AC Size Calculator: Central AC Tonnage for Your House
A 1,500 ft² house in a mixed climate (zone 4) with average insulation needs about 33,000 BTU/h, or 2.75 tons. Both a 2.5-ton and a 3-ton unit fall inside the ACCA sizing window; it is a tie, so the calculator picks the smaller 2.5 t. The rule of thumb runs from about 400 ft² per ton in South Florida to 750 ft² per ton in the far North.
How the tonnage is estimated
A ton of air conditioning is 12,000 BTU/h of cooling. That is the heat it takes to melt one ton of ice in a day, and it has nothing to do with the weight of the unit. The calculator turns your floor area into a cooling load, then into tons, in four steps:
Tons = load ÷ 12,000
- Zone BTU per square foot. A hotter, sunnier climate needs more cooling per square foot. The values below are the planning figures we use for an average house.
- Envelope factor. Insulation, window quality and air leakage change the load by up to 30% either way.
- Ceiling and sun. Each foot of ceiling above 8 ft adds 12.5% of air volume. Full sun adds 10% and deep tree shade takes off 10%, the same corrections the ENERGY STAR room chart uses.
- Pick a real unit. Central air conditioners come in half-ton steps from 1.5 to 5 tons. ACCA Manual S, the industry's equipment-selection standard, accepts a cooling unit rated at 90–115% of the calculated load. The calculator lists every size in that window and picks the one closest to the load. When two sizes are equally close, it picks the smaller one, because the smaller unit removes more humidity.
| Zone | Example city | BTU/h per ft² | ft² per ton |
|---|---|---|---|
| 1 | Miami, FL | 30 | 400 |
| 2 | Houston, TX | 28 | 429 |
| 3 | Atlanta, GA | 25 | 480 |
| 4 | Baltimore, MD | 22 | 545 |
| 5 | Chicago, IL | 20 | 600 |
| 6 | Minneapolis, MN | 18 | 667 |
| 7 | Duluth, MN | 16 | 750 |
| Envelope | Load factor |
|---|---|
| Poor (pre-1980, little insulation, single-pane) | × 1.20 |
| Average (some insulation, double-pane) | × 1.00 |
| Good (built to 2010s code) | × 0.85 |
| High-performance (tight, R-49+ attic) | × 0.70 |
AC size chart by square footage and climate zone
Most AC size charts give one tonnage per house size for the whole country. This one runs our calculator for each zone (average envelope, 8 ft ceilings, average sun) and shows the nominal unit it would pick. "Split" means the load is over 5 tons, the largest common residential size, so the house needs two systems.
| House | Zone 1 | Zone 2 | Zone 3 | Zone 4 | Zone 5 | Zone 6 | Zone 7 |
|---|---|---|---|---|---|---|---|
| 800 ft² | 2 t | 2 t | 1.5 t | 1.5 t | 1.5 t | 1.5 t | 1.5 t |
| 1,000 ft² | 2.5 t | 2.5 t | 2 t | 2 t | 1.5 t | 1.5 t | 1.5 t |
| 1,200 ft² | 3 t | 3 t | 2.5 t | 2 t | 2 t | 2 t | 1.5 t |
| 1,500 ft² | 3.5 t | 3.5 t | 3 t | 2.5 t | 2.5 t | 2.5 t | 2 t |
| 1,800 ft² | 5 t | 4 t | 3.5 t | 3.5 t | 3 t | 2.5 t | 2.5 t |
| 2,000 ft² | 5 t | 5 t | 4 t | 3.5 t | 3.5 t | 3 t | 2.5 t |
| 2,400 ft² | split | split | 5 t | 4 t | 4 t | 3.5 t | 3 t |
| 2,800 ft² | split | split | split | 5 t | 5 t | 4 t | 3.5 t |
Why a bigger AC is not safer
Oversizing is the most common sizing mistake, and installers have a reason for it: nobody calls to complain that the house is too cold. The costs show up elsewhere:
- Short cycling. The unit hits the thermostat setting in a few minutes and shuts off. Starts are the hardest part of a compressor's life.
- Clammy air. An AC removes moisture only after the coil has been cold for several minutes. With short runs the air gets cold but stays humid, which is a real problem in zones 1–3.
- Bigger ducts and a higher price. Every extra half ton needs about 200 CFM more airflow. That means larger ducts or more noise, and the unit itself costs more.
Undersizing has a cost too. On the hottest afternoons the house drifts a few degrees above the setting, and the unit runs nonstop. A unit sized 90–100% of the load, in a house with sealed ducts, usually holds temperature fine.
Rule of thumb vs. Manual J
The square-foot method is quick, but it knows nothing about your windows, which way the house faces, or whether the ducts run through a 130 °F attic. A contractor's ACCA Manual J calculation goes wall by wall and window by window. For houses built or renovated to recent energy codes, Manual J often comes out well below the rule of thumb. Set the envelope to "Good" or "High-performance" to see that range. If a quote is a full ton above both our "Average" and "Good" results, ask for the Manual J report before you sign.
From tons to airflow and duct size
A central system moves about 400 CFM of air per ton. In humid zones it is often set nearer 350 CFM per ton, so the coil stays colder and pulls out more moisture. A 3-ton system therefore needs roughly 1,050–1,200 CFM through the return and supply ducts. The duct size calculator turns that airflow into a trunk and branch size. Undersized ducts are a common reason a new AC never delivers its rated capacity.
Heat pumps and efficiency (SEER2)
A heat pump is sized like an AC in warm climates. In cold climates, size it for the heating load from the BTU calculator and check its capacity at your design temperature, because heat pump output falls as it gets colder outside. Efficiency (SEER2) does not change the size you need; it changes the electric bill. As a rough example, a 3-ton unit running 1,000 full-load hours a year uses about 36,000 × 1,000 ÷ (15.2 × 1,000) ≈ 2,370 kWh at SEER2 15.2.
Sources
- ACCA Manual S, Residential Equipment Selection: cooling capacity window of 90–115% of the Manual J load.
- ACCA Manual J, Residential Load Calculation, 8th edition (reference method this tool simplifies).
- ENERGY STAR room AC sizing chart: ±10% sun and shade corrections.
- IECC 2021 climate zone map (zones 1–8) and DOE Building America climate region guide.
Questions people ask
What size AC do I need for a 1,500 sq ft house?
About 2.5 t in a cool climate such as Chicago, 2.5 t (2.5 or 3 tons fit) in a mixed climate such as Baltimore, and 3.5 t to 4 tons in South Florida, for a house with average insulation and 8 ft ceilings.
What size AC for a 2,000 sq ft house?
Roughly 3 t in zone 6 (Minneapolis), 3.5 t in zone 4 (Baltimore) and 5 t in zone 1 (Miami) with average insulation. A newer, well-sealed house can often go half a ton to a full ton smaller.
How many square feet does 1 ton of AC cool?
About 400 ft² in the hottest US climate and up to about 750 ft² in the coldest, for an average house. With the High-performance envelope setting, the calculator gives roughly 570 to 1,070 ft² per ton from zone 1 to zone 7, and a Manual J for a tight house can land in that range too.
What size AC for a 1,000 sq ft house?
About 22,000 BTU/h in climate zone 4, which is 1.83 tons, so a 2 t unit. A 1,000 ft² apartment or addition is also a good fit for a ductless mini-split.
Is it better to oversize an air conditioner?
No. A unit more than 15% above the load short-cycles, removes less humidity and costs more. If you fall between two sizes, the smaller one usually feels better in humid climates.
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