How Do I Know What Size Central Air Conditioning Unit I Need?

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Sizing your central air conditioning unit can be one of the most frustrating parts of the process of installing a new air conditioner. Customers that solicit price quotes from multiple companies are routinely given different sizing recommendations and different reasonings for those sizes. So how are you supposed to know what size air conditioner you actually need? I’ll give you some simplified guidelines for how central air conditioning units are sized here in Denver, but…

The bottom line is that it’s not your responsibility to try and figure out what size your central air conditioner should be. Find a company you can trust, make sure they offer 100% money-back satisfaction guarantee in writing, and let them take responsibility for sizing the unit properly!

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The company you choose to install your new ac unit will use a long-form calculation called a Manual J Load Calc to determine the optimal size of your new unit. But it’s understandable to want a general idea of the approximate size your central air conditioner should be without going to an engineering class, and that’s actually a pretty simple equation.

Here in Denver, you can get a rough approximation of how many tons your central air conditioner should be by dividing the square footage of your home by 600. It’s important that you not include any square footage in the basement in this calculation, regardless of whether your basement is finished or not.

Dividing your home’s square footage by 600 does not work for homeowners outside the metro Denver area. The 600 figure is higher than the figure that would be used in other parts of the country because it has been adjusted upwards to account for Denver’s higher altitude.

Approximate Sizing Equation For Central Air Conditioners In Denver


  • 1,500 s.f. / 600 = 2.5 ton ac unit
  • 1,800 s.f / 600 = 3 ton ac unit
  • 2,100 s.f. / 600 = 3.5 ton ac unit
  • 2,400 s.f. / 600 = 4.0 ton ac unit
  • Over 2,4000 s.f. / 600 = 5.0 ton ac unit

Newer, more energy efficient homes can sometimes require slightly smaller units than the approximate sizing equation would suggest, but that depends on a number of factors related to the construction and style of home.

Please keep in mind, this gives you a general approximation on sizing for your air conditioner. Your heating and air company should attempt to fine-tune the size by performing a Manual J Load Calc. The load calculation may confirm that your home needs the same size as the Approximate Sizing equation produced, or it could indicate that you need a slightly smaller size.

If you are given a quote that recommends installing an ac unit that is more than 1/2 ton smaller than what the approximate sizing equation suggests, and that quote does not include a 100% money-back satisfaction guarantee, you are taking a huge risk. It’s pretty rare that a load calculation will suggest installing a unit that is more than 1/2 ton smaller than the size you’ll get from the approximate sizing equation, but if it happens, you’ll want the contractor to make sure you have sufficient return air to handle the extra cooling capacity.

To be honest, allowing any company that doesn’t offer a 100% money-back satisfaction guarantee in writing to install your new air conditioner is like rolling the dice. You have to cross your fingers and hope that they will install the correct size ac unit, as well as doing a good job on the installation itself. Without that 100% money-back guarantee you are basically on your own if there’s a problem.

If you haven’t read it yet, check out our blog post titled How Does Size Affect The Cost Of A Central Air Conditioning Unit?

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