The short answer
An 1,800 sq ft one-story house with average insulation usually needs a 3 tons (36,000 BTU) air conditioner in Houston, where the summer design temperature is 97°F and the air is humid. The same house in Seattle (85°F, marine climate) needs about 2 tons.
The same house in every state
An 1,800 sq ft, one-story house with average insulation, sized for the climate of each state's largest city.
| Alabama Huntsville | 3A | 91 | 2.4 | 2.5 tons |
|---|---|---|---|---|
| Alaska Anchorage | 7 | 72 | 1.6 | 2 tons |
| Arizona Phoenix | 2B | 110 | 2.9 | 3 tons |
| Arkansas Little Rock | 3A | 95 | 2.6 | 2.5 tons |
| California Los Angeles | 3B | 86 | 2 | 2 tons |
| Colorado Denver | 5B | 92 | 2.2 | 2.5 tons |
| Connecticut Bridgeport | 5A | 86 | 2.1 | 2.5 tons |
| Delaware Wilmington | 4A | 90 | 2.3 | 2.5 tons |
| District of Columbia Washington | 4A | 92 | 2.4 | 2.5 tons |
| Florida Jacksonville | 2A | 94 | 2.6 | 2.5 tons |
| Georgia Atlanta | 3A | 92 | 2.5 | 2.5 tons |
| Hawaii Honolulu | 1A | 89 | 2.4 | 2.5 tons |
| Idaho Boise | 5B | 97 | 2.4 | 2.5 tons |
| Illinois Chicago | 5A | 89 | 2.3 | 2.5 tons |
| Indiana Indianapolis | 4A | 89 | 2.3 | 2.5 tons |
| Iowa Des Moines | 5A | 90 | 2.3 | 2.5 tons |
| Kansas Wichita | 4A | 97 | 2.6 | 2.5 tons |
| Kentucky Louisville | 4A | 91 | 2.4 | 2.5 tons |
| Louisiana New Orleans | 2A | 93 | 2.6 | 2.5 tons |
| Maine Portland | 6A | 84 | 2 | 2 tons |
| Maryland Baltimore | 4A | 92 | 2.4 | 2.5 tons |
| Massachusetts Boston | 5A | 88 | 2.2 | 2.5 tons |
| Michigan Detroit | 5A | 88 | 2.2 | 2.5 tons |
| Minnesota Minneapolis | 6A | 88 | 2.2 | 2.5 tons |
| Mississippi Jackson | 3A | 94 | 2.5 | 2.5 tons |
| Missouri Kansas City | 4A | 92 | 2.4 | 2.5 tons |
| Montana Billings | 6B | 92 | 2.2 | 2.5 tons |
| Nebraska Omaha | 5A | 92 | 2.4 | 2.5 tons |
| Nevada Las Vegas | 3B | 108 | 2.8 | 3 tons |
| New Hampshire Manchester | 5A | 90 | 2.3 | 2.5 tons |
| New Jersey Newark | 4A | 92 | 2.4 | 2.5 tons |
| New Mexico Albuquerque | 4B | 94 | 2.3 | 2.5 tons |
| New York New York | 4A | 88 | 2.2 | 2.5 tons |
| North Carolina Charlotte | 3A | 93 | 2.5 | 2.5 tons |
| North Dakota Fargo | 6A | 88 | 2.2 | 2.5 tons |
| Ohio Columbus | 4A | 89 | 2.3 | 2.5 tons |
| Oklahoma Oklahoma City | 3A | 97 | 2.6 | 3 tons |
| Oregon Portland | 4C | 90 | 2.2 | 2.5 tons |
| Pennsylvania Philadelphia | 4A | 92 | 2.4 | 2.5 tons |
| Rhode Island Providence | 5A | 86 | 2.1 | 2.5 tons |
| South Carolina Charleston | 3A | 92 | 2.5 | 2.5 tons |
| South Dakota Sioux Falls | 6A | 90 | 2.3 | 2.5 tons |
| Tennessee Nashville | 3A | 93 | 2.5 | 2.5 tons |
| Texas Houston | 2A | 97 | 2.7 | 3 tons |
| Utah Salt Lake City | 5B | 98 | 2.4 | 2.5 tons |
| Vermont Burlington | 6A | 87 | 2.1 | 2.5 tons |
| Virginia Virginia Beach | 3A | 92 | 2.5 | 2.5 tons |
| Washington Seattle | 4C | 85 | 2 | 2 tons |
| West Virginia Charleston | 4A | 89 | 2.3 | 2.5 tons |
| Wisconsin Milwaukee | 5A | 87 | 2.2 | 2.5 tons |
| Wyoming Cheyenne | 5B | 88 | 2.1 | 2 tons |
How it works
The formula
We add up the heat that gets into your house on a design-temperature afternoon, the same idea as ACCA Manual J, then pick the standard size that ACCA Manual S allows.
walls = U × wall area × (ΔT + 10°F sun)
ceiling = U × ceiling area × (ΔT + 40°F hot attic)
glass = U × window area × ΔT
sun = window area × 70 × SHGC
air leaks = 1.08 × CFM × ΔT
people and appliances = 230 × people + 1,200
ducts = 15% of the above
humidity = 200 × people + 0.68 × CFM × grains
tons = total ÷ 12,000
Assumptions
- Indoors is kept at 75°F, the Manual J design condition.
- The house is a simple box: a square footprint, window area 10%, 15% or 20% of the floor area, and walls the height of the ceiling on each story.
- Insulation levels set the U-factors (heat flow per square foot per °F), the windows' solar heat gain coefficient and air leakage. Average is R-13 walls (U 0.075), an R-30 attic (U 0.033), double-pane windows (U 0.5, SHGC 0.55) and 0.45 air changes an hour.
- "Sun on the house" scales the sun through windows and on walls: 0.7 for shaded, 1.3 for full sun.
- Humidity load uses the climate zone: humid "A" zones add moisture from outside air (50 grains per pound in zone 2A, 30 in 4A), dry "B" zones add none.
- Ducts run through an attic or crawlspace. Ducts inside the conditioned space would lower the load by about 15%.
- Sizes are central split systems from 1.5 to 5 tons in half-ton steps (no 4.5-ton units, as most brands don't make them). The recommended size is the smallest one between 95% and 114.99999999999999% of the load.
Worked example: 1,800 sq ft in Houston
One story, 8 ft ceilings, average insulation, average windows and sun, 4 people. Summer design 97°F, zone 2A, so ΔT = 22°F.
- Geometry: perimeter 4 × √1,800 = 169.7 ft; windows 15% × 1,800 = 270 ft²; net walls 169.7 × 8 − 270 = 1,088 ft²; air leaks 108 CFM.
- Walls 0.075 × 1,088 × 32 = 2,600; ceiling 0.033 × 1,800 × 62 = 3,700; glass 0.5 × 270 × 22 = 3,000.
- Sun 270 × 70 × 0.55 = 10,400; air 1.08 × 108 × 22 = 2,600; people and appliances 2,100.
- Ducts +15% = 3,700; humidity 4 × 200 + 0.68 × 108 × 50 = 4,500.
- Total 32,500 BTU/h = 2.71 tons. 95-115% of that is 2.6 to 3.1 tons, so the right size is a 3 tons unit.
Too big or too small
- Too big: clammy air. An oversized AC reaches the thermostat setting before it has run long enough to dry the air. Rooms feel cold and damp, and mold can grow.
- Too big: short cycling. Frequent on-off starts wear the compressor and waste energy; the bigger unit also costs more to buy.
- Too big: uneven rooms. Short runs don't move enough air to mix the house, so far rooms and upstairs stay warm.
- Too small: it can't keep up on the hottest afternoons, running nonstop while the house drifts a few degrees above the setting. That's acceptable for a few hours a year; it isn't if it happens every week.
- Between sizes? A two-stage or variable-speed unit runs at low speed most of the time, so the larger size causes fewer problems.
Questions people ask
What size AC do I need for an 1,800 sq ft house?
It depends on your climate and how well the house is built. With average insulation and windows, an 1,800 sq ft one-story house needs about a 3 tons (36,000 BTU) central air conditioner in Houston (summer design temperature 97°F) but only about 2 tons in Seattle (85°F). A well-insulated, newer house needs less; an older, leaky one needs more.
How many square feet does a 3-ton AC cool?
In a hot, humid climate like Houston, about 2,000 sq ft for an average house; in a mild one like Seattle, about 2,750 sq ft. The old "500 to 600 sq ft per ton" rule comes from leaky houses with single-pane windows and oversizes most homes built since the 1990s.
Is it better to oversize an air conditioner?
No. An oversized AC cools the air quickly and shuts off before it has pulled enough moisture out, so the house feels clammy. It also turns on and off more often, which wears it out sooner and wastes energy. ACCA Manual S, the industry sizing standard, allows a cooling unit between 95% and 115% of the calculated load.
What is a design temperature?
The outdoor temperature a system is sized for. The summer design temperature is exceeded in only 1% of the hours of a typical year, about 88 hours. Sizing for the single hottest day would leave the AC oversized for the rest of the summer. We compute it for your county from ten years of hourly NOAA weather station readings.
How many BTU is a ton of air conditioning?
One ton of cooling is 12,000 BTU per hour: the heat it takes to melt a ton of ice in a day. A 2-ton AC is 24,000 BTU/h, a 3-ton is 36,000 and a 5-ton is 60,000, the largest single residential unit.
Do I need a Manual J load calculation?
For a new or replacement central system, yes: many building codes require one, and a good contractor does it anyway. It measures each wall, window and duct run of your house. Our calculator gives the same kind of estimate from a few inputs, so you can tell whether a quote is in the right range.