A typical 2,000-square-foot house lands at 3 to 4 tons of cooling, or 36,000 to 48,000 BTU, across most of the country. The honest spread is wider than that: the same floor area runs closer to 2.5 tons in a tight northern house and 4 tons or more on the Gulf Coast, because square footage is one input out of a dozen. Rules of thumb like one ton per 400 to 600 square feet get you into the neighborhood and no further, and they skew high often enough to be worth distrusting. The real size comes from a Manual J load calculation, the ANSI-recognized standard that weighs your specific walls, windows, insulation, ducts, and local design temperature, and getting it right matters more than which brand ends up in your yard.
The Short Answer: Square Footage Gives a Starting Range, Not a Final Size
Cooling capacity is measured in tons, and one ton equals 12,000 BTU per hour of heat removal. Residential central systems come in half-ton steps: 1.5, 2, 2.5, 3, 3.5, 4, and 5 tons. A 4.5-ton size exists in some product lines but not most, so a house that calculates out at 3.7 tons is choosing between 3.5 and 4 tons rather than getting a perfect match.
Every online AC unit size calculator does the same thing: multiplies square footage by a BTU-per-square-foot factor and rounds to the nearest half ton. Sometimes that lands close. Often it misses by a full ton. Say one 2,000-square-foot house has 9-foot ceilings, a west-facing wall of glass, R-19 attic insulation, and leaky ducts baking in an attic, while another has 8-foot ceilings, small north-facing windows, R-49 above, and ducts inside conditioned space. Same floor area, very different loads.
Use the table below to judge whether a proposed size is plausible. Use a load calculation to know whether it is right.
Square Footage to Tons and BTU: A Starting Table
These ranges assume 8-foot ceilings, average window area, and insulation typical for the era the house was built. Climate zones follow the standard US energy code map: zones 1 to 3 cover the Gulf Coast, Florida, Texas, and the Southwest, zones 4 and 5 cover the mid-Atlantic and Midwest, and zones 6 to 8 cover the northern tier and mountain states.
| Conditioned Square Feet | Starting BTU Range | Hot South (Zones 1-3) | Mixed (Zones 4-5) | Northern (Zones 6-8) |
|---|---|---|---|---|
| 600 to 900 | 12,000 - 24,000 | 1.5 - 2 tons | 1.5 tons | 1 - 1.5 tons |
| 900 to 1,200 | 18,000 - 30,000 | 2 - 2.5 tons | 1.5 - 2 tons | 1.5 - 2 tons |
| 1,200 to 1,500 | 24,000 - 36,000 | 2.5 - 3 tons | 2 - 2.5 tons | 2 tons |
| 1,500 to 1,800 | 30,000 - 42,000 | 3 - 3.5 tons | 2.5 - 3 tons | 2.5 tons |
| 1,800 to 2,100 | 30,000 - 48,000 | 3.5 - 4 tons | 3 - 3.5 tons | 2.5 - 3 tons |
| 2,100 to 2,400 | 36,000 - 54,000 | 4 - 4.5 tons | 3.5 - 4 tons | 3 - 3.5 tons |
| 2,400 to 3,000 | 42,000 - 72,000 | 5 tons or two systems | 4 - 5 tons | 3.5 - 4.5 tons |
| 3,000 to 3,600 | 54,000 - 84,000 | Two systems | 5 tons or two systems | 4.5 - 5 tons |
Count only conditioned square footage. An unconditioned garage, an unfinished attic, and a crawlspace stay out of the number; a finished basement or converted attic goes in. Anything above roughly 5 tons usually means two separate systems, since residential equipment tops out at 5 tons and a two-system layout handles a multi-story house better than one large unit fighting stack effect.
To find what you already have, read the model number on the outdoor condenser's data plate, where most brands encode nominal capacity in hundreds of BTU: 036 means 3 tons. Treat it as context, not an answer, since plenty of older systems were oversized on installation day.
What a Manual J Load Calculation Actually Accounts For
Manual J is the residential load calculation standard from the Air Conditioning Contractors of America. Most model codes require equipment to be sized from a Manual J load and selected under Manual S, its companion equipment-selection standard, with Manual D covering duct design. The three run in sequence.
What separates a real Manual J from a square-footage guess is the input list:
- Local design temperatures. Not record highs, but the 1% summer design dry bulb and 99% winter design temperature for your location. Sizing to an all-time record guarantees an oversized system for the hours that actually matter.
- Orientation and window area per wall. Each window's size, direction, glazing type, and shading gets logged. West-facing glass carries far more afternoon solar gain than the same area facing north.
- Insulation R-values by assembly. Attic, walls, floors, and slab edge entered separately, not averaged.
- Infiltration and air sealing. Estimated from construction era, or measured with a blower door test.
- Ceiling heights and volume. Cathedral ceilings and two-story great rooms change the load, not just the floor area.
- Duct location, insulation, and leakage. Ducts in a hot attic add load. Ducts inside conditioned space add almost none.
- Occupancy and internal gains. People, lighting, cooking, and equipment each contribute measurable heat.
- Sensible and latent load, separately. Sensible is the temperature load, latent is the moisture load. Humid climates carry a much larger latent share, which is why oversizing hurts more in Houston than in Denver.
The output is a report showing whole-house sensible and latent load in BTU per hour, a room-by-room breakdown, and the airflow in CFM each room needs. That last column is what a duct designer works from. A size handed to you with no document behind it is not a calculation, it is a habit.
Why Oversizing Causes Short Cycling and a Clammy House
An oversized air conditioner cools the air fast and stops. That sounds efficient and is the opposite. Dehumidification only happens while the indoor coil is cold and air keeps moving across it, and a coil needs several minutes of continuous runtime to get cold enough to condense meaningful moisture. A unit that satisfies the thermostat in six or eight minutes never gets there.
The result is a house that reads 72 degrees on the thermostat and still feels damp. Indoor humidity drifts past 55 or 60 percent, which is where musty smells, condensation on registers, and mold complaints start. Compressor startup also draws the highest current in the cycle, so a unit stopping and starting several times an hour ages fast, showing up as early compressor failure and burned contactors.
Undersizing has its own failure mode: the system runs continuously and still loses a few degrees on design days. Uncomfortable, but it dehumidifies well and it is more forgiving than the oversized case. Given two available tonnages with the calculated load between them, most experienced installers round down in humid climates and consider rounding up only in hot, dry ones. Variable-speed and two-stage equipment softens the tradeoff because it can run at partial capacity for long stretches, though it does not excuse skipping the calculation.
Climate Zone, Insulation, Window Area, and Ceiling Height
Of everything on that list, four factors move the tonnage most.
Climate zone sets the base. The design temperature difference between indoors and out drives the whole calculation, so the same house needs meaningfully more capacity in Phoenix than in Portland. Humidity counts separately: Gulf Coast homes carry a heavy latent load a dry-climate home of identical size does not.
Insulation and air sealing separate a 1960s house with R-11 walls and single-pane windows from a code-built house with a sealed attic. The gap is wide enough that adding attic insulation before replacing the system sometimes drops you a half ton, which is why envelope work first is the cheaper order of operations.
Window area and orientation are the biggest single gain in most homes. Solar heat through glass dwarfs conduction through an insulated wall, and a large west or southwest exposure can add the better part of a ton by itself.
Ceiling height matters because you condition volume, not floor area. A house with 10-foot ceilings throughout holds about 25 percent more air than the floor plan suggests, and load calculation software works from that volume rather than the square footage on the listing. Cathedral ceilings compound it, since the hottest air stacks up where nobody is sitting.
Ductwork and Electrical Limits That Cap Your Size
Two physical constraints can override the calculated tonnage, and both tend to surface late.
Ducts have a fixed airflow capacity. A central system needs roughly 400 CFM per ton, a bit less in humid climates where slower air across the coil helps dehumidification, a bit more in dry ones. A duct system built for 2.5 tons cannot move 1,600 CFM for a 4-ton unit no matter what you connect to it. Push it anyway and you get high static pressure, weak airflow at the far registers, a coil that freezes over, and a compressor working against conditions it was never rated for. Most residential air handlers are rated near 0.5 inches of water column of total external static pressure, and undersized ducts blow past that easily. If the calculation calls for more system than the ducts support, the honest options are upgrading trunks and returns or looking at ductless mini split systems for the rooms the ducts serve worst.
Electrical service has to match. The outdoor unit's data plate lists a minimum circuit ampacity and a maximum overcurrent protection rating, and those set the required wire gauge and breaker size. Stepping from 2.5 tons to 4 tons can push required ampacity past what the existing 240-volt circuit and its conductors are rated for, meaning new wire from the panel, a larger breaker, and possibly a panel with room to spare. All of it belongs on the written quote before work starts, and the central AC installation cost breakdown covers how those upgrades typically appear on a bid.
Sizing a Furnace or Heat Pump Is a Different Calculation
If you are replacing a full system, AC tonnage does not set the furnace size. Manual J produces two separate numbers, a cooling load and a heating load, and they are rarely proportional.
Furnaces are sized on the heating load and rated two ways: BTU input and BTU output. Output is what heats the house, and it equals input multiplied by the AFUE rating, so an 80,000 BTU input furnace at 95 percent AFUE delivers about 76,000 BTU of usable heat. Starting ranges run from roughly 30 BTU of input per square foot in mild southern climates to 60 or more in the far north, but the heat loss calculation governs. Oversized furnaces short-cycle for the same reasons oversized air conditioners do.
Heat pumps are sized on the cooling load first across most of the country, since a unit sized to the heating load would be badly oversized for summer. Below the balance point, the outdoor temperature where output stops matching heat loss, supplemental heat covers the gap. Cold-climate models hold capacity at lower temperatures and get sized closer to the heating load, with the cooling side checked so summer performance still works. The types of air conditioners compared guide covers which system suits which house before you get to sizing.
Sizing One Room: Window Units, Portables, and a Single Mini Split Head
If you are cooling one room rather than a house, tonnage is the wrong unit and the whole-house table above does not apply. Room equipment is sold in BTU per hour, and the long-standing federal guidance for room air conditioners starts at about 20 BTU per square foot of floor area.
| Room Square Feet | Starting Capacity |
|---|---|
| 100 to 150 | 5,000 BTU |
| 150 to 250 | 6,000 BTU |
| 250 to 350 | 7,000 to 8,000 BTU |
| 350 to 450 | 9,000 to 10,000 BTU |
| 450 to 550 | 12,000 BTU |
| 550 to 700 | 14,000 BTU |
| 700 to 1,000 | 18,000 BTU |
Then adjust for how the room actually behaves:
- Very sunny room: add about 10 percent.
- Heavily shaded room: subtract about 10 percent.
- More than two people in the room regularly: add about 600 BTU per additional person.
- Kitchen: add about 4,000 BTU for the range and refrigerator.
Two cautions. A portable delivers noticeably less cooling than a window unit of the same rating, since its hose and cabinet both sit inside the room, so size up a step. And a mini split head needs a per-zone calculation that balances every head against the outdoor unit's capacity.
How to Ask a Contractor for a Written Load Calculation
You do not need to run the math yourself. You need to confirm someone did. Use these questions, in this order, on every bid.
- Will you run a Manual J load calculation before quoting equipment, and can I have a copy of the report? The correct answer is yes to both. A calculation you cannot see is a calculation you cannot check.
- Are you entering my actual window and insulation values, or software defaults? A tech who spends 45 minutes walking the house, counting windows, checking attic insulation depth, and measuring rooms is doing it properly.
- What design temperatures did you use for my area? They should cite local design conditions, not a record high.
- What are the sensible and latent loads separately? In a humid climate especially, a contractor who cannot split the two has not opened the report.
- How does the selected equipment compare to the calculated load? Equipment-selection guidance keeps total cooling capacity close to the load, commonly within about 15 percent above it in humid climates and with more latitude in dry ones. A proposal 40 percent above the load is a red flag.
- Can my existing ducts carry the airflow this size needs? The answer should reference static pressure or CFM, not a shrug.
- What electrical changes does this size require? Wire, breaker, and disconnect changes belong on the written quote.
Answers that should send you to another bidder: "same size as what you have now" with no other reasoning, "we always put 3 tons in houses like this," a refusal to share the report, or a calculation produced in five minutes from square footage typed into a phone. A size quoted over the phone without a site visit is not sizing at all.
Get at least two proposals and compare the tonnage each lands on. Contractors who actually run the numbers usually finish within half a ton of each other. A full ton of disagreement means one of them guessed, so ask the higher bidder to explain the difference.
Frequently Asked Questions
How many square feet will a 3 ton AC unit cool?
Roughly 1,500 to 2,100 square feet, depending on climate and construction. In a hot, humid climate with average insulation, 3 tons covers closer to 1,500 square feet. In a northern climate with tight construction and modern windows, the same 3 tons handles 2,000 or more. That spread is the point: square footage alone cannot give you the answer.
Is it better to oversize or undersize an AC unit?
Neither, though oversizing causes more day-to-day discomfort. An oversized unit satisfies the thermostat in short bursts, never runs long enough to pull humidity out of the air, and leaves the house cold and clammy while wearing the compressor out faster. An undersized unit dehumidifies well but loses ground on the hottest afternoons. A correct load calculation avoids both.
How do I know what size AC unit I already have?
Read the model number on the data plate of the outdoor condenser. Most major brands encode nominal capacity in hundreds of BTU inside that string: 018 is 1.5 tons, 024 is 2 tons, 030 is 2.5, 036 is 3, 042 is 3.5, 048 is 4, and 060 is 5 tons. The plate also lists cooling capacity in BTU per hour, and 12,000 BTU per hour equals one ton.
How many BTU do I need per square foot?
Starting points run from about 20 BTU per square foot in cool northern climates to about 30 in hot, humid southern ones, the same as saying one ton per 400 to 600 square feet. Use it as a range check on a contractor's number, not a specification. Ceiling height, window area, insulation, and duct leakage push the real figure outside that band regularly.
Does a Manual J load calculation cost extra?
Most contractors include it in a replacement bid at no separate charge, since they need the number to select equipment anyway. A standalone report from a third-party energy rater or mechanical engineer is billed as a flat fee that scales with home size. Charging for the calculation and crediting it against the installation is also normal.
Can I just replace my old AC with the same tonnage?
Only if the old one was sized correctly and the house has not changed. Plenty of systems installed decades ago were oversized by a ton or more, and matching that mistake locks it in for another 15 years. If you have added insulation, replaced windows, air-sealed the attic, or finished a basement since, the load is different now.
Once you know the right size, the full HVAC installation cost guide explains what drives the rest of the quote. When you are ready to move, get quotes from licensed local pros for professional AC installation services, and make the written load calculation a condition of the bid.