How the calculator sizes your AC
Air conditioners are rated by how much heat they remove in an hour, in BTU per hour. One ton of cooling is 12,000 BTU/h. The calculator starts from the ENERGY STAR room sizing chart, then adjusts for the things the chart leaves out. Here is the math for the default room.
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1
Look up the floor area
A 300 sq ft room falls in the ENERGY STAR row for 300 to 350 sq ft: 8,000 BTU/h.
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2
Scale for ceiling height
The chart assumes 8 ft ceilings, so an 8 ft room stays at 8,000. A 10 ft ceiling holds 25% more air and would raise it to 10,000.
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3
Adjust for climate, insulation and sun
Zone 4, average insulation and average sun leave it unchanged. A very sunny room would add 10%. A shaded one would take off 10%.
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4
Add people, kitchen heat and extra windows
Two people and two windows add nothing. A third person would add 600 BTU/h, and a kitchen adds 4,000. The load stays at 8,000 BTU/h, or 0.67 tons.
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5
Pick the nearest stock size
The calculator picks the smallest standard size that comes within 5% of the load. Here that is an 8,000 BTU window unit.
The ENERGY STAR room chart
These capacities assume an 8 ft ceiling, two people and average sun. Tap a size to load it.
| Area to cool | Capacity needed |
|---|---|
| 100 to 150 sq ft | 5,000 BTU/h |
| 150 to 250 sq ft | 6,000 BTU/h |
| 250 to 300 sq ft | 7,000 BTU/h |
| 300 to 350 sq ft | 8,000 BTU/h |
| 350 to 400 sq ft | 9,000 BTU/h |
| 400 to 450 sq ft | 10,000 BTU/h |
| 450 to 550 sq ft | 12,000 BTU/h |
| 550 to 700 sq ft | 14,000 BTU/h |
| 700 to 1,000 sq ft | 18,000 BTU/h |
| 1,000 to 1,200 sq ft | 21,000 BTU/h |
| 1,200 to 1,400 sq ft | 23,000 BTU/h |
| 1,400 to 1,500 sq ft | 24,000 BTU/h |
| 1,500 to 2,000 sq ft | 30,000 BTU/h |
| 2,000 to 2,500 sq ft | 34,000 BTU/h |
Above 2,500 sq ft the calculator extends the chart's last step at 13.6 BTU/h per square foot. Notice how the rate per foot drops as spaces grow: a small bedroom needs about 33 BTU/h per square foot, a big open area closer to 15.
What each adjustment does
Four adjustments come straight from ENERGY STAR. The others are our own estimates, shown here so you can judge them.
| Factor | Change | Basis |
|---|---|---|
| Very sunny / heavily shaded | +10% / −10% | ENERGY STAR |
| Each person beyond two | +600 BTU/h | ENERGY STAR |
| Kitchen | +4,000 BTU/h | ENERGY STAR |
| Ceiling height | × height ÷ 8 ft | Chart assumes 8 ft |
| Climate zone 1 to 7+ | +15% to −15% | HomeOutlay estimate |
| Poor / good insulation | +15% / −10% | HomeOutlay estimate |
| Windows beyond 2, or 1 per 100 sq ft | +500 BTU/h each | HomeOutlay estimate |
The chart already allows for a couple of windows. Past two, or past one per 100 sq ft in bigger spaces, each extra window adds 500 BTU/h for the sun that pours through a typical 3 by 5 ft window on a summer afternoon. Fewer windows than average never lowers the result, because a single west window can still heat a room.
Climate zones and cooling hours
The state list picks the zone of each state's biggest metro area, using the 2021 IECC county map. Many states span two or three zones, so pick your zone by hand if you live far from that city. The cooling hours drive the running cost only, not the size.
| Zone | Examples | Size factor | Cooling hours |
|---|---|---|---|
| 1 | Miami, Honolulu | +15% | 3,000 |
| 2 | Houston, Dallas, Phoenix, Orlando, New Orleans | +10% | 2,200 |
| 3 | Atlanta, Charlotte, Nashville, Las Vegas, Los Angeles | +5% | 1,500 |
| 4 | New York, Washington DC, St. Louis, Seattle | 0 | 1,000 |
| 5 | Chicago, Boston, Denver, Detroit, Pittsburgh | −5% | 750 |
| 6 | Minneapolis, Portland ME, Burlington VT, Billings | −10% | 500 |
| 7+ | Anchorage, far northern Minnesota | −15% | 300 |
Cooling hours here are full-load equivalents: the hours the unit would need at full blast to do a year's cooling. They are our rough values per zone. Your real number can easily be half or double, depending on how you set the thermostat. If you know your own, type it in.
Tons, stock sizes and what they cost
Window units come in steps of 5,000, 6,000, 8,000, 10,000, 12,000, 14,000, 18,000 and 24,000 BTU/h. Ductless mini-splits usually start at 9,000 and go up in 6,000 steps. Central systems are sold in half-ton steps from 1.5 to 4 tons, then 5 tons.
| Central AC | BTU/h | Installed cost range |
|---|---|---|
| 1.5 tons | 18,000 | $2,000–$5,000 |
| 2 tons | 24,000 | $2,500–$5,500 |
| 2.5 tons | 30,000 | $2,700–$6,000 |
| 3 tons | 36,000 | $3,000–$6,500 |
| 3.5 tons | 42,000 | $3,100–$7,000 |
| 4 tons | 48,000 | $3,500–$7,500 |
| 5 tons | 60,000 | $4,000–$8,000 |
Those ranges cover an AC-only swap on existing ducts. Many 2026 quotes run higher, often $5,000 to $12,000 or more, once you add a new coil, a heat pump, ductwork or premium equipment. A window unit runs from under $200 for a basic 5,000 BTU model to about $650 to $1,000 for 18,000 BTU. A single-zone mini-split usually costs $3,000 to $5,000 installed. The calculator's default prices sit in the middle of those ranges. Change them to match your quotes.
Why central AC needs a Manual J
A chart only knows floor area. A real load calculation also counts which way each window faces, the glass type, wall and attic insulation, air leakage, duct losses and your local design temperature. That calculation is ACCA Manual J. The International Residential Code (section M1401.3) requires heating and cooling equipment to be sized with Manual S, based on loads from Manual J.
The test of a good contractor is simple: ask for the Manual J report. A bid that sizes the system by square footage alone, or matches the old unit's tonnage, often oversizes it. Manual S lets the installed capacity land a little over the load (about 15% for a cooling-only system). It does not allow a full extra ton "just in case."
Oversizing is the most common mistake. A big unit cools the air quickly and shuts off before its coil has pulled much moisture out. You get a cold, damp house, more on-off cycles, more wear and higher bills. An undersized unit runs nonstop on the hottest days but usually keeps humidity in check. If you are torn between two sizes, the smaller one is usually the safer bet.
What it costs to run
Yearly use in kWh is the capacity in BTU/h times the cooling hours, divided by the efficiency rating times 1,000. Central and mini-split systems are rated in SEER2. Window units are rated in CEER, which assumes 750 hours of use a year. For the default room: 8,000 × 1,000 ÷ (13 × 1,000) = 615 kWh. At 18.2 cents per kWh that is about $112 a year.
Since 2023 the federal minimum for new split central AC is 13.4 SEER2 in the North and 14.3 SEER2 in the South. Mini-splits commonly rate 18 to 25 SEER2. New room AC standards took effect on May 26, 2026, so a new window unit will usually beat an older one on efficiency.
Tips before you buy
- Measure the whole open space. If a living room opens into a dining room with no door, size for both.
- Check the outlet. Many window units of 18,000 BTU and up run on 230 volts and need a dedicated circuit. Never run a window AC on an extension cord.
- Seal the gaps. Foam strips and a side panel around a window unit stop hot air from leaking in around it.
- Fix the house first. Attic insulation and air sealing can drop the load enough to buy a smaller system, which costs less up front and every summer after.
- Compare portable units carefully. A portable AC needs more nameplate BTU than a window unit for the same room, because it sits inside and leaks some heat back in.
- One head per room for mini-splits. A single wall head will not cool rooms behind closed doors.
Sources and assumptions
- Room sizing chart and the sun, people and kitchen adjustments: ENERGY STAR, Room Air Conditioners (checked October 2026).
- Climate zones by county: PNNL, Guide to Determining Climate Zone by County: Building America and IECC 2021 Updates (October 2022), and the DOE Building America climate zone lookup.
- Load calculation and equipment sizing: ACCA Manual J and Manual S, as required by IRC section M1401.3.
- Electricity price: 18.2 cents per kWh, the 2026 U.S. residential average in the EIA Short-Term Energy Outlook, October 2026.
- Central AC minimum efficiency (13.4 / 14.3 SEER2 since January 1, 2023): Carrier summary of the DOE standard. Room AC standard, compliance from May 26, 2026: DOE final rule, 88 FR 34298. The 750-hour basis of CEER comes from the DOE test procedure.
- Central AC installed cost by tonnage: PickHVAC replacement cost guide (June 2025). Window and mini-split prices: HomeOutlay estimates from 2026 retailer listings and installer price guides. All prices as of October 2026.
- HomeOutlay assumptions: climate, insulation and window factors, and full-load cooling hours per zone, as shown in the tables above. Defaults: CEER 13 for window units, 20 SEER2 for mini-splits, 14.3 SEER2 for central AC.
Common questions
What size AC do I need for a 12x12 room?
A 12x12 room is 144 square feet, which falls in the ENERGY STAR row for 100 to 150 square feet: 5,000 BTU per hour. Go up to 6,000 BTU if the room is very sunny or regularly holds more than two people.
How many BTUs do I need per square foot?
For a single room, the ENERGY STAR chart works out to roughly 20 to 40 BTU per hour per square foot, and more for very small rooms. Large open areas need less per square foot. Sun, people, ceiling height and climate shift the number from there.
What size central AC do I need for a 2,000 square foot house?
With this calculator's defaults, a 2,000 square foot house comes out at about 3 tons in a mixed or hot climate and about 2.5 tons in a cold northern one. Treat that as a starting point. A Manual J load calculation is what should set the final size.
Is it bad to oversize an air conditioner?
Yes. An oversized unit cools the air fast and shuts off before it pulls much moisture out, so the house feels cold and clammy. The short on-off cycles also waste energy and wear parts sooner. Pick the smallest size that covers the load.
How much does it cost to run a window air conditioner?
An 8,000 BTU window unit with a CEER of 13 that runs the equivalent of 1,000 full-load hours a year uses about 615 kWh. At the 2026 national average of 18.2 cents per kWh, that is about $112 a year. Hot climates and lower-efficiency units cost more.
How many square feet does a 1-ton AC cool?
One ton is 12,000 BTU per hour. The ENERGY STAR room chart assigns 12,000 BTU to rooms of 450 to 550 square feet. In a whole house with good insulation and tight ducts, one ton often covers more, which is why old rules of thumb tend to oversize central systems.