What Size AC Do I Need?
A Round Rock Contractor Explains Manual J
By Scott Feller, Owner & CEO of Koala Cooling & Plumbing | Updated June 27, 2026
The short answer most people want first: a common rule of thumb is one ton of cooling for every 400 to 600 square feet of conditioned space. By that math, a 2,000-square-foot Central Texas home usually lands somewhere between 3.5 and 5 tons.
Now the part that actually matters: that rule of thumb is the single biggest reason so many Round Rock homes clammy, and uncomfortable - the ac system is too big. Square footage alone cannot tell you the right size. A contractor who quotes you a tonnage off your address and a Zillow listing isn't measuring anything — he's guessing, and in this climate the guess is usually too big.
Here's how sizing actually works, why getting it wrong costs you comfort and money, and how to land on the right number.
The rule of thumb, and why it falls apart in Texas
One "ton" of air conditioning equals 12,000 BTU per hour of cooling capacity. The square-foot rule of thumb exists because it's easy: pick a number of square feet per ton, divide, done. The trouble is that the same 2,000-square-foot house can need anywhere from 3 to 5 tons depending on factors that square footage never sees.
In Central Texas specifically, the rule of thumb tends to push sizing up — and bigger is the wrong instinct here. Our problem isn't just heat, it's heat plus humidity. An oversized unit handles the heat fine and fails the humidity, which is exactly the failure mode that makes a house feel worse, not better. More on that below.
What a Manual J actually is
Manual J is the residential load calculation standard published by ACCA (the Air Conditioning Contractors of America). Instead of guessing from square footage, it calculates how much heat your specific house gains on a design day — and the system is sized to remove exactly that much, no more.
A real Manual J accounts for things like:
- Window area, orientation, and type. A wall of west-facing single-pane glass is a completely different load than the same wall facing north with low-E double-pane.
- Insulation levels in the attic, walls, and floor — the actual R-values, not a guess.
- Air infiltration. How leaky the house is. A 1995 build and a 2022 build of identical size are not the same load.
- Ceiling height and volume. Cooling is about the cubic feet of air, not the floor area. Vaulted ceilings change the answer.
- Ductwork and where it runs. Leaky ducts in a 130°F attic add load that floor area can't predict.
- Shade, roof color, and exposure. Mature trees on the south side change your number.
- Occupancy and internal gains — people, appliances, and how the home is used.
Manual J is one of three companions, and a good contractor uses all three: Manual J sizes the load, Manual S selects the equipment that matches that load, and Manual D designs the duct system to deliver it. Skip J and the other two are built on a guess.
Why bigger is not better — the oversizing trap
This is the part homeowners are rarely told, and it's the most important thing on this page. An oversized AC is a worse machine than a right-sized one. Here's why.
An air conditioner does two jobs at once: it lowers the temperature, and it pulls moisture out of the air. The dehumidifying happens slowly, while the system runs. An oversized unit blasts the air cold so fast that it hits the thermostat setpoint and shuts off before it has run long enough to wring the humidity out.
The result is a house that's technically at 74 degrees but feels cold and clammy — and in a humid climate, that clammy 74 feels worse than a properly dehumidified 76. You crank the thermostat down chasing comfort that's really a moisture problem, the unit runs even shorter cycles, and now you're paying more for less comfort. Persistent indoor humidity also feeds mold and dust mites.
That rapid on-off pattern has a name — short cycling — and it doesn't just hurt comfort. It hammers the compressor with start-stop wear, the most expensive part to replace, and it drives up your electric bill because the start of every cycle is the least efficient moment of operation. An oversized system is a more expensive unit up front that delivers a clammier house, a shorter equipment life, and a higher bill. There is no upside.
Undersizing has its own problems — a unit that runs constantly and still can't keep up on the worst afternoons of August — but in Central Texas, oversizing is the far more common mistake, and it's the one the square-foot rule of thumb pushes you toward.
So what size do I actually need?
If you want a ballpark to sanity-check a quote, the 400-to-600-square-feet-per-ton range is fine for that — and only that. Use it to catch a wildly off number, not to choose your system.
The honest answer to "what size AC do I need" is that nobody can tell you precisely without measuring your specific home. The number that matters comes from a load calculation, not a chart. Two identical-looking houses on the same street can need different tonnage because one has been re-insulated and re-glazed and the other hasn't.
And one trap to avoid: don't just match the size of your old unit. If the home was sized by the rule of thumb 15 years ago, you may simply be replacing one oversized system with another. A replacement is the right moment to size the home correctly, especially if you've added insulation, replaced windows, or finished out a space since the last install.
How we do it at Koala
When we quote a system replacement, we run the load calculation on your actual home — windows, insulation, orientation, ductwork, the whole picture — and size to that. It's also how we catch the short cycling and humidity complaints that turn out to be an oversizing problem from a previous install, not a broken unit.
That's the whole idea behind our pricing: no two-hour in-home sales pitch, no upsizing you into a bigger unit than your house needs. Just the right size for your home and a straight price for it.
If you're weighing a new system and want it sized correctly the first time, we're happy to take a look.
Frequently asked questions
As a rough sanity-check, a 2,000-square-foot Central Texas home usually falls between 3.5 and 5 tons — but the right number depends on insulation, windows, ceiling height, and ductwork, which is why a load calculation rather than a square-foot estimate is the only way to size it correctly.
Often you shouldn't. If the original unit was sized by a rule of thumb, matching it just carries an old mistake forward. A replacement is the right time to recalculate, especially if you've changed insulation, windows, or square footage since the last install.
No. An oversized AC cools fast but shuts off before it can pull humidity out of the air, leaving the house cold and clammy while short cycling wears out the compressor and raises your bill. In a humid climate, right-sized beats oversized every time.
Yes. Cooling is about the volume of air in the home, not just the floor area, so vaulted or high ceilings increase the load that a floor-area-only estimate would miss.
We run the calculation as part of quoting a replacement so the system is sized to your home, not to a chart. Reach out and we'll walk you through it.
Want your home sized right the first time - no upsell, no two-hour pitch? Call us or book online.