As you start shopping for a new central air conditioner, heat pump, or gas furnace, chances are you’ll come across the acronym BTU. But what does BTU mean?
BTU stands for British Thermal Unit, a unit of measurement that shows just how much energy your heating or cooling unit uses to remove heat from your home within an hour. Specifically, one BTU equals the amount of heat required to lower or raise the temperature of one pound of water by one degree Fahrenheit.
It’s a standard unit for the measurement of heat energy. That means it can be used to rate energy transfer for air conditioners, heat pumps, furnaces, cooking devices, and other heating and cooling appliances.
By Anne Fonda
Air conditioners and heat pumps remove heat from the air inside your home and move it outside. Using an intricate system of coils, fans, and refrigerant, they replace that hot and humid air with cooler air that travels throughout your home via ductwork.
When we talk about AC units and heat pumps removing heat from your home, it’s the same as saying that they’re moving energy. BTUs tell you how much energy your air conditioner is using to do all that. So, if you see that your air conditioner has 12,000 BTUs, that means it’s absorbing 12,000 British Thermal Units of heat each hour and moving it outside so your environment inside feels better.
When you get a new HVAC system, you want to ensure correct HVAC sizing. If your system is too big or too small for your home, you’ll run into problems. Work with an HVAC professional to ensure you get a Goldilocks system – just the right size to heat and cool your home efficiently, with maximum comfort.
BTUs are also part of the Manual J Calculation, a mathematical method HVAC professionals use to accurately determine how much heating and cooling a home your size requires. You can request a Manual J calculation from an experienced HVAC technician.
Getting an air conditioner or heat pump with too many BTUs for a home your size can increase your energy costs. That’s because your air conditioner will run for a shorter period and waste more energy doing what it’s built to do.
An air conditioner with BTUs that exceed the recommended square footage in your home may also take on more wear and tear over time because it will initiate its on/off cycle more frequently. It could also create a subpar cooling experience because your AC’s compressor may turn off much sooner than it should. So, instead of your AC removing an adequate amount of heat from your home, it leaves some behind, creating a hot, sticky, humid environment you probably won’t enjoy.
Getting a central air conditioning unit or heat pump with too few BTUs for a home your size isn’t much better, and can also result in the need for AC troubleshooting. Your air conditioner may never stop running because it’s trying to reach a temperature in a large space that it just can’t cool.
It may never sufficiently cool your home, creating uneven temperatures across your house. And it could cost you more in energy bills because your AC or heat pump is expending extra energy to do what you tell it to do via your thermostat.
That’s a good question. Generally speaking, a cooling unit needs about 20 BTUs for each square foot of living space that it’s cooling. To get an approximate idea of how many BTUs you need, multiply the square footage of your space by 20.
For example, let’s say you need to find an air conditioner or heat pump with enough capacity to cool your 1,400-square-foot home. You should look for a unit that’s about 28,000 BTU (1,400 x 20 = 28,000.)
Remember, this is just a simple, general estimate that doesn’t apply to all situations. A true BTU calculation will take your ceiling height and the size of your doorways and windows into consideration, as well. This BTU rating chart gives you a rough guide to follow based on square footage.
| Home size (sq. ft.) | Unit Size (tons) | Unit Size (BTUs) |
| 1,200 sq. ft. | 2-ton air conditioner/heat pump | 24,000 BTUs |
| 1,800 sq. ft. | 3-ton air conditioner/heat pump | 36,000 BTUs |
| 2,400 sq. ft. | 4-ton air conditioner/heat pump | 48,000 BTUs |
| 3,000 sq. ft. | 5-ton air conditioner/heat pump | 60,000 BTUs |
Cooling units also come in half-ton sizes, including 2.5-ton and 3.5-ton.
This BTU/square footage calculation has evolved over the years due to improved construction material and insulation requirements used today vs historically. Thirty years ago, the rule of thumb was 500 SF/ton. More recently, this estimation would be 600 SF/ton for existing residential applications..
However, with the updated insulating materials used in new construction, it may be closer to 1000 SF/ton for residential. An HVAC contractor may also need to adjust the SF/ton for your home if you’ve installed energy-efficient replacement windows and doors, made insulation upgrades, and more.
So if one HVAC contractor recommends one unit size and another recommends a larger capacity cooling system, ask them to show you their calculations to back up their recommendation.
If you’re still with us, you need to know that calculating the BTUs you need for a gas furnace uses a different formula. The gas furnace BTU calculator uses a different conversion factor. According to the Energy Information Administration, the conversion factor is
1 cubic foot = 1.036 BTU and 1 therm = 100,000 BTU
Please work with an experienced HVAC professional for the proper sizing of your gas furnace. There is no one-size-fits-all answer. Two homes of the exact same size in the same neighborhood might need different size furnaces.
Common furnace sizes include:
Your climate zone, the size of your house, home insulation, furnace efficiency (AFUE rating), and more will all affect the furnace size that is appropriate for your home And just like with an undersized or oversized air conditioner, a furnace that’s the wrong size can waste energy and decrease your comfort level. Work with HVAC professionals to get the right furnace.
The BTU to tons conversion is 12,000 BTUs = 1 ton. HVAC tonnage is not a measurement of your air conditioner or heat pump’s weight. In HVAC terms, it’s another way to describe your unit’s cooling capacity, or how much heat your cooling unit is capable of absorbing to reduce a room’s temperature. One ton of cooling (or refrigeration) refers to the amount of heat required to melt a ton of ice over 24 hours.
Wait, why ice? Good question! Before the advent of electric air conditioning, people cooled their homes and businesses with large blocks (or tons) of ice. As it slowly melted, the ice absorbed the heat in the room. The process worked just like the modern-day air conditioner.
The ice didn’t produce cool air but instead removed hot air from the room to make it feel cooler and more comfortable. The HVAC industry continues to use “tons” of ice to measure how much heat air conditioners and heat pumps remove. Tons are not used to describe how much heat a furnace can produce.
The BTU to square feet conversion is 24,000 BTUs per 1,000 sq. ft. So, 12,000 BTUs for 500 sq ft, or 48,000 BTUs for 2,000 sq. ft, and so on.
Before you commit to a particular AC unit or heat pump, you may want to know how much electricity it may consume to cool your home. That way, you get an idea of how it will influence your utility costs. BTUs, when converted to kilowatts and kilowatt-hours, can help you figure that out.
First, a quick explainer: A kilowatt is 1,000 watts, which is a measurement of power. A kilowatt hour measures the amount of energy needed to run an appliance, tool, or machine for one hour. When it comes to your air conditioning, a kilowatt hour tells you how much energy your AC is using, which could be instrumental in helping you decide what unit to buy.
For a simple BTU to kilowatt conversion, take your BTUs and multiply them by 0.000293 — the number of kilowatt-hours in one BTU. For example, if you’re looking for an air conditioner with 18,000 BTU, you’d get 5.27 kilowatts or 5,270 watts. Let’s say your AC runs for four hours a day. Within that period, your air conditioner would use 21.08 kilowatt hours (5.27 kilowatts x 4 hours).
18,000 BTU x 0.000293 = 5.27 kilowatts
5.27 kilowatts x 4 hours a day = 21.08 kilowatt hours for your air conditioner
If you want to determine your estimated monthly cooling costs, multiply your kilowatt hours (21.08, in this case) by 30 for 30 days in a month. That gives you 632 kilowatt hours per month. You’ll need to multiply this figure by the electricity rate you’re charged each month.
According to the Bureau of Labor Statistics (BLS), as of July 2024, the average electricity rate in the United States was 17.8 cents per kilowatt hour. Rates vary by state – check the table or your utility bill for your rate. By multiplying your electricity rate by your kilowatt hours, you can determine your energy costs:
21.08 kilowatts x 30 days in a month = 632 kilowatt-hours per month
632 kilowatt-hours x 0.17 cents per kilowatt hour = $107.44
We know that’s a lot of number-crunching. But hopefully, those calculations can help you make a more informed purchasing decision.
When we compile information like this, our goal is to help homeowners make informed decisions. We work with HVAC engineers and HVAC professionals with decades of experience. Our glossary items and blogs undergo expert review to ensure you get the information you need.
To help ensure you get the HVAC system that’s appropriately sized to offer the comfort solutions you crave, consult with your local Trane dealer. They’ll answer any questions you have and guide you through the process of finding the right system for your home size, energy efficiency requirements, and budget.
Anne Fonda, Content Writer at Trane Technologies
A Content Writer with Trane Technologies, Anne Fonda researches topics and writes for Trane® and associated residential HVAC brands. She works in collaboration with Trane Technologies subject matter experts, offering easy-to-understand, informative content on complex topics. Her goal is to help consumers make informed decisions on the products and services they need.
She has written for HVAC and other service provider websites for over 16 years. Before transitioning to web content writing, Anne had a 14-year stint as an award-winning journalist. She graduated cum laude from the University of Missouri-Columbia School of Journalism. When she’s not working, Anne enjoys playing word games, reading, gardening, spending time with family, and visiting gardens and museums.
Expert review by: Kimberly Sexton, Senior Systems Engineer
An extension of BTU, BTUH stands for British Thermal Units per Hour. This metric is used to determine how much heat an air conditioner can remove from a room within an hour.
HVAC capacity is the production capability of a heating or cooling unit. Air conditioning capacity and heating capacity are measured in tons or BTUs.
A ton in HVAC terms refers not to the weight of your central air conditioner or heat pump but to its cooling capacity. One ton corresponds to 12,000 BTUs.
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