source }
 For example, a 100,000 BTU/h input furnace would not be enough to heat a home needing an output of 100,000 BTUs per hour. An 80% efficient furnace would deliver an output of only 80,000 BTU/h (100,000 x 0.8). To find an 80% efficient furnace that does provide enough power, divide the BTU/h rating you need by 0.8. So, 100,000 BTU/h ÷ 0.8 = 125,000 BTU/h, meaning you'd need a furnace rated to 125,000 BTU/h input.
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Method 2
Method 2 of 2:Figuring out the Correct Cooling Capacity

1Measure the square footage of the space you plan to cool. If you want to install a central airconditioning unit, measure the square footage of each room and add all of the numbers together to find the total square footage of your home. Alternatively, if you’re only going to install an A/C unit to cool a single room, find the square footage of that room.^{[6] X Research source }
 Multiply the length by the width, in feet, to find the square footage of a rectangular room.
 Multiply the length and width of a triangular room, then divide that number by 2 for a triangular room.
 Measure the radius of a circular room (“r” which is the distance from the center to the edge), square the number, then multiply that by π (3.14) (the formula is: πr^{2}).
 Divide rooms with odd shapes or alcoves into regular shapes and measure each shape separately.

2Multiply your square footage by 20 BTUs to get a general estimate. Typically, plan to purchase a heating or cooling unit with 20 BTUs for every square foot of space you have. However, keep in mind that factors such as your climate zone, sun exposure, and the number of people living in your home may require you to adjust this figure.^{[7] X Trustworthy Source Consumer Reports Nonprofit organization dedicated to consumer advocacy and product testing Go to source }
 For instance, if you live in a home that’s 800 square feet, you’ll need a unit with 16,000 BTUs. On the other hand, if you live in a 5,000squarefoot home, get a unit with 100,000 BTUs.

3Select an A/C unit with a lower capacity if you don’t have a lot of sun exposure. The position of your home also affects how many BTUs you need to properly heat or cool it. Reduce the capacity by 10% if your home or room gets a lot of shade, or increase the capacity by 10% if your home or room is usually in the sunshine.^{[8] X Trustworthy Source Energy Star Program run by the U.S. Environmental Protection Agency and Department of Energy focused on educating consumers about energy efficiency Go to source }
 Take a look at your sun exposure during the middle of the day in the summer season to help you gauge this accurately.

4Increase the capacity if more than 2 people live in your home. If you have a lot of family members, you’ll need an air conditioner that has a higher capacity. Add 600 BTUs per person for a family of more than 2.^{[9] X Trustworthy Source Energy Star Program run by the U.S. Environmental Protection Agency and Department of Energy focused on educating consumers about energy efficiency Go to source }
 For instance, if there are 6 people living in your home, multiply 600 by 4 to get 2,400. Add 2,400 BTUs to the number you calculated by multiplying your square footage by 20 to ensure your unit will cool your space well.

5Get a system with more BTUs if it will be in the kitchen. Kitchens have lots of appliances that give off heat, including stoves and dishwashers. If you’re installing an A/C unit in your kitchen, select one with 4,000 more BTUs than you need based on the square footage alone.^{[10] X Trustworthy Source Energy Star Program run by the U.S. Environmental Protection Agency and Department of Energy focused on educating consumers about energy efficiency Go to source }

6Factor in the unit’s efficiency rating. While furnaces are rated by their effectiveness in delivering the heat they generate, air conditioners are rated by how efficiently they use electricity over the course of a typical operating year. One such rating is the Seasonal Energy Efficiency Ratio (SEER) rating, created by the Air Conditioning, Heating, and Refrigeration Institute, which is the ratio of the unit's cooling output in BTUs divided by the energy in watthours needed to run it for the entire operating year. (Note that 1 kilowatthour equals 1,000 watt hours.)^{[11] X Research source }
 For example, take a 4,000 BTU/hour air conditioner run for 1,000 hours during an operating year using 400,000 watthours of electric power. This air conditioner would have a SEER rating of 10, since 4,000 x 1,000 / 400,000 = 10.
 To find the average power consumption, divide the unit's power in BTUs per hour by the SEER rating. Since the SEER rating is in units of BTU per Watthour, your answer will be in terms of watts. In the example above, (4,000 BTU/h) / (10 BTU/Wh) = 400 W.
 Central air conditioners manufactured in the United States since January 2006 are required to have a SEER rating of at least 13, or 14 to be Energy Star qualified. Room air conditioners are currently exempt from this requirement; many have SEER ratings closer to 10.
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BTU Cooling Capacity Chart
Community Q&A

Question32,000 BTU equals what in tons?Community AnswerThe math indicates 2.6; however, a 2.6 is not manufactured, so you're dealing with a 2.5 T unit. Take the rated BTUs, 32 K/12,000. 12,000 represents BTUs per ton.

QuestionWhat size air conditioner should I buy for a 1,446 square foot home that has a vaulted ceiling in the living room?Community Answer30,00036,000 BTU, depending on how many square feet the living room takes up, how much higher than a normal 8' ceiling the vault is, how much glass you have, and how well insulated the home is.

QuestionHow many BTUs do I need for a 906 square foot house?Community AnswerAnywhere from 17,500  18,500 BTUs. It depends on how well insulated the home is and how much glass you have.

QuestionIf I'm buying a house that is 2145 sq. ft., will a 2 ton heat pump heat and cool it adequately?Community AnswerDefinitely not. Depending on how well the house is insulated, I'd say you need anywhere between 3.5 and 4 tons.

QuestionDo I include the basement area in the total heating area in calculations to size the furnace?DonaganTop AnswererYes, if you intend to heat the basement.

Question63,000 btu with seer of 14  what tonnage a/c would you suggest in an older home?Community AnswerIt depends on your prevailing climate. For an average US climate, a twoton A/C unit would be sufficient.

QuestionHow do I calculate how much baseboard heater capacity I can connect to a 15 AMP breaker?Community AnswerPer the National Electrical Code (NEC), in general, the calculated load cannot exceed 80% of the circuit rating. Therefore, 80% of 15 AMP = 12 AMP max load. Assuming U.S. nominal voltage of 120V, the maximum heater rating is 120 Volts x 12 Amps max which means 1440 Voltamps max assuming it is a standard resistance heater. If the circuit breaker (CB) is a 15 Amp 2pole CB and the nominal voltage between the two poles is 240V; the maximum heater rating would be 80% x 15A x 240V = 2880 VoltAmps max or 2880 Watts max for a standard resistance heater.

QuestionWill the height of the room affect the calculation?Community AnswerIt will not normally, just as a basement does not affect the furnace requirements. However, if the ceilings are over 12 feet, you might want to double up the square footage for that space.

QuestionWill an air conditioner with a cooling capacity of 27,600 cool a 22,500 square foot home during hot summers?Community AnswerYou will need several AC units. 22,500 square feet. is about the size of about 10 large houses.

QuestionThe gas guy ordered my new Intertherm furnace for my mobile home. I told him the square footage is 1150, but didn't tell him that 200 of that is an addon with no vents from the furnace. Is this dangerous?Community AnswerNo, it should be fine.
Tips
 There are many online BTU calculators you can utilize.Thanks!
 If you're mounting a room air conditioner in a corner window, look for one that can send airflow throughout the room, not into the wall.Thanks!
 Central air conditioning units frequently have the capacity coded into the model number; for example, model RDR36 would be a 36,000 BTU unit. They may also report their size as tonnage, with 12,000 BTUs equal to 1 ton (the amount of power needed to melt 1 ton of ice in 24 hours); thus the model RDR36 would be a 3ton unit.Thanks!
 A poorly insulated home will lose a lot of heat to drafts. Insulating may allow you to use a furnace with a smaller BTU rating.Thanks!
Warnings
 Don't buy a room or central air conditioner with a greater capacity than you need. If you do, the unit will cool the room too rapidly to also take the humidity out of the air. An oversized central air conditioner will also turn on and off more frequently, which costs more and wears out the unit more quickly.Thanks!
References
 ↑ https://krec.ky.gov/legal/Legal%20Forms%20%20Contracts/calc_sqfootage.pdf
 ↑ https://www.inchcalculator.com/calculatemanybtusneededheathome/
 ↑ https://energy.ces.ncsu.edu/hvacheatingandcoolingsystems/
 ↑ https://energy.ces.ncsu.edu/hvacheatingandcoolingsystems/
 ↑ https://www.eeducation.psu.edu/egee102/node/2066
 ↑ https://krec.ky.gov/legal/Legal%20Forms%20%20Contracts/calc_sqfootage.pdf
 ↑ https://www.consumerreports.org/windowairconditioners/howtosizeawindowairconditioner/
 ↑ https://www.energystar.gov/index.cfm?c=roomac.pr_properly_sized.
 ↑ https://www.energystar.gov/index.cfm?c=roomac.pr_properly_sized.
About This Article
To calculate BTU per square foot, start by measuring the square footage of each room you want to heat or cool. Then, add the square footage for each room together. Once you have the total square footage, just multiply that number by 20 to find how many BTUs per hour you'd need to heat or cool the space. For example, if you're trying to heat or cool 1,000 square feet, you would multiply 1,000 by 20 and get 20,000 BTUs per hour. To learn how to adjust BTU per square foot for special conditions, scroll down!
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