Property managers inherit HVAC systems more often than they select them, which means most of you are living with a tonnage decision someone else made, sometimes decades ago, sometimes with a rule of thumb instead of a real calculation. Understanding how sizing is actually supposed to work makes it a lot easier to spot when your building’s system was never right to begin with.
How tonnage is actually calculated
Square footage — the starting point, not the answer
A rough rule of thumb in Florida’s climate is somewhere around 350 to 500 square feet per ton of cooling for a typical office or retail space, but that range is wide enough to be nearly useless on its own — a 10,000 square foot building could reasonably need anywhere from 20 to 28 tons depending on everything else on this list. Square footage sets the ballpark; it does not set the number.
Occupancy load
Every person in a building adds heat — body heat plus the additional ventilation air needed to keep CO2 and odor levels acceptable. A densely occupied space like a call center or a restaurant dining room needs meaningfully more cooling capacity per square foot than a warehouse or a lightly staffed office of the same size, because the occupant load and the fresh-air requirement that comes with it are both significantly higher.
Equipment load
Kitchens, server rooms, and spaces with significant electronics or process equipment generate heat that has to be removed regardless of occupancy. A restaurant kitchen or a small data closet can add tonnage requirements well beyond what the square footage and occupancy alone would suggest, and a load calculation that ignores equipment heat gain will undersize the unit for that space every time.
Insulation and building envelope
An older building with single-pane storefront glass, minimal roof insulation, and gaps around doors and loading areas loses and gains heat far faster than a newer, tightly built structure of the same footprint. Two identical-looking buildings on the same block can have meaningfully different tonnage requirements purely because of envelope quality.
Orientation and glazing
A building with substantial west- or south-facing glass takes on significant solar heat gain through the afternoon, especially in Central Florida’s intense summer sun. A space with the same square footage but north-facing or minimal glazing can run with meaningfully less tonnage. Orientation is one of the more commonly overlooked factors in a rough sizing estimate.
What happens with an undersized unit
An undersized unit runs continuously during peak conditions and still fails to reach setpoint on the hottest afternoons, which shows up as tenant complaints and a thermostat that never quite satisfies. It also runs longer cycles under sustained high load, which accelerates wear on the compressor and blower motor compared to a properly sized unit cycling normally. An undersized system is not just an uncomfortable building — it is one that burns through its own service life faster.
What happens with an oversized unit
Oversizing seems like the safer mistake, but it creates its own problems. An oversized unit satisfies the thermostat quickly and shuts off before it has run long enough to properly dehumidify the space, which in Florida’s climate means a building that feels cold and clammy rather than comfortable. It also short-cycles more than a correctly sized unit would, which brings the same startup-wear problem covered in our RTU warning signs article. Bigger is not automatically better; it is a different set of problems.
Why a like-for-like tonnage swap isn’t always correct
When a unit fails and needs replacement, the fastest path is ordering the same tonnage as the nameplate on the old unit. That is sometimes right and sometimes just repeating a mistake made years earlier. Buildings get renovated — walls come down, occupancy changes, a restaurant tenant moves into a former retail space and brings a kitchen’s worth of equipment load with it. A replacement is the right moment to ask whether the original sizing was correct for the building’s current use, not just match the old number and move on. This is exactly the kind of check we do before quoting a replacement — measuring the actual current load rather than assuming the old tonnage was ever right.
What we check before recommending a tonnage
We look at square footage, occupancy pattern, any kitchen or equipment loads, envelope condition, and glazing orientation before recommending a tonnage on a replacement or new installation. If the existing unit was undersized or oversized for the building’s actual use, we will say so and explain the tradeoff — sometimes correcting a decades-old sizing mistake is worth a slightly higher equipment cost, and sometimes the building has changed enough that it genuinely needs re-evaluation. Full detail on our installation process is on our RTU installation page.