How Do You Size A Commercial HVAC System?

If you’re ready to upgrade your building’s HVAC system; or thinking about buying a new one, you first have to consider the building’s size, the usual number of occupants, and so on.

Correct HVAC sizing is important. It influences not just the installation costs in the short term, but also the energy efficiency of the system and its overall performance.

But, just to give you a quick heads up, what follows is not an in-depth guide on how to size a commercial HVAC system, but we will be able to provide you with the main steps required to ascertain the correct size.

HVAC Sizing

HVAC sizes are measured in tons. You will need 1 ton of HVAC for every 12,000 BTU (British Thermal Units) of heat generated in the building every hour.

So, you will have to calculate the building’s cooling load to ascertain the capacity of the HVAC system you will need. And, as a very general rule, it takes about 25 BTU to cool down 1 square foot of a building.

There’s also another quick sum that HVAC professionals use to make a quick estimate of the size of the HVAC system required, and that’s to use 1 ton per 5000 square foot of floor area.

However, please note that these are quick guidelines only. We’ll be introducing our 6 step method later for a more accurate estimate.

Moreover, there are many different factors that can affect the size of the HVAC required besides the square footage of the building. And we’ll be coming onto those in good time a little later.

Why Accurate HVAC Sizing is Important

If you were to get an HVAC system that is too big or too small, you will run into several problems.

An oversized HVAC system could have poor humidity control. This in turn could risk getting mold in the building and lead to discomfort or asthma in the occupants.

An oversized HVAC would also come at an increased installation cost, and could also cause energy wastage, which in turn means higher energy bills. There will also be increased wear and tear over time, which means it will require more maintenance.

With an undersized HVAC system, on the other hand, the units won’t be able to cool the place efficiently. The units will have to work harder than they’re actually designed to and the compressor will be pushing its limits.

Not only does it mean more general wear and tear, but you get higher energy costs and you’ll have to wait longer to reach your desired temperature.

Factors that Can Affect the Size of HVAC Required

There are several different factors that can affect the size of HVAC that you need. We’ll list some of the main ones here for reference.

● The activities carried out in each room (e.g. are they going to be hot due to cooking)

● The position of the building in relation to the sun (e.g. is it in the shade)

● The total number of occupants in each room and the average size of each room

● The difference between the desired indoor temperature and the temperature outside

● The type of lighting in the various rooms, the type of windows in each room

● The amount of insulation in the walls and ceilings

Main Steps Involved in HVAC Sizing

  1. Find the number of square feet of the entire building

  2. Divide this number by 500

  3. To get the exact space cooling load, multiply the result by 12,000

  4. Add 1200 BTU for every kitchen, and 380 BTU per person in the building

  5. Add a further 1000 BTU to take in to account solar radiation from windows

  6. And finally, divide the number by 12000 to get the size of the HVAC in tons


Getting a rough estimate of what size HVAC you need is relatively easy. If you have the information laid out in the main steps that we provided earlier, then you can get a rough estimate of your HVAC system you need in just 6 quick steps.

And once your calculations are done, you can give us a call here at Sovereign, and we’ll see what we can do it help. You can provide us with your calculations, and leave us to worry about the exact heat load and cooling load calculations together with the heat flow calculations.

If you jot us a quick email at, we can call you back at a convenient time, and discuss your needs.


Author: Gareth Whalley
Sharing my vast knowledge in the Air Conditioning and Mechanical services field especially in HVAC installation, service and maintenance across a broad range of sectors, including Healthcare, Education, Transport, Banking, Retail and Commercial developments.