AC Size Calculator: What Size Air Conditioner Do I Need?
Enter your home's size, insulation and windows to get the cooling load in BTU per hour and the AC size in tons, using NOAA climate data for your state.
Safety: Replacing or adding a central air conditioner involves refrigerant and high-voltage wiring and usually needs a permit. A licensed HVAC contractor should run a Manual J load calculation before you buy equipment.
The right air conditioner is the smallest one that keeps your house at 75 °F on a hot, humid day where you live. Bigger isn’t better: an oversized unit short-cycles, leaves humidity behind and costs more. This calculator estimates your home’s cooling load from its size, insulation, windows and air leakage, using a design day for your state from NOAA climate records. It then picks the nominal size that covers it.
How it works
The cooling load is the heat that gets into the house on the design day, plus the moisture the air conditioner has to remove. We add up eight sources:
walls, windows U × area × (outdoor − 75 °F)
ceiling U × area × (outdoor + 35 °F − 75 °F) (hot attic)
sun window area × SHGC × 70 BTU/h·ft² × exposure
air leakage 1.08 × CFM × ΔT + 0.68 × CFM × Δgrains (humidity)
people 230 sensible + 200 latent BTU/h each
appliances 1,200 BTU/h
ducts +15% of envelope gains when ducts run through the attic
size (tons) total ÷ 12,000, smallest unit covering 95% or moreThe design day. For each state we use the nearest NOAA station with hourly climate normals to a major city. The design temperature is the highest hourly 90th-percentile temperature from June through September. The design dew point is the highest hourly dew point normal. That lands within a few degrees of the design values HVAC engineers use.
The house. We model each story as a square box. Insulation sets the U-values: an average house means about R-13 walls and R-30 in the attic. Windows set the U-value and the solar heat gain coefficient (SHGC). Air leakage is in air changes per hour, from 0.25 for a tight new house to 0.7 for a drafty older one.
Humidity. In humid climates, outdoor air leaking in carries moisture that the AC has to condense out, and that’s part of the load. The sensible heat ratio shown in the results tells you how much of the load is temperature rather than moisture.
What it doesn’t do: room-by-room loads, orientation of each window, basements and the equipment’s actual performance at your conditions. Those are Manual J and Manual S work.
Worked example
A one-story, 2,000 sq ft house near Atlanta has average insulation, double-pane windows (300 sq ft, 15% of the floor), some shade, four people and ducts in the attic. NOAA normals at Atlanta’s Fulton County airport give a design day of 95 °F with a 70 °F dew point.
- Sun through windows: 300 × 0.65 × 70 = 13,650 BTU/h, the biggest single source
- Ceiling: 0.033 × 2,000 × (95 + 35 − 75) = 3,630 BTU/h. Windows add 3,000 and walls 1,810
- Air leakage: 120 CFM, so 2,590 BTU/h of heat plus about 3,700 BTU/h of humidity
- People and appliances: 920 + 800 + 1,200 BTU/h. Duct gains add 3,700
- Total: about 35,000 BTU/h, or 2.92 tons. A 3-ton unit covers it with 3% to spare
Switch to low-E windows and good insulation and the load drops to about 24,100 BTU/h, which a 2-ton unit covers. In this house the windows matter more than anything else.
AC size by state: reference table
The same 2,000 sq ft home in every state, with design conditions from the NOAA station listed.
| State (station) | Cooling design | Dew point | Load (BTU/h) | AC size |
|---|---|---|---|---|
| Alabama (Birmingham AP, AL) | 96 °F | 71 °F | 35,900 | 3 t |
| Alaska (Anchorage Merrill Fld, AK) | 75 °F | 52 °F | 21,400 | 2 t |
| Arizona (Phoenix Sky Harbor Intl AP, AZ) | 111 °F | 61 °F | 40,600 | 3.5 t |
| Arkansas (Little Rock AP Adams Fld, AR) | 99 °F | 72 °F | 37,700 | 3 t |
| California (Los Angeles Intl AP, CA) | 82 °F | 62 °F | 26,300 | 2.5 t |
| Colorado (Aurora Buckley Fld ANGB, CO) | 95 °F | 52 °F | 31,300 | 2.5 t |
| Connecticut (Hartford Bradley Intl AP, CT) | 93 °F | 64 °F | 32,400 | 3 t |
| Delaware (Philadelphia Intl AP, PA) | 94 °F | 67 °F | 33,600 | 3 t |
| District of Columbia (Washington Reagan AP, VA) | 96 °F | 68 °F | 35,000 | 3 t |
| Florida (Jacksonville NAS, FL) | 96 °F | 74 °F | 36,900 | 3 t |
| Georgia (Atlanta Fulton Co AP, GA) | 95 °F | 70 °F | 35,000 | 3 t |
| Hawaii (Honolulu Intl AP, HI) | 90 °F | 69 °F | 32,200 | 3 t |
| Idaho (Boise Air Terminal, ID) | 100 °F | 46 °F | 33,800 | 3 t |
| Illinois (Chicago Midway AP, IL) | 92 °F | 64 °F | 31,700 | 3 t |
| Indiana (Indianapolis, IN) | 92 °F | 66 °F | 32,300 | 3 t |
| Iowa (Des Moines Intl AP, IA) | 95 °F | 67 °F | 34,000 | 3 t |
| Kansas (Wichita, KS) | 102 °F | 68 °F | 37,900 | 3.5 t |
| Kentucky (Louisville Intl AP, KY) | 95 °F | 68 °F | 34,500 | 3 t |
| Louisiana (New Orleans Lakefront AP, LA) | 94 °F | 76 °F | 36,400 | 3 t |
| Maine (Portland Intl Jetport, ME) | 86 °F | 62 °F | 28,300 | 2.5 t |
| Maryland (Baltimore Wash Intl AP, MD) | 95 °F | 68 °F | 34,300 | 3 t |
| Massachusetts (Boston Logan Intl AP, MA) | 91 °F | 63 °F | 31,100 | 2.5 t |
| Michigan (Detroit City AP, MI) | 91 °F | 63 °F | 31,000 | 2.5 t |
| Minnesota (Minneapolis/St Paul AP, MN) | 91 °F | 62 °F | 30,800 | 2.5 t |
| Mississippi (Jackson Intl AP, MS) | 96 °F | 73 °F | 36,500 | 3 t |
| Missouri (Kansas City Downtown AP, MO) | 99 °F | 68 °F | 36,300 | 3 t |
| Montana (Billings Logan Intl AP, MT) | 96 °F | 51 °F | 31,900 | 3 t |
| Nebraska (Omaha Eppley Airfield, NE) | 96 °F | 68 °F | 35,100 | 3 t |
| Nevada (Las Vegas Mccarran AP, NV) | 110 °F | 48 °F | 38,800 | 3.5 t |
| New Hampshire (Manchester AP, NH) | 92 °F | 63 °F | 31,400 | 2.5 t |
| New Jersey (Newark Intl AP, NJ) | 95 °F | 65 °F | 33,700 | 3 t |
| New Mexico (Albuquerque Intl AP, NM) | 97 °F | 55 °F | 32,300 | 3 t |
| New York (New York Laguardia AP, NY) | 93 °F | 65 °F | 32,600 | 3 t |
| North Carolina (Charlotte Douglas AP, NC) | 95 °F | 69 °F | 34,800 | 3 t |
| North Dakota (Fargo Hector Intl AP, ND) | 89 °F | 62 °F | 29,800 | 2.5 t |
| Ohio (Columbus Port Columbus Intl AP, OH) | 91 °F | 65 °F | 31,600 | 3 t |
| Oklahoma (Oklahoma City Will Rogers AP, OK) | 102 °F | 69 °F | 38,100 | 3.5 t |
| Oregon (Portland Intl AP, OR) | 94 °F | 57 °F | 31,000 | 2.5 t |
| Pennsylvania (Philadelphia Intl AP, PA) | 94 °F | 67 °F | 33,600 | 3 t |
| Rhode Island (Providence T F Green AP, RI) | 91 °F | 65 °F | 31,500 | 2.5 t |
| South Carolina (Charleston Intl AP, SC) | 95 °F | 74 °F | 36,400 | 3 t |
| South Dakota (Sioux Falls, SD) | 92 °F | 65 °F | 32,000 | 3 t |
| Tennessee (Nashville Intl AP, TN) | 95 °F | 70 °F | 34,900 | 3 t |
| Texas (Houston Intercont AP, TX) | 99 °F | 76 °F | 38,900 | 3.5 t |
| Utah (Salt Lake City Intl AP, UT) | 100 °F | 49 °F | 33,800 | 3 t |
| Vermont (Burlington Intl AP, VT) | 89 °F | 61 °F | 29,700 | 2.5 t |
| Virginia (Oceana NAS, VA) | 94 °F | 72 °F | 35,100 | 3 t |
| Washington (Seattle Boeing Fld, WA) | 87 °F | 55 °F | 27,300 | 2.5 t |
| West Virginia (Charleston Yeager AP, WV) | 92 °F | 68 °F | 32,900 | 3 t |
| Wisconsin (Milwaukee Mitchell AP, WI) | 89 °F | 63 °F | 30,100 | 2.5 t |
| Wyoming (Cheyenne, WY) | 91 °F | 50 °F | 29,300 | 2.5 t |
Tips and common mistakes
Don’t size from square footage alone. Two houses of the same size can differ by a ton or more because of windows, shade and insulation. The breakdown table shows what’s driving your load.
Fix the envelope before you buy. Air sealing, attic insulation and window film or shades can drop the load enough to buy a smaller unit, which costs less up front and runs better.
Ask for a Manual J. The IRC requires heating and cooling equipment to be sized with ACCA Manual S, based on a Manual J load. A contractor who quotes a size without measuring the house is guessing.
Match the indoor and outdoor units. A new condenser on an old coil rarely performs as rated. Look for a matched system listed in the AHRI directory.
Mind the ducts. Ducts in a hot attic can add 15% or more to the load and lose cooling before it reaches the rooms. Sealing and insulating them is often cheap capacity.
Frequently asked questions
What size AC do I need for a 2,000 sq ft house?
For a typical one-story, 2,000 sq ft home with average insulation and double-pane windows, our model gives about 1.8 to 3.4 tons depending on the state, so a 2 to 3.5 ton unit. Hot, humid places like Phoenix and Houston sit at the top of that range, and Alaska, the Pacific Northwest and northern New England at the bottom.
How many BTUs per square foot do I need for AC?
Most homes in our model come out between about 11 and 21 BTU per hour per square foot. Rules of thumb of 25 to 30 BTU per square foot usually oversize the system, especially in newer or well-insulated homes.
Is it bad to oversize an air conditioner?
Yes. An oversized unit cools the air quickly and shuts off before it has run long enough to pull much humidity out, so the house feels clammy. It also cycles on and off more, which wastes energy and wears out parts. Pick the smallest size that covers the load.
What's the difference between this and Manual J?
Manual J is the ACCA standard the building code requires for sizing equipment. It works room by room with detailed construction, orientation and duct data. Our calculator uses the same physics in simplified form for the whole house. Use it to check a contractor's number or to see what an upgrade would change, not as a substitute for a permit-ready Manual J.
What does one ton of air conditioning mean?
One ton is 12,000 BTU per hour of cooling. The name comes from the heat it takes to melt a ton of ice over 24 hours. A 3-ton air conditioner removes about 36,000 BTU of heat per hour at its rated conditions.