Too Big, Too Small, or Just Right? The Guide to Picking the Right Size Central AC

What’s Up, I’m Jake 💬

Listen—if you’re shopping for a central AC system and you’re thinking, “I’ll just go bigger to be safe,” I need you to stop right there. You wouldn’t buy clown shoes for a marathon, right? Same deal with AC. Sizing it wrong means high bills, humidity problems, short cycling, and regret. Don’t worry—I’ve got your back. Let’s size this baby up the right way. 💪

 

1. Why AC Size Really Matters

Central AC systems are rated by tonnage, which translates to BTUs (British Thermal Units) of cooling per hour. One ton = 12,000 BTUs. But it’s not just about square footage. It's about how your home behaves in the heat.

If your AC is too small, it’ll run constantly, never cool your home, and wear out fast.
If it’s too big, it’ll short-cycle—turning on and off constantly—leaving your home clammy and your system stressed.

The U.S. Department of Energy nails it: oversized systems waste energy, cost more, and don't dehumidify properly (DOE).

 

2. Rule of Thumb ≠ Rule of Jake 😤

You’ll hear people say “1 ton per 500–600 square feet.” That’s lazy math. Sizing depends on:

• Home insulation

• Window quality and direction

• Local climate (Phoenix ≠ Portland)

• Ceiling height

• Shade from trees or overhangs

• Number of people in the home

• Appliances generating heat (oven, dryer, etc.)

Jake’s Rule: Don’t wing it. Get a Manual J Load Calculation.

 

3. Manual J: The Gold Standard of Sizing

Manual J is an industry-standard HVAC calculation used to determine the exact cooling load your home requires. It considers dozens of real factors—not just square footage.

You can hire a local HVAC contractor or use online tools (some even free). My go-to recommendation: the calculator from HVAC Load Explorer, which helps DIYers estimate their own load. It’s not perfect, but it’s miles better than guessing.

Heads-up: If a contractor doesn’t do a Manual J? Walk away. Red flag 🚩

 

4. Real-World Sizing Examples

Let’s say you’ve got a 2,000 sq ft home in a moderate climate with average insulation.

• Ballpark: ~3.5 tons or 42,000 BTUs

• But… with new windows, ceiling fans, and some shade trees? Maybe just 3 tons is enough

• Poor insulation, south-facing windows? Might need 4 tons or more

This is where Manual J saves your bacon.

Also: add 600 BTUs for each extra person who lives in the home full-time. Why? We’re little heat generators. 😅

 

5. What Happens If You Oversize?

You might think “bigger is better.” But with AC, that’s dead wrong.

Here’s what goes wrong with oversized units:

• Short cycling: turns on and off too fast, leading to wear and tear

• Poor humidity removal: it cools too fast to dehumidify properly

• High energy bills: that startup surge? You’re paying for it repeatedly

• Uneven cooling: upstairs sauna, downstairs icebox

The folks at Carrier explain how short cycling ruins efficiency and comfort—and they’re not wrong.

 

6. Undersizing Isn’t Cute Either

You’ll run your unit into the ground trying to cool a house it can’t handle.
If your system:

• Runs all day and still doesn’t hit the set temp

• Never gets below 75° even at night

• Blows weak or lukewarm air

…it’s likely undersized (or clogged, or leaking, or worse).

Trane has a great explainer on how to recognize this, and I agree with most of it.

 

7. Sizing Tips for Additions, Basements, & Bonus Rooms

Doing a home addition? Don’t just tap into your existing ductwork. Your AC was sized for the original footprint.

• Additions: May require a mini-split or separate zone

• Basements: Often overcooled unless ducted and zoned properly

• Sunrooms: Need their own strategy—usually high heat gain

If your system isn’t zoned or modulating, you’ll struggle to get even comfort across varied spaces.

 

8. Don’t Forget Ductwork Size

Even if your AC is perfectly sized, bad ductwork kills performance. Undersized or poorly designed ducts restrict airflow and boost static pressure. That makes your blower motor work harder—and your bills go up.

The Air Conditioning Contractors of America (ACCA) created Manual D, which handles duct sizing. Pro installers should follow this along with Manual J.

 

9. Indoor Coil and Air Handler Matching

You can’t pair just any air handler or coil with a condenser. The system’s total capacity depends on matchups—based on airflow, refrigerant type (👋 R-32 lovers), and coil design.

Always check the manufacturer’s AHRI match for your exact model. That ensures your system hits the right SEER2 rating and doesn’t void the warranty.

If you're buying from The Furnace Outlet, they pre-match components for you—huge win for DIYers and pros alike.

 

10. Jake’s Quick Sizing Checklist ✅