What Size AC Do I Need?

What size AC do I need for my house? (Quick reference table)

Here are the most common combinations people ask about. These use the mixed-climate baseline with average insulation and 3 bedrooms. Use the calculator above for your specific situation.

Table assumes: 3 bedrooms, average insulation (1980-2010 construction). Add half a ton for older/poorly insulated homes. Subtract half a ton for new construction with good insulation.

What does "tons" mean for an AC?

One ton of cooling equals 12,000 BTU of heat removed per hour. The word "ton" comes from the old days of using ice blocks for cooling. One ton of ice melting over 24 hours removed 12,000 BTU of heat per hour. The name stuck even though modern AC uses refrigerant, not ice.

Common residential AC sizes in tons: 1.5, 2, 2.5, 3, 3.5, 4, 5. Most single-family homes need 2 to 4 tons. Window units and small ductless mini-splits are often rated in BTU/hr rather than tons.

Why size matters so much

Too small: it runs forever and can't keep up

An undersized AC runs continuously on a hot day without ever reaching your set temperature. Your house stays warm, your electricity bill climbs, and the compressor wears out faster from constant use. On a 100-degree day in Phoenix, a 2-ton AC in a 2500 sqft house is going to lose the battle by afternoon.

Too big: it short-cycles and leaves your home humid

An oversized AC is actually a more common problem than undersizing. A unit that is too large for your home reaches the set temperature fast, shuts off, then kicks back on again 10 minutes later. This is called short-cycling. The issue is that AC pulls humidity from your air during the cooling process, but that process needs time. When the unit short-cycles, your home stays at the right temperature but feels clammy and sticky because the moisture never gets removed. Short-cycling also hammers the compressor with repeated starts and shortens equipment life.

The goal: 80-90 percent runtime on your hottest day

HVAC contractors size for the "design day": the hottest outdoor temperature your area hits in a typical year (usually the 99th percentile temperature from weather data). A properly sized AC should run about 80-90 percent of that design day to maintain your indoor set temperature. That means it runs a long time, but it does cycle off. If it runs 100 percent of the time and still can't keep up, it is undersized. If it cycles off every 10-15 minutes on a design day, it is oversized.

The 4 things that matter most for AC sizing

1. Square footage

The biggest factor. The standard starting point is 20 BTU per square foot of living space. A 1500 sqft home starts at 30,000 BTU = 2.5 tons. A 2000 sqft home starts at 40,000 BTU = 3.3 tons (rounds to 3.5).

2. How hot your summers get (climate zone)

This is the second-biggest variable. A home in Phoenix hits 115 degrees and needs to maintain a 50-degree indoor-to-outdoor temperature difference. The same home in Minneapolis might only need to handle a 90-degree day. That difference matters a lot for sizing. Hot climates need 15-20 percent more cooling capacity than mixed climates. Cold climates need 10-15 percent less cooling capacity (though they need much more heating capacity, which is a separate calculation).

3. Insulation and how tight your home is

A drafty older home with single-pane windows leaks cooled air out and lets hot air in much faster than a modern home built to the 2012 International Energy Conservation Code (IECC). That can swing your AC requirement by 15-20 percent in either direction. If you have an older home and you've added blown-in insulation or replaced windows, you may land between the "poor" and "average" categories.

4. How many people live in your home

People generate heat. Each person adds roughly 250-400 BTU/hr of sensible heat to a space at rest (more when active). Bedrooms are a good proxy for household size. Three bedrooms and under is a baseline; each extra bedroom adds about 1500 BTU/hr to the load, which is roughly another person's heat output plus electronics.

What this calculator does not account for

This is a simplified calculator designed to give you a reliable starting point. Before buying equipment, you should know it does not account for:

• Ceiling height above 8 feet: vaulted ceilings and two-story great rooms hold more air volume and need more cooling capacity.
• Number and size of windows: south- and west-facing windows add significant solar heat gain in the afternoon. Lots of glass can push your load up by 10-15 percent.
• Duct condition: leaky ducts in an unconditioned attic can lose 20-30 percent of your cooling before it reaches the rooms.
• Local humidity: high-humidity climates (FL, LA, coastal southeast) add latent cooling load because your AC has to remove moisture from the air, not just lower the temperature.
• Shade and tree cover: a home under heavy tree cover on the east and west sides needs less cooling than an identical exposed home.

For a calculation that accounts for all these variables, that is a full ACCA Manual J load calculation. Your HVAC contractor runs one before sizing your system. You can also use our HVAC Load Sizing Calculator which includes climate zone, window load, and door load.