Central AC Size Calculator

Understanding your result

The headline is the equipment size in tons, rounded up to the next half ton, because that is how central air conditioners are sold. Under it, the sub-line gives the calculated cooling load, which is the number the house actually needs, and the capacity of the rounded equipment in BTU per hour. The gap between those two is the oversize margin in the breakdown table: at 10 to 15% the system will run long enough to dehumidify well, while at 25% or more it will short-cycle on most days and you should look hard at the half size down.

The three figures beneath the headline are the load, the load per unit of floor area, and the design airflow. The airflow is what the ducts, the return grilles and the supply registers must pass; if the house has one undersized return in a hallway, a 3-ton system cannot move 1,200 CFM and will not deliver 3 tons. The CFM per room calculator splits that airflow between rooms, and the return grille calculator checks whether the returns can carry it.

The breakdown table shows every factor on its own line, so you can see which input moved the answer. If the insulation factor is 1.2 and the sun factor is 1.1, those two together add a third to the load, and improving either is cheaper than buying a bigger machine. To check an existing system against the same arithmetic, use the “Is my AC the right size?” calculator, which takes the nameplate capacity and compares it.

How we calculate this

The load per square foot comes from the BTU per square foot table, which lists the cooling and heating figures for the seven DOE climate zones at average insulation and 8 ft ceilings. The ceiling factor adds 10% for each foot above 8 ft, because the air volume to be cooled grows and tall rooms stratify; the insulation, sun and window factors are the standard ±10 to 20% adjustments used in published sizing charts and in the ENERGY STAR room air conditioner guidance, and the 600 BTU/hr per person and 4,000 BTU/hr per kitchen are from the same guidance.

In metric mode the floor area is converted from square metres to square feet and the ceiling from metres to feet before the arithmetic, and the results are shown in kilowatts (1 kW = 3,412 BTU/hr) and cubic metres per hour (1 CFM = 1.7 m³/h). The unit conversions page lists the factors, and the tonnage converter turns any result into any other unit.

The assumptions behind the numbers

Assumptions last reviewed October 7, 2026.

The calculator does not model the orientation of individual windows, the colour of the roof, the leakiness of the ducts, internal gains from large appliances or the latent (moisture) share of the load, all of which a Manual J calculation includes. It also assumes the whole area is served by one system; a two-storey house often has a separate load per floor. The guide on how to size an air conditioner explains what Manual J adds and how to read one.

Two worked examples

Where to find your inputs

Conditioned floor area. Use the heated and cooled area from the listing, the appraisal or the plans, and leave out the garage, an unfinished basement or attic, and porches. If you are measuring, multiply the outside length and width of each floor and subtract the garage.

Climate zone. The DOE map assigns every US county a zone from 1 (hot, Miami) to 7 (very cold, northern Minnesota); zone 8 is Alaska. The BTU per square foot table lists example cities for each zone. If you are on the border between two, run both.

Insulation. A house built before 1980 with little attic insulation and single-pane windows is “poor”. A house built to code since then with double glazing is “average”. A house with R-38 or more in the attic, air sealing and low-e windows is “good”. An energy audit report will say which you have.

People. Count the people who are usually at home on a hot afternoon, not the number who live there.

Common mistakes

• Sizing from the old unit. The previous system may have been oversized for decades; the house’s load is what matters, not the nameplate on the unit being replaced.
• Counting unconditioned space. Garages, unfinished basements and attics add area but not load. Including them can add a full ton.
• Using 20 BTU per square foot everywhere. That figure is right for a mixed climate with average insulation and wrong by 30% or more at either end of the country.
• Rounding up twice. Rounding the load to the next half ton is one step. Adding another half ton “for the hottest day” is how 3-ton houses end up with 4-ton units.
• Ignoring the ducts. A 3-ton unit on ducts that pass 900 CFM delivers about 2.3 tons. Size the ducts and returns with the airflow figure before buying the equipment.
• Skipping the Manual J. The calculator is a starting estimate. For a purchase of several thousand dollars, a room-by-room load calculation is cheap insurance.

Questions people ask

How many tons of AC do I need per square foot?

It depends on the climate. The rule of thumb this calculator uses runs from about one ton per 450 sq ft in the hot-humid zone 1 (24 BTU/hr per sq ft before occupants and kitchen) to one ton per 1,000 sq ft in the very cold zone 7 (12 BTU/hr per sq ft). A mixed climate such as Washington DC or St. Louis is about one ton per 650 sq ft. Poor insulation, high ceilings and a lot of west-facing glass can add 30% or more to those figures.

Is it better to oversize or undersize an air conditioner?

Neither, but a modest undersize is the lesser fault. An undersized unit runs long on the hottest days and may fall a degree or two behind, while an oversized unit cools the air fast, shuts off before it has removed much moisture, and leaves the house cold and clammy, with more wear from frequent starts. Industry guidance (ACCA Manual S) allows a cooling unit up to about 15% above the calculated load; the calculator shows your margin.

What is a Manual J load calculation and do I need one?

Manual J is the Air Conditioning Contractors of America's room-by-room method for working out heating and cooling loads from the actual construction of a house, its orientation, its windows, its insulation and its leakage. It is the standard most building codes require for new equipment. This calculator is a rule-of-thumb estimate that gets you to the right conversation; a contractor who refuses to do a Manual J and sizes by square feet alone is sizing by guesswork.

How many CFM does the system need?

About 400 cubic feet per minute per ton, so 1,200 CFM for a 3-ton system. Humid climates use 350 CFM per ton to make the coil run colder and remove more moisture; dry climates can use 450. The ducts, return grilles and registers all have to pass that airflow, which the duct size and return grille calculators check.

Does a finished basement count in the square footage?

Only if it is cooled by the same system. Basements below grade gain very little heat from outside and are usually cooler than the rest of the house, so counting a basement at the full per-square-foot rate oversizes the system. If the basement has its own supply registers, add about half its area; if it is not conditioned, leave it out.

Why does the calculator give a different answer from my contractor's quote?

Three common reasons. The contractor may have done a Manual J, which uses your actual windows, orientation and leakage and is more accurate than any rule of thumb. The quote may be for the same size as the old unit, which may itself have been oversized. Or the quote may round up a half size "to be safe". Ask for the load calculation; if the quoted size is more than 15% above it, ask why.