What is BTU? Ultimate Guide to BTU Rating 2024

what is btu

This guide to BTU meaning answers your questions and is designed to help you understand the BTU or heat capacity options for furnaces, air conditioners, boilers and other home heating and air conditioning appliances.

BTU is a type of rating.

You might be surprised that BTU ratings apply to air conditioners (where’s the heat?), not just to heat-producing equipment.

What is BTU / What are BTUs

BTU Meaning – BTU stands for British thermal unit. OK, but what are British thermal units?

A BTU, also spelled Btu, is defined as the amount of heat required to heat or cool one pound of water 1 degree Fahrenheit when the water is at sea level.

A gallon of water weighs 8 pounds. And there are almost 16 cups in a gallon. So one pound of water is about 2 cups of water.

That’s quite a lot of heat! That gives us an idea of just how much heat a gas furnace can create. They are built in various sizes. The smallest produce about 40,000 BTU per hour The largest produce 120,000 or more per hour.

Wait a minute. The definition includes “cool” too? Heat cools? Obviously not. Why does BTU apply to air conditioners too? That’s explained below, but you may already know the answer or perhaps you can figure it out.

What is BTU/H?

It has already been alluded to – BTU/H stands for British Thermal Unit per Hour. It is also written BTU/h and Btu/h.

To say a furnace creates 80,000 BTU is rather meaningless. At every instant? Over its lifetime?

To standardize the sizing of furnaces, boilers, air conditioners, water heaters, gas ranges, gas dryers and a hundred other appliances that burn gas/propane/oil, the rating is “per hour.”

How many BTUs of heat does the appliance produce if it runs steady for one hour?

That’s BTU/h.

Air Conditioner BTU – What’s That About?

We just asked this question: “How many BTUs of heat does the appliance produce if it runs steady for one hour?” Then said, “That’s BTU/h.”

All true but for one omission. We should have added “or remove” after the word “produce.” The most complete question should be, “How many BTUs of heat does the appliance produce or remove if it runs steady for one hour?”

That’s what air conditioning does. There’s no way to “make cold air.” What we call “cold” is just the absence of heat. If you want to cool the air around you, the only way to do it is to remove heat. And the time-tested best way of doing it is through evaporation.

How ACs and heat pumps work: We’ll make this brief.

Both ACs and heat pumps use refrigerant to collect heat. Once captured, the heat can be circulated and dumped elsewhere.

When refrigerant evaporates inside a closed coil, it collects heat – just like water evaporating off your skin takes heat and cools your skin.

Heating: The refrigerant expands in the outside coil – evaporates. It collects heat from outside. The refrigerant is pumped to the inside coil where it condenses, releasing heat.

Air Conditioning: The physics are opposite. The refrigerant evaporates in the indoor coil, carries it to the outside coil and dumps it as it condenses.

BTU in Heating Equipment – And AC

heating units and ac

Here are common heating and cooling equipment components and the amount of BTU/h they are capable of.

This is residential equipment; Commercial equipment can be more powerful.

Gas and Oil Furnaces

Gas furnaces burn either natural gas (NG) or propane (LP for liquid propane). NG furnaces are the most common. Oil furnaces burn home heating oil.

  • Gas furnace BTU Range: 40,000 to 140,000 BTU/h
  • Oil furnace BTU Range: 60,000 to 160,000 BTU/h

These ranges are approximate and vary slightly by manufacturer.

How big should my furnace be?

How many BTU/h do I need?

These are common questions. It depends on your climate. Obviously, the colder your climate is, the more BTU/h you’ll need.

The answer is 30 to 60 BTU per hour per square foot of space.

On our main Furnace Guide page, we have a climate zone map of the US. It shows 7 zones plus a list of how many BTU/h are recommended by furnace specialists.

In the warmest climates, a 2,000 square foot home would need a furnace with an output of 60,000 BTU furnace.

The same home in the coldest climate would require an output of 120,000 BTU furnace.

Get a Load Calculation Done! We highly recommend asking your HVAC contractor to do a load test, there are several options including the popular Manual J load calculation. It’s the best way to ensure your furnace – or any heating or air conditioning equipment – is sized properly for your home. On the main Furnace Page full discussion of the topic, “What Size Furnace Do I Need?

What does furnace output mean? That topic is discussed after we’ve covered the BTU range of residential heating and cooling equipment.


Boilers heat water, though most don’t bring it to an actual boil. The water is circulated through pipes to radiators or tubes where the heat releases to warm the air. Most burn gas or oil, but a few are electric.

Combi boilers – combination boilers for heating your home and your hot water – are quite popular.

  • Standard boiler BTU range: 45,000 to 300,000 BTU/h, but most are in the 75,000 to 180,000 BTU/h range.
  • Combi boiler BTU range: 100,000 to 199,000 BTU/h.

Input / Output – The same efficiency issue applies to any equipment that burns fuel, and it is covered below.

How big should my boiler be?

The range is from about 20 BTU/square foot to 60 BTU/square foot in the coldest climates.

Our main Boiler Buying Guide page also has the climate zone map and suggested boiler size for your area. If you choose a combi boiler, the size will be larger than if just using it to heat your home (not water too).

We’ve reviewed all the top boiler brands too, and you’ll find links on the Boiler Buying Guide page.

Outdoor Wood Furnaces & Boilers

Outdoor wood furnaces provide heat only. Boilers provide heat and hot water. The water from the boiler doesn’t flow into your water lines. Rather, it travels through piping in your water heater’s tank, heating the water as heat transfers through the pipe into the water in the tank.

  • Wood furnace and boiler BTU range: 120,000 to 400,000 BTU/h. Most are between 150K and 240K BTU/h.

Our main Outdoor Wood Furnace and Boiler Guide has full details plus links to the PickHVAC Reviews of the top boiler brands.

Indoor Wood and Pellet Stoves

Stoves produced for use indoors are smaller than outdoor wood stoves and boilers. Full details are found on our Wood & Pellet Stove Buying Guide.

  • Indoor wood & pellet stove BTU range: 40,000 to 100,000 BTU/h.

There are various efficiencies too, so yes, the “input vs output” difference applies, and it is discussed below.

Electric Furnaces

These all-electric units have huge heating coils, like really big space heaters. A fan then pushes the heat either directly into the air of your home or more commonly through ductwork to the various rooms.

According to our Electric Furnace Buying Guide, this equipment ranges from 10kW/hour to about 25kW/hour.

KW to BTU: Each kW, or kilowatt, is equivalent to about 3,400 BTUs.

  • Electric furnace BTU range: 34,000 to 85,000 BTU/h.

Electric furnaces are used in warm to moderate climates. They are “100% efficient,” but electricity is expensive to produce. These furnaces have the highest operating cost of any furnace type. If you are shopping for any electric heating equipment – an electric furnace, electric heat strips for your heat pump air handler or even a space heater and want to know how much heat it creates, use the kW to BTU formula: 1kW = 3,400 BTUs.

For example: A 1.5 kW space heater (usually written as 1,500 watts) is equal to approx. 5,100 BTU/hour of heating capacity.

What size electric furnace do I need?

The recommendation starts at about 30 BTU/square foot in hot climates to 45 BTU/square foot in moderate regions.

Conventional Water Heaters

Your options are tank style water heaters and the type that is growing in popularity – tankless water heaters.

  • Tank style water heater BTU range: 40,000 to 199,000 BTU/h.
  • Tankless water heater BTU range: 10,000 to 199,000 BTU/h.

10K BTU/h seems small. Yes. These are primarily “point of use” models. They’re dedicated to one bathroom, a hot tub or jetted tub, pool heater, etc.

What size water heater do I need?

The answer isn’t calculated by home size but by how many bathrooms you have and the size of your household – in other words, how much hot water is likely to be needed at any given time.

Our Water Heater Buying Guide has full details that will help you choose the right water heater for your household. We’ve reviewed the top brands too, and you’ll find links on the Buying Guide page. Rinnai, Rheem, Noritz, Navien, etc., the reviews are available.

Central Air Conditioners & Heat Pumps

This section is about split system equipment and ductless or mini split equipment.

Air conditioners move heat outside in warm weather. Heat pumps have something called a reversing valve that allows them to reverse the flow of refrigerant and move or “pump” heat indoors in cold weather. They are remarkable pieces of equipment.

This equipment does not burn fuel! You knew that, of course. The fuel is burned at the electric plant. Electricity arrives at your home through the grid and powers your AC or heat pump.

Heat pumps are more efficient than gas/oil furnaces for this reason: The amount of fuel burned to produce the electricity they need to operate is a fraction of what a furnace burns.

Rather than burning fuel to make heat, they use a small amount of electricity to circulate refrigerant – and the refrigerant captures and moves heat.

So, when we talk about BTU/h, it’s not heat created but heat moved.

  • Split system central air conditioner BTU range: 18,000 to 60,000 BTU/h.
  • Split system central heat pump BTU range: 18,000 to 60,000 BTU/h.
  • Ductless mini split system AC and heat pump BTU range: 6,000 to 48,000 BTU/h (though a few are larger, this is the most common range).

What size heat pump or AC do I need?

Heat pumps: 16 to 30 BTU/square foot is recommended. Heat pumps aren’t typically used in cold climates. There, when used, models that can produce up to 50 BTU/square foot are required.

Air conditioners: 12 to 30 BTU/square foot is recommended. The warmer the climate, the more powerful the AC should be.

See our main pages for each equipment type for details.

Our Guides are more comprehensive than anything else found online. We think you’ll agree.

BTUs and Tons – In central heat pump and air conditioner lingo, the term “tons” is used to describe heating and cooling capacity.

12,000 BTUs equals 1 ton of heating or air conditioning capacity.

Room Air Conditioners – Window ACs & Portable ACs

These room air conditioners have limited cooling capacity due to their smaller size and their purpose – cooling one room or small zone.

There are differences in the number of recommended BTUs per square foot needed between window air conditioners and portable air conditioners. We’ll explain why in a minute.

  • Window air conditioner BTU range: 5,000 to 34,000 BTU/h. The most popular models are 8,000 to 12,000 BTU/h.
  • Portable air conditioner BTU range: 6,000 to 14,000 BTU/h.

What size window air conditioner do I need?

30 to 40 BTU/square foot is recommended. Much of it depends on how hot the room gets, ceiling height, how many people are in the room and other factors.

Our Window Air Conditioner Reviews and Guide tells the complete story and reviews the best window air conditioners available.

What size portable air conditioner do I need?

If you look at the standard manufacturer ratings, then it is the same as window air conditioners.

However, in the last few years, rules for rating portable air conditioners have changed for one simple reason: They are about half as efficient as window air conditioners.

Why? Because as they are blowing heat out of your home, they are sucking in heat through cracks and drafty windows/doors.

So the US DOE has created a second rating system just for portable air conditioners. It is called Seasonally Adjusted Cooling Capacity. The old standard – the standard for window air conditioners – is ASHRAE.

Here is the difference – quoted from our Portable Air Conditioner Review and Guide.

  • 14,000 ASHRAE = 7,500 SACC
  • 12,000 ASHRAE = 6,500 SACC
  • 10,000 ASHRAE = 6,000 SACC
  • 8,000 ASHRAE = 4,500 SACC
  • 6,000 ASHRAE = 3,000 SACC

Not all product pages list the SACC rating. Many list them both. Only a few list just the SACC rating. If there is only one BTU rating given, it is probably the ASHRAE rating and you should consider that the SACC rating is about half.

So again, what size portable air conditioner do I need?

60 to 80 BTU/h is recommended.

BTU/h Needed: Central vs Window vs Portable ACs

We’ve listed three recommended BTU/h ratings:

  • Central air conditioners: 12 to 30 BTU/h
  • Window air conditioners: 30 to 40 BTU/h
  • Portable air conditioners: 60 to 80 BTU/h

You’re likely wondering what the difference is. There are a few.

1. Effectiveness: Central air conditioners are more efficient at cooling the air and removing humidity from it to make you comfortable.

2. Climate Spread: Central air conditioners are used in every climate, from the hottest to regions with mild summers when the unit doesn’t have to work hard. The 12 BTU/h rating is for those cool climates where just a little air conditioning can make the air comfortable.

Think of a high mountain desert region like eastern Oregon. The air is dry, so little dehumidification is needed. A little AC goes a long way, especially if you’re only trying to drop the temperature from 80F to 75F or similar range.

In hot, muggy areas like the Southeast US – they don’t call it “Hotlanta” for nothing – you’ll want every one of those 30 BTU/h.

3. Hot Space: Window air conditioner and portable air conditioners are typically used in rooms that are overly hot. So you’ll want 30-40 BTU/h for a window air conditioner.

4. Portable AC Inefficiency: As just discussed, the ASHRAE BTU/h rating used for window air conditioners isn’t suitable to portable air conditioners because portable ACs are less efficient. It takes more BTU/h power to achieve the same level of cooling. So, 60 to 80 BTU/h is better – because about 25 to 50 BTU/h of heat gets pulled in.

BTU Input vs BTU Output – Efficiency Explained

We’ve finally made it to this topic!

It explains the BTU/h ratings for gas and oil burning equipment. Wood too.

The ratings of how many BTU/h are produced are based on efficiency. No fuel-burning furnace is 100% efficient, though the most efficient at almost 99% are getting close.

Instead of 100% efficiency, some of the heat is lost in exhaust gases. See, the heat created isn’t blown directly into your ducts or home air. Combustion gases contain carbon monoxide, so the gases have to be vented. The gases travel through one or two heat exchangers where the heat is transferred. Some heat is lost.

Input vs Output Heat

Input heat: Most furnaces and boilers are rated by the heat they create.

Output heat: The percentage of the heat produced that is not lost with the exhaust gases.

The math: It is simple:

Input heat x Efficiency = Output heat

Example: 100,000 BTU input x 90% efficiency = 90,000 BTU output.

This chart shows common fuel burning equipment and their efficiency ratings.

EquipmentLow EfficiencyAverageHigh
Gas & Propane Furnaces0.880% or 90%0.99
Oil Furnaces0.8183-86%0.95
Gas Boilers0.8285-90%0.95
Gas Water Heaters0.8288-92%0.96
Outdoor Wood Furnaces0.780-90%0.98

What Size Equipment Do I Need?

Have a load calculation done.

It will tell you the Heat Output required to heat your home.

Example: Load Calculation determines that 95,000 BTU/h are required to heat your home.

Let’s use common furnace sizes. They are usually found in increments of 5K or 10K BTU/h.

Input x Efficiency = Output

  • 100,000 BTU/h input x 95% efficiency = 95,000 BTU/h output
  • 105,000 BTU/h input x 90% efficiency = 94,500 BTU/h output
  • 120,00 BTU/h input x 80% efficiency = 96,000 BTU/h output

Any of these furnaces from 100K to 120K BTU will do, depending on their efficiency.

The less efficient the furnace is, the bigger it has to be to achieve 95,000 BTU/h output.

The next section has a chart that will assist you in converting BTU Input ratings to Output ratings.

BTU Input Output Conversion Chart

You can find the BTU input for any piece of equipment on its product page. Most also have efficiency ratings. See this Goodman GMVC96 page.

Right at the top it says, “Up to 96% AFUE”. That is the annualized fuel utilization efficiency rating – efficiency in plain language.

Scroll down to find the Dealer Specification Sheet, and click it open. On page 3 of the pdf, you’ll find the models: 40K, 60K, 80K (2), 100K (2) and 120K BTU/h High Fire Input. This is the amount of heat created when running at full capacity.

Next is listed High Fire Output – These figures across the board are Input x 96% efficiency.

Here are samples that show Output based on Input.


The Bottom Line

BTU is British Thermal Units

BTU/h is BTU per hour, the rating used for all heating and air conditioning equipment.

This guide will help you size the equipment you need based on:

BTU per square foot, often based on your climate.

The Output BTUs of the equipment you are considering.

There’s loads more information in the guides we’ve linked to throughout our What is BTU Guide.

Written by

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree and EPA & R-410A Certifications.

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