Why A/C Sizing Isn’t One-Size-Fits-All

Buying an air conditioner sounds simple—until you try to choose the right size.

Walk into any store or search online, and you’ll find charts telling you how many BTUs you need based on your room size. But what those charts don’t tell you is that cooling performance isn’t just about square footage. It’s about how your space behaves thermally—which depends on far more than just its dimensions.

If you've ever installed an A/C unit and found that it runs constantly, fails to cool evenly, or short cycles without removing humidity, chances are it was improperly sized.

Let’s break down why A/C sizing isn’t one-size-fits-all—and why that matters more than most people realize.

The Common Advice: BTU Charts and Square Footage

Most guides offer a table like this:

Room Size (sq. ft.)	Recommended BTUs
100–200	6,000
250–300	7,000
400–450	10,000

It seems helpful. But these charts assume every room:

• Has average insulation

• Is shaded most of the day

• Has 8-foot ceilings

• Isn’t affected by appliances or people

• Isn’t facing direct sun

That’s a lot of assumptions. And in real life, your room probably breaks one—or all—of these rules.

The Reality: Sizing Depends on Thermal Behavior

Let’s say you and your neighbor both have a 300 sq. ft. room. You buy the same 8,000 BTU unit. Yet your space never cools down properly while theirs is freezing. Why?

Here are just a few variables that affect heat load (how much cooling your A/C must provide):

• Insulation quality (walls, ceiling, floor)

• Sun exposure and window orientation (especially west-facing windows)

• Number and type of windows

• Ceiling height (higher ceilings mean more air to cool)

• Appliances and electronics (TVs, ovens, computers give off heat)

• Number of occupants (yes, people add to the heat load)

• Local climate and humidity

A 300 sq. ft. sun-soaked room with poor insulation needs a much stronger unit than a well-insulated, shaded one. That’s why square footage alone can lead you in the wrong direction.

The Cost of Getting It Wrong

A poorly sized air conditioner doesn’t just lead to discomfort—it hits your wallet, too.

• Oversized units short cycle, wear out faster, and fail to dehumidify.

• Undersized units run continuously, driving up energy bills and still failing to cool.

• Either way, you’re paying more for worse performance—and shortening the unit’s lifespan.

So, What’s the Right Way to Size an A/C?

The gold standard is a full thermal analysis of your space: one that considers your specific room layout, insulation, climate, sun exposure, and internal heat sources.

Historically, this level of analysis was time-consuming and expensive—something reserved for large buildings or industrial applications.

But that’s changing.

A Smarter Approach: Real-Time Thermal Simulation

To solve this problem, we built a sales agent that simulates your room's thermal behavior during the sales conversation. It accounts for dozens of environmental and structural factors to recommend the right size—not just a rough guess based on floor space.

It’s like giving your room a thermal stress test before you buy anything.

Bottom Line

If you're still using square footage charts or guessing based on what “seems about right,” you're setting yourself up for higher bills, lower comfort, and a shorter equipment lifespan.

Every space is unique. Your A/C sizing should be too.

👉 Not sure what size air conditioner you actually need?

Try our A/C sizing agent—it models your space in real-time and gives you a personalized recommendation based on real thermal behavior. No guesswork. No one-size-fits-all shortcuts.