Central Air Sizing Guide: How to Calculate the Right AC Ton for Your Home

Key Takeaways

• Start with sq. ft. × 20 BTU for a rough estimate.

• Adjust for climate, insulation, windows, and people.

• Remember: 12,000 BTU = 1 ton.

• Oversized = wasted energy; undersized = overworked unit.

• Not sure? Get a Manual J from a pro.

Why Central AC Size Matters

A central air conditioner is a big investment—usually the most electricity-hungry appliance in a U.S. home. If it’s too small, it runs nonstop and still can’t beat the heat. If it’s too big, it blasts cold air in short bursts, leaves the house clammy, and racks up huge power bills. Industry surveys show that many homes end up with mismatched equipment because they skip basic load math. In the next sections, you’ll learn a step-by-step way to hit the sizing sweet spot, using numbers any seventh-grader can follow. We’ll also point you to detailed resources, like our BTU & tonnage sizing guide, should you want a deeper dive.

Need help after sizing? Browse R32 AC condensers made for exact-fit performance.

Measure Your Home’s Square Footage

Grab a tape measure or your phone’s laser app. For each room that needs cooling, jot down the length and width in feet, multiply those two numbers, then add up every room. That grand total is the cooled square footage. Garages and unconditioned basements don’t count unless they will be cooled, too. Write the final number down—it drives every calculation that follows. A 2 000 ft² single-story ranch, for instance, has exactly twice the load of a snug 1 000 ft² bungalow, all else equal. This step may feel basic, yet it corrects the most common sizing error: guessing based on the home’s listing size, which often inflates heated areas by including garages, lofts, or unfinished attics.

Installing soon? Don’t forget a proper line set to preserve cooling power.

Turn Square Feet into BTUs

With the square footage in hand, multiply by 20 BTU if your insulation is average and your ceilings are 8 ft high. Live in Phoenix or Miami? Bump the multiplier up to 22–25. Chilly Seattle or Buffalo homes can drop to 18. For our 2 000 ft² ranch in a moderate climate:
2 000 × 20 BTU = 40 000 BTU.
This number represents the heat you need to remove each hour on a typical summer day. You can also explore our straightforward AC sizing guide for more real-world examples.

Convert BTUs to Tons

HVAC pros talk in “tons,” a nod to the ice blocks used before modern refrigeration. One ton equals 12 000 BTU per hour. Divide your BTUs by 12 000 to get tons:
40 000 BTU ÷ 12 000 = 3.33 tons.
Because systems are sold in half-ton steps, you’d round to a 3.5-ton unit after fine-tuning for other factors. A common mistake is rounding up “just to be safe.” Don’t. Too much capacity means short cycling, poor humidity removal, and a shorter compressor life.

Shop packaged AC systems that align with your tonnage.

Fine-Tune for Climate, Insulation, and Ceilings

Numbers on a chart can’t see your attic insulation or feel your desert sun. If you’re in a hot-humid zone (Louisiana, Florida), increase your BTU estimate by 10–20 %. In dry-hot desert zones, bigger temperature swings may demand a similar bump. Super-insulated walls, sealed attics, and triple-pane windows let you trim 10–15 %. Ceiling height matters, too: every foot above 8 ft adds roughly 10 % load because heat rises and stays there. Think of your home like a cooler—thicker walls and lids (insulation) keep the ice longer. For exact regional charts, check our Manual J demystified.

Windows, People, and Appliance Heat Gains

Sun-soaked windows act like tiny space heaters. Add 1 000 BTU for every large south- or west-facing pane. Kitchens pile on more load—stoves and fridges bleed about 1 200 BTU each. Each person after the first two contributes 400 BTU, because humans are basically 98 °F radiators. Families who love big weekend cookouts or have wall-to-wall glass need a bigger unit than the same-size house with shaded awnings and minimal cooking.

Oversizing vs. Undersizing: Common Mistakes to Avoid

Oversizing feels safe but costs you twice: higher upfront equipment cost and higher monthly bills due to short cycling. Humidity stays high, leading to mold and that sticky “heavy” air. Undersizing forces the compressor to run nonstop, wearing it out early and still failing to hit the setpoint on hot July afternoons. Aim for a run time of 15–20 minutes per cycle on design-day temperatures; that’s the sweet spot for efficiency and dehumidification. Homeowners who skipped math often regret it when utility bills or repair costs pile up.

Choose a system built for your load. Shop complete matched systems.

Quick Reference Sizing Table You Can Trust

Below is a handy chart for 8 ft-ceiling homes. Adjust up or down using the tips above.

For more on energy savings once you pick a size, read SEER2 explained.

FAQs: Your Top Sizing Questions Answered

Q1. Do I really need a professional Manual J calculation?
If your home has unusual architecture, vaulted ceilings, or heavy glass walls, yes. A Manual J ensures no surprises. Our full Air Conditioner Sizing Guide walks through DIY steps and tells you when to call in a pro.

Q2. What happens if I skip the window BTU adjustment?
Your AC may cool the air but fail to lower humidity, leaving rooms muggy in late afternoon sun.

Q3. Can I oversize slightly and just let the thermostat handle it?
Short answer: no. Thermostats can’t fix rapid on-off cycles that waste energy and reduce dehumidification.

Q4. How often should I recalculate my load?
After major changes—new windows, added insulation, room additions, or a switch to electric cooking.

Q5. Is SEER2 rating connected to size?
 Not directly. SEER2 measures efficiency, while sizing measures load. Choose the right size first, then pick the highest SEER2 you can afford.