About ACSizeCheck

Why this exists

Almost every AC sizing chart says the same thing: 20 BTU per square foot, or a ton for every 500 square feet. That chart has no idea where the house is. It gives a 2,000-square-foot house in Seattle and one in Phoenix the same air conditioner, and one of them ends up with a unit that short-cycles, runs up the bill and leaves the rooms damp. ACSizeCheck sizes the unit for the summer a city actually gets, for 3,206 US cities with ten thousand or more people.

What goes into the number

Climate is the NOAA NCEI 1991-2020 U.S. Climate Normals at the weather station nearest each city (1,260 stations in all): the normal high and overnight low of the hottest month, the number of days at or above 90°F and 100°F, and cooling degree days for every month. City locations and populations come from the Census Bureau's 2025 Gazetteer and 2024 population estimates. Electricity prices are the average residential price by state from the EIA's Electric Power Monthly.

The model is a simplified version of the ACCA Manual J heat balance contractors use. It estimates a design-day outdoor temperature a few degrees above the hottest month's normal high, works out heat coming through walls, windows and roof against a 75°F indoor setting, adds sun, people and appliances, then raises the total for humidity where summer nights stay warm. It picks the smallest stocked unit that covers the load, never more than 15% over it, which is ACCA Manual S's limit for a single-stage system. Running cost uses the city's cooling degree days, the federal minimum efficiency for a new unit (14.3 SEER2) and the state's electricity price.

What the data shows

For a typical 2,000-square-foot house, the 20 BTU per square foot chart oversizes the unit in 2,150 of the 3,206 cities, lands close in 885, and falls short in 171. The square feet one ton of cooling covers runs from about 513 in the hottest, most humid places to about 1,105 in the mildest (middle 90% of cities), which is the spread the chart flattens into a single number.

Known limits

This is a screening estimate, not a Manual J. It assumes an average house: average insulation and air sealing, average sun on the windows, four people. A house with a wall of west-facing glass, a vaulted ceiling or a leaky attic can need much more; a tight new build can need much less. The calculator on the home page lets you change the floor area. Climate normals describe a typical summer, not a record heat wave. Several cities share a weather station, so their climate figures match even where the towns differ. Before you buy a central system, ask the contractor for a room-by-room Manual J and compare it with the number here.

Corrections

If a city's figures look wrong, say so through the contact page. Confirmed errors are fixed in the data or the model, which corrects every page that depends on it at once.

Who runs this

ACSizeCheck is independently run. It is not affiliated with any air conditioner maker, retailer, HVAC contractor, utility or government agency, and nobody pays to change a size.

Written by

Each city page is generated by AI software from the cited NOAA climate normals, Census place data and EIA electricity prices, and no journalist or editor reviews a page before it publishes. The loads, sizes and costs come from that data and the model described above, not from AI; the AI only assembles the sentences around them.