There is no honest way to size an air conditioner from square footage alone. The rule of thumb you will find online — one ton per 400 to 600 square feet — consistently oversizes systems in this climate, and an oversized system is the single most common reason a Charleston house feels cool and damp at the same time. The correct answer comes from a Manual J load calculation, which looks at your insulation, windows, orientation, air leakage, duct losses and local design conditions, and it frequently comes out smaller than the system you already have.
That last point surprises people, so here is the reasoning.
Why bigger is not better
An air conditioner does two jobs. It lowers air temperature, and it removes moisture. The first is instant. The second takes time.
Moisture removal only happens while the system is running and the evaporator coil has been cold long enough to condense water out of the air passing over it. A short run cools the air and leaves the water in it.
An oversized system satisfies the thermostat in a few minutes, shuts off, and repeats. That is short cycling. The house hits 73°F quickly and sits there at 65% relative humidity, feeling like the inside of a cooler. Which is precisely the complaint we hear most often, and which we wrote about in more detail in why Charleston houses feel clammy with the AC running.
A correctly sized system runs long and steady on a hot afternoon. Long runs are what dry a house out. Correct sizing is not the smallest unit that works; it is the one that runs nearly continuously on a design day.
The other costs of oversizing
- More starts per day, and starting is when a compressor wears.
- Larger temperature swings between cycles, so rooms feel uneven.
- Noise, because a big single-stage unit announces every start.
- Higher installed cost for worse comfort.
Why it happens so often here
Three reasons, all of them understandable and all of them wrong.
Rounding up at replacement. The old unit was a four-ton, so the new one is a four-ton. Nobody asks whether the four-ton was right in 1998, or whether the windows and insulation have changed since.
Playing it safe. Nobody gets a complaint call for a system that cools too fast. They get one for a system that cannot keep up. So the incentive runs one direction.
Production building. In a new subdivision the same equipment package goes into every floor plan regardless of orientation. The house whose glass faces west and the one whose glass faces north get the same system, and one of them is wrong. It is why brand-new houses in Nexton, Cane Bay and Carolina Park can feel muggier inside than a century-old house downtown.
What a real load calculation looks at
A Manual J calculation is a room-by-room heat gain and heat loss model of your specific house. The inputs that actually move the number:
- Design conditions. Charleston’s summer design temperature and, critically, our design wet bulb. The humidity load here is a large share of the total, and rules of thumb ignore it entirely.
- Window area, orientation and glazing. West-facing single-pane glass is an enormous gain. The same window on the north side is a fraction of it.
- Insulation levels in walls, ceiling and floor, as they actually are rather than as the builder specified.
- Air leakage. A 1962 raised ranch on James Island and a 2021 build in Summerville are different houses at the same square footage.
- Duct location and leakage. Ducts in a vented crawlspace or a 130°F attic lose capacity before the air reaches a room. That loss belongs in the calculation.
- Internal gains. Occupants, cooking, appliances, and how much of the house you actually condition.
- Shading. A live oak over the west side of the house is worth real tonnage.
The output is a sensible heat load and a latent (moisture) load in BTU per hour, room by room. That is what equipment gets selected against, and it also tells you how much air each room needs — which is the input for sizing the ducts, a step that gets skipped far more often than it should.
Tonnage is only half the specification
Once you know the load, two further choices matter as much as the number of tons.
Staging
A single-stage system is either fully on or fully off. A two-stage runs at roughly 65% most of the time. A variable-capacity system modulates continuously, often down to 25% or 30%.
In a humid climate, modulation is worth more than almost anything else on the spec sheet, because it converts a system that blasts and stops into one that runs long and slow. Long and slow is what dries a house. It also means a slightly generous sizing decision is far less damaging, because the system simply spends its life at low capacity.
Airflow per ton
Standard practice is around 400 CFM per ton. In a humid climate, dialling that back toward 350 CFM per ton makes the coil colder and pulls more moisture out for the same tonnage. It is a configuration choice made at commissioning, it costs nothing, and it is routinely left at the default.
The ducts are part of the sizing question
You cannot push the air from a correctly sized system through undersized or leaky ducts and get the result you paid for. If the existing duct system has high static pressure, crushed flex runs, disconnected boots or leaking returns, the new equipment inherits all of it.
So a proper installation quote includes a measurement of static pressure and an assessment of the duct system, and where necessary the cost of ductwork repair alongside the equipment. A quote that covers equipment only is not a quote for a working system.
When the answer is not one bigger box
Sometimes the load calculation shows the problem is distribution rather than capacity. A bonus room over a garage, a sunroom, an addition on a long duct run, or a converted attic bedroom will be uncomfortable no matter what size the main system is, because the ducts cannot deliver the air that room needs.
In those cases a ductless mini split on the problem space, with a correctly sized main system for the rest of the house, delivers better comfort for less money than upsizing everything. It also means the main system is not oversized for the other eleven months.
If the load says you are still humid
Some houses have a latent load that no reasonably sized air conditioner can handle on its own, particularly houses over vented crawlspaces or with a lot of outdoor air infiltration. The honest answer there is not more tonnage — that makes it worse — but a whole-home dehumidifier that runs on its own humidistat independently of the cooling, including during our mild damp shoulder seasons when the air conditioner barely runs at all.
What to ask any contractor
- Will you perform a Manual J load calculation on my house, and can I see it?
- What did you measure, rather than assume?
- What is the latent load, and how does the equipment you are proposing handle it?
- Did you measure static pressure, and what did it read?
- What airflow per ton will you commission it at, and why?
- What does the duct system need, and is that in the quote?
A contractor who answers “it is a 2,400 square foot house so you need four tons” has not done the work. A contractor who says the calculation came out smaller than your existing system, and can show you why, probably has.
How we quote it
We do the load calculation before we propose equipment, and you get the result. The quote is written and itemized — equipment, labour, duct work, commissioning — so you can see what each part costs rather than one number at the bottom. We will not give you a figure over the phone based on square footage, because that number would be a guess and you would be entitled to hold us to it.
Call or text 843-609-5740 or book an in-home assessment. We cover Daniel Island, Mount Pleasant, Charleston, Summerville and the rest of the Lowcountry.
