What Size AC Do I Need? Why Square-Footage Rules Get Florida Homes Wrong

Quick answer: There is no reliable square-footage rule for AC sizing. Correct sizing comes from a Manual J load calculation that accounts for your home's insulation, windows, orientation, air leakage, and our humid North Florida climate. Oversizing — the most common sizing mistake we find — causes short-cycling, poor humidity removal, and higher bills. If a contractor quotes you a tonnage without measuring anything, get a second opinion.

Homeowners across Wakulla, Leon, and Franklin Counties ask us this every week, usually after reading a "one ton per 500 square feet" rule online — or getting a number from an AI chatbot trained on those same articles. After 30+ years of fixing comfort problems caused by wrong-sized equipment, here's what actually determines the right size for your home.

Why Square-Footage Rules Fail

Two 1,800 sq ft homes in Crawfordville can need meaningfully different systems. What the rule of thumb ignores:

• Insulation and construction era. A 1970s block home with original attic insulation carries a very different load than a 2020 build to current Florida code.
• Windows and orientation. Big west-facing glass in the Florida afternoon sun can add a huge load the rule never sees.
• Ceiling height, air leakage, and shade. Vaulted ceilings, leaky can lights, and whether your lot is full-sun or under live oaks all move the number.
• Ductwork. The best-sized unit can't overcome ducts that leak a chunk of its output into the attic.
• Humidity. In our climate, moisture removal (latent load) is a major share of the work — and it's precisely what generic rules ignore.

Oversizing: The Expensive "Upgrade" That Makes Comfort Worse

The instinct is understandable — bigger sounds safer. In a humid climate it backfires:

• Short cycles = clammy house. The system cools the air fast and shuts off before wringing out moisture. You get 74°F and sticky.
• More wear. Starts and stops are the hardest thing on a compressor. Short-cycling equipment dies younger.
• Higher bills. Startup draws are inefficient; a right-sized system running long, steady cycles costs less to operate.
• You paid more for it. Bigger equipment costs more up front, then delivers worse comfort. Twice the loss.

In North Florida, the right-sized AC runs longer cycles on the hottest days — and that's a feature, not a flaw. Long cycles are what dry the air out.

What a Real Load Calculation Looks At

When we size a system, we measure the home — wall and window areas by orientation, insulation levels, ceiling heights, duct location and condition — and run it against local climate design conditions. The output is a heating and cooling load in BTUs, which we match to equipment. It takes real time on-site. That's the difference between an estimate built on your house and a guess built on averages — the same reason AI chatbots can't price HVAC work accurately either.

Red Flags When You're Getting Quotes

• A tonnage quoted over the phone from square footage alone
• "We'll just match what you have" — the existing unit may have been wrong for 15 years, and homes change (new windows, added insulation, enclosed porches)
• No measurements taken, no questions about hot rooms or humidity

Replacing a system? We size it to your home, in writing — call (850) 926-3546 or request an estimate. Already have a quote? Bring it in for a free second opinion.

Frequently Asked Questions

Is it better for an AC to be slightly oversized or undersized?

In our humid climate, modest undersizing usually beats oversizing. A slightly small system runs longer cycles and controls humidity well, falling behind only on the most extreme days. An oversized one delivers clammy comfort all season.

Should my new AC be the same size as my old one?

Not automatically. If you've added insulation, replaced windows, or the original unit was sized by rule of thumb, the correct size may be different — sometimes smaller and cheaper.

Does a load calculation cost extra?

A proper sizing evaluation is part of every replacement estimate we do. A quote without one isn't a quote — it's a guess with a dollar sign.

---

Written by the Keith Key Heating & Air team — state-certified Florida HVAC contractor (Lic. CAC1818432), serving Wakulla, Leon & Franklin Counties since 1991.