Air conditioners are sized in BTU per hour, and for central systems in tons (one ton is 12,000 BTU). Get the size right and the house is cool, dry and quiet. Get it wrong in either direction and you pay for it every month in comfort and electricity. This guide gives you the chart, the adjustments, the warning signs of a mis-sized unit and the questions to ask before you buy.
Want the number for your home?
Our calculator adjusts for climate, sun, insulation, ceiling height and occupants and gives you BTU and tonnage in seconds.
Open the AC size calculatorThe quick answer: BTU by room size
These figures assume 8 foot ceilings, average insulation and a moderate climate. They are the starting point, not the final answer.
| Room or area | Square feet | BTU/hr | Typical unit |
|---|---|---|---|
| Small bedroom, home office | 100 to 200 | 5,000 to 6,000 | 6,000 BTU window |
| Large bedroom | 250 to 350 | 7,000 to 8,000 | 8,000 BTU window |
| Living room | 350 to 550 | 10,000 to 12,000 | 12,000 BTU window or 1 ton mini-split |
| Open-plan living and kitchen | 600 to 900 | 14,000 to 18,000 | 18,000 BTU mini-split |
| Apartment or small home | 1,000 to 1,400 | 21,000 to 24,000 | 2 ton central |
| Average home | 1,500 to 2,000 | 30,000 to 36,000 | 2.5 to 3 ton central |
| Large home | 2,000 to 2,600 | 36,000 to 42,000 | 3 to 3.5 ton central |
| Very large home | 2,600 to 3,500 | 48,000 to 60,000 | 4 to 5 ton, often two systems |
What moves the number
Climate
A home in Phoenix or Houston needs 15 to 25 percent more cooling than the same home in Denver, and a home in Seattle needs about 15 percent less. Humidity matters as much as heat: in the Gulf states the system spends much of its time removing moisture, which is why sizing errors show up as a clammy house there.
Sun and windows
A room with large west-facing windows gains heat all afternoon. Add about 10 percent. A room that is shaded by trees or faces north can drop 10 percent.
Insulation and age of the home
Older homes with single-pane windows and thin attic insulation leak cooling. Add 10 to 15 percent. A newer, sealed home with double-pane windows can take 10 percent off. If you plan to insulate the attic anyway, do it before the AC is sized; it can drop you a full half ton.
Ceiling height
Nine or ten foot ceilings mean more air to cool. Scale the number by the height divided by eight.
People and kitchens
Each person beyond two adds about 600 BTU. A kitchen adds about 4,000 BTU for the cooking appliances and the refrigerator.
Signs your current AC is the wrong size
Too big
- The system turns on and off every few minutes (short cycling).
- The house reaches the set temperature fast but feels humid or sticky.
- Some rooms are cold while others never get there.
- Electricity bills are high for the amount of time the unit seems to run.
Too small
- The system runs constantly on hot days and never reaches the set temperature.
- It keeps up in June but not in August.
- The upstairs is always warmer than the thermostat reading.
If the symptoms are the "too big" ones, replacing the unit with the same size because "it worked before" repeats the problem. Have the load calculated properly this time.
Central air, mini-split or window unit
| Option | Good for | Sizing note |
|---|---|---|
| Window unit | Single rooms, renters, under 700 sq ft | Over 12,000 BTU often needs a 220 V outlet |
| Ductless mini-split | Additions, homes without ducts, zoning one or two rooms | One outdoor unit can feed several indoor heads; size each room |
| Central air | Whole homes with ducts | Sized in half-ton steps; ducts must match the size |
| Heat pump | Whole-home cooling and heating | Sized the same way for cooling; check heating capacity too |
Why bigger is not safer
Homeowners and some installers round up "to be safe". An oversized unit cools the air quickly and shuts off before it has run long enough to dehumidify, so the house is cold and damp. The constant stopping and starting wears the compressor, the most expensive part, and uses more power than a correctly sized unit running longer, steadier cycles. If you are between sizes, go smaller in a humid climate and larger in a dry one.
What to ask the contractor
- "Will you do a Manual J load calculation?" This is the industry method. It measures walls, windows, orientation, insulation and ducts. The answer should be yes, and good contractors will show you the report.
- "What size are you quoting and why?" Two quotes a ton apart mean one contractor did not measure.
- "Are the ducts sized for this unit?" A bigger unit on old ducts starves for airflow and loses capacity in the attic.
- "What is the SEER2 rating?" 15 to 16 is standard now; 18 and up is premium and worth it in hot climates.
- "Does the quote include permit, disposal of the old unit and a new thermostat?"
Get three quotes and ask every contractor the same five questions. The one who measures, explains and writes it all down is usually the one to hire, even if not the cheapest.
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