Two HVAC units outside a house with the caption "Why HVAC Is Measured in Tons"

The three primary functions of an HVAC system are heating, ventilation, and air conditioning. The purpose of an HVAC system is to provide thermal comfort – and acceptable indoor air quality – to both commercial buildings and homes.

The heating element typically comprises a furnace or boiler, while the mechanical ventilation system uses fans and ducts to draw in fresh air and distribute it throughout the indoor space. (Click here to learn more about ventilation systems and how to keep them clean.)

The last element is AC, which stands for air conditioning. Its main purpose is to cool down your living space.

While air conditioning (AC) is only one part of the HVAC system, it is often used to refer to all types of residential heating and cooling devices. Even more confusingly, air conditioner capacity is often measured in “tons.”

What’s A Ton Of Air Conditioning?

You probably don’t need me to tell you that your AC doesn’t weigh anywhere near 2,000 pounds. So, what does your HVAC contractor mean when they say that they’ll be installing a new three-ton unit in your home?

Well, that’s the question we will be answering in this article. So let’s dive straight in!

Before electric air conditioners were invented, people used large blocks of ice to cool their homes. To melt one ton of ice in 24 hours requires 12,000 BTUs (or British Thermal Units) of heat per hour. Therefore, one ton of air conditioner capacity simply means that the AC can remove 12,000 BTUs of heat from your house in one hour.

In other words, if your HVAC contractor says that they will be installing a three-ton AC in your home, it does not mean that the device will weigh 6,000 lbs. Rather, it simply refers to the fact that your new AC can remove 36,000 BTUs of heat from your living space in one hour.

Why Use An Old Metric To Measure AC Capacity?

You might be wondering why HVAC professionals have continued to measure air conditioner capacity in terms of tons of ice, despite the fact that we switched to more modern home-cooling methods generations ago.

Well, that’s a fair question.

The fact is that the use of tons to measure AC capacity has its roots in historical custom, rather than a deliberate decision made by HVAC professionals or by any institution governing the industry.

Some sources claim that the use of “tons” to measure AC capacity was standardized by the American Society of Mechanical Engineers as far back as 1912. However, this claim is not universally accepted by HVAC professionals today.

Regardless of when and where the practice originated, it persists because modern, electric air conditioners function somewhat like the blocks of ice that were used to keep homes cool in the 19th century.

In other words, an AC does not produce cool air. Rather, much like a block of ice melting slowly as it absorbs the ambient heat from its surroundings, a modern air conditioner cools down your living space by removing the existing heat from your interiors.

The capacity of an air conditioner, therefore, simply measures how much heat it can remove from its surroundings in an hour.

Guy sitting on a couch using a remote control on a wall HVAC unit.
We’ll explain how to find the right size AC for your space.

How Many Tons Should Your AC Be?

Now that you understand what the phrase “AC tonnage” means, and why HVAC systems are measured in this way, you might still be wondering how these concepts apply to your own home.

How much AC tonnage do you need, in order to adequately cool your property with minimal waste? How do you calculate your own AC requirements properly, so as to avoid wasting your hard-earned dollars on an unnecessarily large AC or bloated electricity bills?

Well, we’re here to answer those questions, and to help you accurately calculate how much air conditioner capacity is optimal for your property.

As mentioned above, a ton of air conditioner capacity is measured in British Thermal Units or BTUs. One BTU is equivalent to the amount of energy needed to heat one pound of water (at sea level) by one degree Fahrenheit. One ton of air conditioning capacity means that 12,000 BTUs of heat are removed from the surroundings every hour.

There is a simple formula that will help you calculate how many tons of air conditioning capacity you need. Just follow the steps below to find out the answer before buying an air conditioner for your home.

  • Find out the size of the space that needs to be cooled, in terms of square feet.
  • Now, you need to multiply the number of square feet by 25. For instance, if you’re cooling a 1,200-square-foot space, then 1,200×25 would equal 30,000.
  • This (30,000) is the total number of BTUs that need to be absorbed per hour, for adequate cooling. Since one ton of air conditioning capacity will absorb 12,000 BTUs of heat per hour, now you need to divide it by 12,000.
  • 30,000 divided by 12,000 gives a result of 2.5. That tells you that a 2.5-ton air conditioner is required to adequately cool the property.
  • Under ordinary circumstances, therefore, you will need a 2.5-ton AC to properly cool a 1,200 square-foot house.

This formula will only give you a general estimate of your air conditioning needs, though. Here are some other factors that might determine your AC tonnage needs.

Factors That Affect AC Needs

Three different houses; a two-story house, a low ranch style house, and a house with thick shrubbery. They will all have different HVAC needs.
Architecture and landscaping will influence how big your AC needs to be.

Insulation

If your property is very well insulated – with double-paned windows, sealed doorways, thermal blinds and curtains, etc. – then your AC tonnage needs would be lower. (Click here to learn more about home insulation.)

This is because well-insulated spaces can effectively keep the air-conditioned air inside from leaking out, while also preventing the air outside from seeping indoors.

As a result, the interior doesn’t heat up quickly if the property is well insulated. Consequently, fewer BTUs of heat need to be removed per hour in order to keep the space cool and comfortable, leading to lower AC tonnage needs. 

Landscaping

The way in which your yard or garden is landscaped will also have an effect on how many tons of air conditioning capacity you need. This is because a property surrounded by lots of trees and vegetation will remain relatively cooler, even during the summer. The shade provided by the trees surrounding the property will keep the temperature under control.

On the other hand, if your property is located on a wide, bare yard with few (if any) trees, then it will heat much more quickly during the summers. As a result, you will need more air-conditioning capacity to keep the same amount of space cool and comfortable.  

Architecture

The architecture of the property – such as the number of floors – will also have an effect on the air conditioning needs. In general, it is harder to cool the second and third floors of a building.

This is because hot air rises upwards. So your second story will already be “preheated” by the warm air rising to the ceiling of the first story. The highest level of the house will also be exposed to the radiated heat from sun on the roof.

As a result, single-story homes require less AC tonnage than multi-story ones. Ranch-style homes, which have a close-to-the-ground architectural style, are quite affordable in terms of their HVAC needs.

Final Thoughts

Installing the right HVAC system on your property will help you save money, while at the same time making your living space (or workspace) more comfortable. You should consult an experienced HVAC technician before choosing an AC for an exact estimate of how much air conditioning capacity your property needs.

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