What "tons" means for an air conditioner
Central air conditioners and heat pumps are sold by cooling capacity, measured in tons. One ton of cooling is 12,000 BTU per hour, the rate of heat needed to melt one short ton of ice in a day. Home systems usually come in half-ton steps from 1.5 to 5 tons (18,000 to 60,000 BTU/h). Larger homes often use two systems or zoning rather than one bigger unit.
Cooling load (BTU/h) = floor area × BTU per sq ft × (ceiling height ÷ 8)
Tons = BTU/h ÷ 12,000 · Square feet per ton = area ÷ tons
Where the BTU-per-square-foot figure comes from
ENERGY STAR's guidance for room air conditioners uses a rule of thumb of about 20 BTU per square foot of living space for a typical room with 8 ft ceilings. This calculator uses that figure as its baseline and scales it for taller ceilings, because a 10 ft room holds 25% more air than an 8 ft one. The 25 and 30 BTU per sq ft options are planning assumptions for hotter climates and poorly insulated houses, not an official standard: use them to see a realistic range, not as a final answer.
Quick reference: tons by house size
| Floor area (8 ft ceilings) | At 20 BTU/sq ft | At 25 BTU/sq ft | At 30 BTU/sq ft |
|---|---|---|---|
| 1,000 sq ft | 1.67 tons | 2.08 tons | 2.5 tons |
| 1,500 sq ft | 2.5 tons | 3.13 tons | 3.75 tons |
| 1,800 sq ft | 3 tons | 3.75 tons | 4.5 tons |
| 2,000 sq ft | 3.33 tons | 4.17 tons | 5 tons |
| 2,500 sq ft | 4.17 tons | 5.21 tons | 6.25 tons |
| 3,000 sq ft | 5 tons | 6.25 tons | 7.5 tons |
Worked example 1: 1,800 sq ft, mild climate
- Load = 1,800 × 20 × (8 ÷ 8) = 36,000 BTU/h.
- Tons = 36,000 ÷ 12,000 = 3 tons, which is a standard size: 600 sq ft per ton.
Worked example 2: the same house with 9 ft ceilings
- Load = 1,800 × 20 × (9 ÷ 8) = 40,500 BTU/h = 3.38 tons.
- That falls between 3 and 3.5 tons. The smaller unit runs longer cycles and removes more humidity; the larger one covers hot afternoons. This is exactly the decision a proper load calculation should settle.
Worked example 3: 1,800 sq ft in a hot climate
At 25 BTU per sq ft the load is 45,000 BTU/h, or 3.75 tons, so the standard choices are 3.5 or 4 tons. At 30 BTU per sq ft it rises to 4.5 tons. The spread between these answers is why square footage alone cannot size a system precisely.
Why oversizing is a problem
A unit that is too large cools the air quickly and switches off before it has run long enough to pull moisture out, so rooms feel cold and clammy. Short cycles also waste energy and wear out compressors sooner. An undersized unit runs almost constantly on the hottest days. The industry method for choosing equipment, ACCA Manual S, starts from a Manual J load calculation and limits how far above that load the equipment may be, which is why bigger is not safer.
What a Manual J load calculation adds
Square-footage rules ignore the things that change cooling loads the most. A contractor's ACCA Manual J calculation, required by many building codes for new systems, accounts for:
- Insulation levels in walls, attic and floors, and how airtight the house is.
- Window area, glass type and which way the windows face.
- Local design temperatures and humidity.
- Number of occupants, appliances and lighting.
- Duct location and leakage, for example ducts in a hot attic.
Use this calculator to understand the ballpark before talking to installers, and to sanity-check a quote that seems far too large or small.
Common mistakes
- Counting unconditioned space such as garages, unheated basements or attics.
- Copying the old unit's size. Many older systems were oversized, and insulation or window upgrades lower the load.
- Ignoring the ductwork. A bigger unit on ducts sized for a smaller one moves air noisily and inefficiently.
- Using this for a single room. For one room and a window or portable unit, use the BTU calculator instead.
Frequently asked questions
What size AC do I need for a 2,000 sq ft house?
Using the 20 BTU per square foot rule of thumb, 2,000 sq ft needs about 40,000 BTU/h, or 3.3 tons, so a 3.5-ton unit is the usual ballpark. Hot climates or older houses can push it toward 4 or 5 tons; a Manual J calculation gives the real answer.
How many square feet does a 3-ton AC cool?
At 20 BTU per sq ft, 3 tons (36,000 BTU/h) covers about 1,800 sq ft with 8 ft ceilings. In a hot climate or a leaky house the same unit may only cover 1,200 to 1,450 sq ft.
How many BTU is one ton of air conditioning?
One ton equals 12,000 BTU per hour, so a 2.5-ton unit is 30,000 BTU/h and a 5-ton unit is 60,000 BTU/h.
Is it better to oversize an air conditioner?
No. An oversized unit short-cycles, removes less humidity and wears faster. Choose the size that matches the calculated load.
Does ceiling height affect AC size?
Yes. Taller ceilings mean more air to cool. This calculator scales the load by ceiling height ÷ 8 ft, so 10 ft ceilings add 25%.
Sources
- ENERGY STAR (US EPA / DOE), Room Air Conditioners sizing guidance: about 20 BTU per square foot of living space.
- ACCA Manual J (Residential Load Calculation) and Manual S (Residential Equipment Selection), Air Conditioning Contractors of America.
- NIST Special Publication 811: 1 ton of refrigeration = 12,000 BTU (IT)/h.
Last reviewed 31 October 2026. Results are estimates; see our disclaimer.