How this AC tonnage calculator works
Air-conditioner sizing is one conversion — a cooling load in BTU, turned into tons at 12,000 BTU each. The load is floor area times a climate factor, adjusted for height and sun:
tons = BTU ÷ 12,000 → rounded to the nearest half-ton unit
At the defaults — 1,000 ft², 25 BTU/ft², 8 ft ceilings — the load is 25,000 BTU, about 2.08 tons, so a 2-ton (24,000 BTU) unit, roughly 500 ft² per ton; the full chain is in the worked example. It is a planning estimate — a Manual J load calculation is the real sizing.
Tons, BTU, and SEER
Cooling has its own vocabulary, and mixing up the units is where sizing goes wrong:
- BTU/hr: the unit of heat moved per hour — the raw cooling load a house imposes.
- Ton: 12,000 BTU/hr, the way condensers are sized — 1.5 to 5 tons in half-ton steps for homes.
- SEER2: the efficiency rating — how many BTU per watt-hour; it sets the power bill, not the size.
- Manual J: the room-by-room load calculation (ACCA) that accounts for windows, insulation and orientation — what a square-footage estimate approximates.
- Air handler & ducts: the indoor half — the CFM calculator sizes the airflow the tonnage has to move.
BTU per square foot, and why not to oversize
The one judgment call is the BTU per square foot — it carries your climate, insulation and sun. The guide:
- Mild / coastal: 18–22 BTU/ft² — long shoulder seasons, moderate peaks.
- Moderate: 22–26 BTU/ft² — the default range for most of the country.
- Hot / humid: 26–35 BTU/ft² — long, intense cooling seasons.
- Sun and glass: lots of west glass or an unshaded roof pushes toward the top of the range.
- Never oversize: a unit that’s too big short-cycles and leaves humidity behind — round down between sizes.
The honest way to read all this: the square-footage method gets you to the right size class, and the adjustments nudge you within it. Where it falls short is a house with unusual glass, a walk-out basement, or one very sunny wing — anywhere the load isn’t spread evenly across the floor. That is exactly the case a room-by-room Manual J is built for, and the reason a good installer runs one before committing to a compressor rather than sizing off the deed.
AC size by square footage, and BTU by climate
Table 1 runs the average 25 BTU/ft² out to common home sizes; Table 2 is the cooling BTU per square foot by climate.
| Area | Cooling BTU | AC tons |
|---|---|---|
| 600 ft² | 15,000 | 1.5 |
| 1,000 ft² | 25,000 | 2 |
| 1,500 ft² | 37,500 | 3 |
| 2,000 ft² | 50,000 | 4 |
| 2,500 ft² | 62,500 | 5 |
| Climate | BTU/ft² |
|---|---|
| Mild / coastal | 18–22 |
| Moderate | 22–26 |
| Warm | 26–30 |
| Hot / humid | 30–35 |
How to size an AC unit
- Measure the conditioned floor area; total the rooms on the square footage calculator.
- Pick the BTU per ft² for your climate and insulation from the sizing guide.
- Set the ceiling height — it scales the load by volume — and the number of occupants.
- Choose the sun exposure — shaded, average or sunny — for the final adjustment.
- Read the tons, round down when between sizes, and confirm with a Manual J before you buy.
Worked example: cooling a 1,000 ft² space
The default, worked line by line — 1,000 ft², 25 BTU/ft², 8 ft ceilings, 2 occupants, average sun:
tons = 25,000 ÷ 12,000 = 2.08 tons
unit = nearest half-ton = 2 tons (24,000 BTU)
coverage = 1,000 ÷ 2 = 500 ft² per ton
Notice the exact 2.08 rounds down to a 2-ton unit, not up to 2.5 — because a slightly undersized AC that runs longer beats an oversized one that short-cycles and never dries the air. Bump the climate to a hot 30 BTU/ft² and the same space needs 30,000 BTU, a 2.5-ton unit.
What this AC estimate doesn’t account for
A square-footage estimate gets you in the right size class; a real system design adds:
- Manual J. The room-by-room load calc that accounts for each window, wall and orientation — the actual sizing method.
- Ductwork. Duct size and layout, and the airflow (CFM) the unit has to push.
- Humidity. Latent load in humid climates, which favors right-sizing over oversizing.
- Windows and insulation. Glass area, orientation and R-values move the load well beyond floor area.
- Zoning. Big or multi-level homes split into zones or two systems rather than one large unit.
AC tonnage FAQ
What size AC do I need for 1000 square feet?
At an average 25 BTU per square foot, a 1,000 ft² space is a 25,000 BTU cooling load — 2.08 tons — which rounds to a 2-ton (24,000 BTU) unit. Milder climates need less (a 1.5-ton may do), hot and sunny ones more. This is a square-footage estimate; a Manual J load calculation is the real answer.
How many square feet does a ton of AC cool?
Roughly 400 to 600 ft² per ton depending on climate, insulation and sun. At an average 25 BTU per square foot it works out to about 480 ft² per ton; a hot, sunny, poorly insulated house drops toward 400, a mild well-sealed one stretches past 600.
How do you calculate AC tonnage?
Multiply floor area by the cooling BTU per square foot for your climate, adjust for ceiling height and sun, then divide by 12,000 — because one ton of cooling is 12,000 BTU per hour. Round to the nearest half-ton, since units are sold in 0.5-ton steps from 1.5 up to 5 tons.
What size AC unit for a 2000 square foot house?
At 25 BTU per square foot a 2,000 ft² home is a 50,000 BTU load — about 4.17 tons, or a 4-ton (48,000 BTU) system. Larger homes are often split into two systems or zoned rather than run on one big unit, so the whole-house number is a starting point, not a spec.
Is it bad to oversize an AC unit?
Yes. An oversized unit cools the air fast, satisfies the thermostat, and shuts off before it has pulled the humidity out — so it short-cycles, leaves the house clammy, and wears out sooner. When your load lands between two sizes, take the smaller one and let it run longer.
What is a ton of air conditioning?
Twelve thousand BTU per hour of cooling. The term is historical: one ton is the heat needed to melt a ton of ice over 24 hours. So a 3-ton unit removes 36,000 BTU of heat an hour — nothing to do with the weight of the equipment.
Does ceiling height affect AC size?
Yes — cooling is really about volume, not floor area, so the calculator scales the load by ceiling height. A room with 10 ft ceilings needs about 25% more capacity than the same floor area at 8 ft, which is why vaulted rooms and open great rooms size up.
Sources and standards
The conversion is fixed; the per-square-foot factors are estimating practice:
- The ton: 12,000 BTU/hr — the definitional cooling unit.
- ACCA Manual J: the residential load-calculation standard this square-footage method approximates.
- Climate BTU/ft²: common HVAC estimating ranges, 18–35 by climate and insulation.
- Unit sizes: residential condensers in half-ton steps, 1.5 to 5 tons.
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