How BTU Is Calculated DIY?
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           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 (0.45 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 cost-effectively.
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1. Measure 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.
● 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: πr2
● For rooms with odd shapes, divide them into regular shapes and 
measure each shape separately.
● If the room is not rectangular, you can try to break it up into 
rectangles so that you can calculate several lengths times widths.
● For example, if your room is L shaped, break that up into two non-
overlapping rectangles. Then calculate length times width for both 
rectangles and add them up.
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2.Find 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.
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3. Multiply 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.
● For instance, if you live in Zone 2 
and have a 1,200-square-foot 
home, multiply 1,200 by 35-40 to 
        
get a BTU range of 42,000-
48,000.
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4. Use 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.
● 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.
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5. Take 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.
● 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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1. Measure the square footage of the 
space you plan to cool. 
If you want to install a central air-conditioning 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.
● 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: 
πr2).
● Divide rooms with odd shapes or alcoves into regular 
shapes and measure each shape separately.
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2. Multiply 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.
● 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 5,000-square-foot 
home, get a unit with 100,000 BTUs.
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3. Select 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.
● Take a look at your sun exposure 
during the middle of the day in 
the summer season to help you 
gauge this accurately.
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4. Increase 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.
● 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.
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5. Get 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.
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INNOVATION PERFORMANCE RESULTS 6. Factor 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 watt-hours needed    
to run it for the entire operating year. (Note that 1 kilowatt-hour equals 
1,000 watt hours.)
● For example, take a 4,000 BTU/hour air conditioner run for 1,000 
hours during an operating year using 400,000 watt-hours 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 Watt-hour, 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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