What Size AC Unit Do I Need

How to make sure your new AC unit is the right size.

It’s not uncommon to have a homeowner call us at Magic Touch and ask “What size air conditioner do I need for a 1500 square foot house?” or, “How many tons of AC do I need per square foot?”

Chances are, if you are asking that question, you are in the process of buying a new AC unit, or recently purchased a new unit and are wondering if it’s the right size because it’s not cooling well. Hopefully you are the former and not the latter, but either way this article is for you.

Beware of rule of thumb sizing when buying a central air conditioner!

While it is mistakenly used by many air conditioning companies in Phoenix and other cities around the country, there is no such thing as a “good rule of thumb” when it comes to properly sizing an air conditioning and heating system.

Consider the following two scenarios and you will see you don’t need to understand anything about HVAC (Heating, Ventilation, and Air Conditioning) to see why rules of thumb don’t work.

For the purposes of these scenarios, we will use the most commonly heard rule of thumb here in the Phoenix, AZ area – 400 square feet per ton.

To put that another way, they’re saying (not me) you need:

1-Ton of cooling for every 400 sq. ft.


Scenario 1:

There are two 2,000 square foot homes with identical floor plans right next to each other, let’s say in Chandler, AZ. Based on the rule of thumb both of these homes would require a 5-ton air conditioner.

Home 1has R-40 Insulation in the attic, new dual-pane low-e windows, solar shade screens on all southern, western, and eastern facing windows, has a 10’patio cover shading 50% of the south facing windows, and has two occupants.

Home 2only has R-19 Insulation in the attic (and the cable-guy moved a bunch of it so there’s only an inch or two in a lot of places), the windows are single-pane and also kind of leaky, there are no shade screens (aka sunscreens or solar shade screens) only insect screens on the windows, and there are some big windows catching the hot afternoon sun. This home has a patio too but the home sits at a different angle than next door so the patio doesn’t shade the windows much. This home is occupied by 6 people.

Do you think these two 2,000 s. f. homes with identical floor plans require the same size AC unit?

If it was 100 degrees outside and both homeowners were trying to maintain 78 degrees indoors, do you think they have the same “load”? Or “heat gain?


The “load” determines the capacity (air conditioner size) needed. Yes, square footage is a factor in the load – but it’s only one of many factors that need to be not only considered, but actually recorded and added into a “load calculation”. A complete and detailed load calculation is the only way to size a new ac unit, period.

Don’t take my word for it, see what the Air Conditioning Contractors of America (ACCA ) has to say about it. The ACCA sets many of the HVAC standards followed by many municipality, city, state, and even federal inspectors and agencies – including the requirement of a Manual-J Load Calculation (often called a load calc.) when installing and even replacing a new AC unit.

This mathematical formula is also known as a “Heat Gain Analysis” as in the scenario above, house #2 would obviously be gaining heat from outside much faster than house #1. So, common sense would dictate house #2 needs a bigger unit (maybe even two units) to do the same job.

Technically, common sense would dictate some energy home improvements be done therefore requiring a smaller HVAC system – but that’s covered in other articles I’ve written. Search our blog for articles about “energy audits” and “home performance” for more information on that topic.


Scenario 2: 

There are two 2,000 square foot homes with different floor plans right next to each other, let’s say in Tempe, AZ this time (this works anywhere for our far away readers). Based on the rule of thumb both of these homes would require a 5-ton air conditioner.

Home 1is a two-story stucco covered framed home with a bedroom over the garage. The front of the home faces north and most of the windows are on the south and west side. There are 5 occupants including three teenagers who each have a friend over most of the time, and a large doggy-door for their two Labradors.

Home 2is a single-story ranch block home. The front of the home also faces north but most of the windows are on the north and east side. There is 1 occupant who occasionally has a guest come by for a few hours. This occupant has one indoor cat who never goes out because of those Labs next door.

Do you think these two 2,000 s.f. homes with very different floorplansrequire the same size AC unit?

If it was 100 degrees outside and both homeowners were trying to maintain 78 degrees indoors, do you think they have the same “load”? Or “heat gain?


Why do so many air conditioning companies use a rule of thumb if it’s wrong?

My first response is – good question!

My next response is – load calculations are time consuming, require a lot of knowledge of buildings, construction, and building science, as well as a lot of training and practice to be done accurately and correctly.

In my thirty-years in HVAC, I have determined a company named Wrightsoft makes the best and most accurate ACCA approved, Manual-J load calculation software on the market (in my humble but very experienced opinion). This software doesn’t come cheap and requires ongoing upgrades and support as building materials and practices change and progress.

We use several of their products including a suite called Right Suite Universal which not only allows us to perform both “block” and “room-by-room” load calculations, we are also able to determine exact duct sizes and lengths needed, equipment selection to match the load and much more.

All of this software requires ongoing training which we often have to travel to attend. In other words, doing things the right way comes at a price. In my opinion however, doing things the wrong way comes at a much higher price…and unfortunately that price is paid by the homeowner in discomfort, high utility bills, shorter unit life, and frequent equipment failures and repairs.


How often does the rule of thumb really work?

Several of our home energy auditors, sales estimators, service technicians and myself were just discussing this a few weeks ago. Often as a result an energy audit or load calculation we find we are able to actually reduce the size of a new air conditioner from what was installed in this home before – and the homeowner reports better comfort and obviously lower energy bills.

We estimate that happens on approximately 30% of our installations. So, it’s probably fair to say about 70% of the load calculations we perform come out the same size as the rule of thumb. The problem is what if youare one of the 30%?


Why would my new AC unit replacement size change if the old one worked fine for years?

I understand why this one confuses people. It stands to reason that if you had a 3-ton air conditioner before and it seemed to work fine for years, why would a new 3-ton not be the right size for the replacement unit?

The answer has to do with the fact that not every 3-ton is a TRUE 3-ton.

1-Ton is “technically” supposed to equal 12,000 BTU’s but “supposed to” and “actually do” aren’t one in the same.

The phrase “ton” in HVAC is actually a made-up concept for easy identification. Load calculations calculate BTU’s not tons. Air conditioner manufacturers like Trane, Carrier, Lennox and Goodman determine what units they classify as a 3-ton.

Note: 3-Ton should technically be 36,000 BTU (12,000 BTUH = 1-Ton)

EXAMPLE:Goodman may call their 32,000 BTU unit a 3-ton, while Trane may call their 38,000 BTU unit a 3-ton. The Trane in this case, while not really big enough to call a 3-1/2-ton unit – is ACTUALLY not just technically, ½ ton bigger than the “3-ton” Goodman!

What if your old system was a 38k BTU (3-ton) and you replaced it with a ½ ton smaller 32k BTU (3-ton)?

Let’s take that one step further, what if your original load calc. actually called for 40k BTU and the builder rounded down, not up (not only common but technically correct). The 2k BTU difference could, and most likely would, go unnoticed by the occupants. Now that same 3-ton Goodman would be almost ¾ of 1-ton too small – and that would most likely notgo unnoticed! Especially when the temps hit 110+ in the Phoenix area.


Why do so many companies selling new AC units online use rule of thumb charts or say it’s okay to replace my old unit with the same size?

The new ac unit retailers that allow to choose your equipment online and then ship the equipment to you directly wouldn’t be in business unless they made it easy for you to purchase. Read the fine print however and you will find out they have no liability if you buy the wrong size…in fact, it can and usually will void any manufacturer warranty. Worse, you don’t find all that out until it’s installed and not keeping up, now used, and non-returnable.

A few of the newer AC unit E-commerce websites popping up on the internet now will sell you both the equipment andprovide a licensed installing contractor in one transaction. They claim your “assigned” contractor will make sure the unit is the right size and you can even get a refund or pay the difference for the size you need.

However, much like their predecessors that didn’t provide the assigned contractor, they are not liable if the right size is not installed – the assigned contractor is! Unfortunately, you do not get to choose your contractor, and most contractors that are willing to work for the low margins these companies make lack the experience or load calculation knowledge needed. I have personally served on arbitration committees for consumer complaints for this exact situation, multiple times.

The online retailer is completely shielded of liability and it is both the homeowner and the installing contractor who suffer the consequences. Think about it like this, it’s not the equipment you “save thousands” on – it’s the labor, i.e. the quality of the design, installation and materials. In other words, you didn’t save money for the same thing – you got less for less!

Choose the right contractor for your new AC unit, and you don’t have to worry about what size you need because he or she will size it properly…by running a load calculation.

Note: Don’t be afraid to ask to see the load calculation – however, keep in mind most contractors aren’t going to produce a load calculation until after you purchase from them. This is for their protection as again, there is time and expense that goes into this process and it’s only fair they don’t do the work only for another contractor that didn’t to use it.


For expert new AC unit installation that include an ACCA Approved Manual-J Load Calculation performed with the latest Wrightsoft Heat Gain Analysis software in Mesa, AZ and all Phoenix suburbs, CONTACT US TODAY!