How do I calculate A/C units and tonnage to square feet?


If you are in the Phoenix Area and are thinking about a new HVAC unit, STOP and CLICK HERE to go to Merritt’s new unit page.  We have every make and model under the sun and I will squeeze every SEER out of your new purchase for the best price. Remember it is not the unit itself but the installer that makes that happen..


Click here to visit the AC load calculator to rough out a basic size  

Ok these are some quick and dirty rules of thumb when describing A/C tonnage, BTU's and how to size a unit for a home.


One ton of cooling = 12000 BTUS and is designed to cool approximately 400 square feet of living space.

So, if you have a 2000 square foot home you might have one 5 ton unit or two units a 2 and a 3 ton.

These are rough numbers so your results may vary.  When your home was built someone (hopefully) did a calculation to determine what size unit to put on that particular home.  If the home was made of glass then obviously the unit would be sized larger, for example.  I live in a 3200 square foot home but get by with a 3 ton split and a 3.5 split system.  A little undersized but I have returns that go under the concrete slab and 2x6 framing with good insulation.  My system is sized properly.


Click here for the SRP calculator to help find cost savings per SEER.  Just plug in your numbers and see what the payoff time is


Now why not just upgrade the unit and stick a couple of 4 ton units at my place? I mean more is better right? No way!  My units run often during the summer but they turn on and stay on.  If I upped the size of the units then they would 'short cycle'.  A fancy way of saying turn on and off all the time.  If I selected 77 degrees at the thermostat and the unit kicked on at 78 it would pull down to 77 in a hurry, no doubt.  But, and here is the kicker, it would not run long enough to dehumidify the air and thus run back up to 78 in short order and the cycle repeats, over and over again.


The hardest thing for any motor, and I don't care if it is your lawn mower, car, ceiling fan or A/C unit is to turn on.  The amp draw of an electric motor is higher at start up to overcome the torque necessary to get up to speed. Example, my compressor will momentarily draw 60+ amps at start up and then run at 14 amps.  That is a lot of heat in a short amount of time and motors HATE heat.  It is their Achilles heel.


Last how can you tell how many tons your unit is.  Look on the name plate on the unit.  You will find on the model number a few numbers and letters and then typically a 0 followed by a number divisible by 12.


Example:  Lennox model 13hpd 036 230


The 036 is the giveaway that this is a three ton unit - it moves 36000 BTU's of heat in cooling mode.  A five ton of the same model Lennox would be  Lennox model 13hpd 060 230.


You may have a unit that you think is undersized.  In Phoenix it runs a lot in the summer but the rest of the year it kicks on and off as needed.  This is ok as long as the unit is mechanically sound and in fact good for it.  Remember the starting and stopping is what is bad for the unit.  Airflow ect.  are totally different issues.


For more helpful tips and questions answered visit my free A/C help site at and please, call me or email if you have any questions..


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Comments: 10
  • #1

    marci (Thursday, 11 July 2013 00:45)

    fresno area two story home, 2400 sq feet one unit presently which is 4 tons. Not cooling enough,condenser coils partly damaged by pets. Before damage it was cooling enough. We want to change should we continue with 4 tons, or 5 tons according to one company??

  • #2

    George Paugh (Wednesday, 01 July 2015 03:26)

    I live in Arizona and I bought a new 3200 sf house that has 1 A/C unit to supply three thermostatically controlled zones. I've been told that the best I can expect is a 30 degree drop from the outside temperature. From the best I can tell it's a Carrier 5 ton unit and I wonder if there is anyway that the tonnage can be increased or anything else that can be done to give more than the 30 degree temperature swing? It's not unusual for up to 120 degree temp. out here and a 90 degree inside isn't very cool.

  • #3

    Arnold Ortiz (Saturday, 25 July 2015 19:19)

    I love your site. I have garnished info that I need and will have a friend who does has the commercial/residential experience to the work for us. Once the project is over and I have sold a few acres (actually this was already in the process) I will send you a few bucks for the paypal "beer fund", but it is my humble opinion that you have not missed out on too many of the suds but the poor puppy is in need of some.
    Take care my "new" friend and may the Lord bless you in the name of Jesus!

  • #4

    bway57 (Friday, 07 August 2015 10:46)

    hey George Paugh you would need to increase your tonnage. that 5 is too small 12 to 15 ton would do the trick.
    12ton = 144000btu
    15ton = 180000btu
    to be on the safe side 15ton is the one.

  • #5

    Robert Jackson (Thursday, 17 September 2015 18:08)

    I just bought a new house in Louisiana. It's 24,000 living space without any shade. What size unit should I have. Some rooms are hotter than others also...

  • #6

    bhagyesh (Wednesday, 11 May 2016 03:35)

    How to calculate Starting and running Ampare of A/C compressor 1ph and 3 ph

  • #7

    Manijeh (Sunday, 19 June 2016 15:31)

    I live in San Antonio and it can get warm up to 110 degree. Every year around this time my AC is working, but not cooling. My house is 2 story 3200sq with only 4tons and 2 zoning.. I had many argue with builder, but they only installed a 4 Tons., however when I turn the breaker off for one hour and then I turn it back on.. It works.
    Is this system small? Would it help if I get a bigger tons for my house?

  • #8

    Ernie (Sunday, 07 August 2016 19:50)

    Always go 400 sq ft per ton bo exceptions. At 400 sqft per ton u wontvget any problems or have to do anything special to make it work right

  • #9

    Harper (Wednesday, 31 August 2016 14:40)

    Updating 2- 25yr 3ton York Heatpump systems. Can I increase the unit tonnage without having to enlarge the duct system or electrical? What is the outdoor unit clearance? Cooling has never been an issue (high elevation) need only heat home will not consider getting gas. What are your suggestions

  • #10

    Wes McFadden (Wednesday, 21 September 2016 10:55)

    home is 1900 sq ft with 11ft ceilings built in late 1800 no insulation. 17 windows 4 walk through 8ft 3 people in house have 6 ton 2 units
    vents sweating we live in Galveston TX



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