The "500 Sq Ft per Ton" Rule is Costing You Money

For decades, HVAC contractors have used a simple "Rule of Thumb" to size air conditioners: 1 Ton of cooling for every 500-600 square feet.

In 1975, this was a reasonable guess. Homes were built with similar (poor) insulation, single-pane windows, and drafty doors. But in 2025? This rule is dangerously outdated.

The Physics Problem: Area vs. Insulation

Your air conditioner doesn't cool "square footage." It removes heat. Heat enters your home through the walls, windows, and roof. The rate at which heat enters is determined by your insulation quality, not just the size of your floor.

In engineering terms, this is defined by the U-Value (Thermal Transmittance). Our calculation engine uses specific U-Values derived from ACCA Manual J standards to calculate your load.

As you can see, House A needs nearly double the cooling capacity of House B, despite having the exact same square footage. The "500 sq ft rule" would suggest a 3.5 or 4-ton unit for both houses.

If you put a 4-ton unit in House B, it would be drastically oversized.

Why Oversizing is Bad (The "Clammy" Effect)

You might think, "Bigger is better, right? It will cool the house faster!"

That is exactly the problem. An oversized air conditioner cools the air too fast. It satisfies the thermostat setting in 5 or 10 minutes and then shuts off. This is called Short Cycling.

• High Humidity: It takes about 15 minutes of running for an AC coil to get cold enough to start pulling moisture out of the air. If it shuts off in 10 minutes, you get cold air, but the humidity stays. This leads to a cold, clammy, swamp-like feeling.
• Higher Bills: AC units use the most electricity upon startup. Frequent starting and stopping spikes your electric bill.
• Wear and Tear: It's like stop-and-go traffic for your compressor. It kills the equipment years early.

How to Get It Right

Before you accept a quote, you need to account for your home's specific "Heat Gain." This includes:

• Insulation Age: As shown above, this changes the multiplier by up to 3x.
• Windows: Single pane vs. Double pane glass.
• Orientation: West-facing windows admit far more afternoon heat than North-facing ones.
• Climate: The design temperature in Phoenix is 108°F, while in Seattle it might be 82°F.

You don't need a degree in physics to get a better estimate than the "Rule of Thumb." Our tool calculates the thermal load based on these specific factors for your Zip Code.