AC Tonnage Calculator
Work out how many tons of cooling your house actually needs, from the envelope and the climate rather than from a square-footage chart.
AC tonnage calculator
Where the cooling load comes from
| Component | Cooling BTU/hr | Heating BTU/hr | Basis |
|---|
Same house, different envelope
| Envelope | Cooling tons | Size to buy | Square feet per ton |
|---|
Every row uses the same area, ceiling, climate, glass and ducts you entered. Only the insulation and air-tightness change.
What a ton actually measures
One ton of cooling is 12,000 BTU per hour, and the number comes from ice. A ton of ice absorbs 12,000 BTU over 24 hours as it melts, so a "3-ton" air conditioner removes heat at the same rate as three tons of ice melting every day. It says nothing about the weight of the equipment, the size of the cabinet, or how much electricity it draws — a 3-ton unit built to a 14.3 SEER2 standard and a 3-ton unit built to 20 SEER2 move the same amount of heat and differ entirely in what they cost to run.
Nameplate capacity is also a rating, not a promise. AHRI 210/240 rates equipment at 95°F outdoor air and 80°F dry bulb / 67°F wet bulb indoors. Capacity falls as the outdoor temperature climbs past that point, by roughly 5 to 10 percent at 105°F for a standard single-stage condenser. A load calculation runs at your design temperature for exactly this reason: the question is not what the unit can do at the rating point, it is what your house needs at 5 p.m. on the hottest afternoon of the decade.
Why every square-footage chart oversizes
The familiar rule — one ton per 500 square feet, or 20 BTU per square foot — dates from an era of single-pane windows, uninsulated walls and no house wrap. Applied to a house built after 2000 it typically specifies about double the equipment the house needs. ACCA's own guidance points the other way: homes sized properly with Manuals J, S and D usually land somewhere above 800 square feet per ton.
Run this calculator on a 2,000 square foot two-story in climate zone 4 with average insulation and it returns 2.45 tons, which the trade would round to 2.5. That is 800 square feet per ton. The chart would have sold you 4 tons, and the extra 1.5 tons would never run long enough to be useful.
Oversizing is not a harmless safety margin. A unit that is 50 percent too large pulls the house down to the thermostat setpoint in a few minutes and shuts off, and the coil never stays cold long enough to condense water out of the air. The result is a house that feels cold and clammy at 72°F, the exact complaint that leads people to lower the thermostat further and spend more. Short cycling also wears the compressor and the contactor, and it makes room-to-room temperature swings worse, not better, because the blower only runs in bursts.
How this calculator builds the number
The model is a single-zone block load, the same structure Manual J uses before it splits a house into rooms. Cooling is the sum of seven terms, and each one is visible in the table above so you can see which input is driving the answer:
- Conduction through walls, ceiling and windows — surface area divided by R-value, multiplied by the difference between outdoor design temperature and 75°F indoors. Studs short-circuit cavity insulation, so cavity R-values are multiplied by 0.85 before use.
- Infiltration — air changes per hour times house volume times 0.018 BTU per cubic foot per degree. An average existing house leaks about 0.8 air changes per hour; a spray-foam house is nearer 0.3.
- People — 230 BTU per hour sensible each, plus a latent share that rises with humidity.
- Appliances and lighting — a fixed allowance, larger when a kitchen vents into the conditioned space.
- Solar through glass — glass area times shading coefficient times a peak gain factor that depends on orientation. West and south glass in full sun is the single largest swing factor on most houses.
- Latent load — the moisture the system must remove, driven by climate zone, occupants and infiltration.
- Duct gain — 18 percent for ducts in a vented attic, zero when they run inside the envelope. This is the term most online calculators leave out, and it is often the reason a contractor's number comes in higher than a web estimate.
Reading the square feet per ton figure
This is the number worth comparing against the chart, because it is the one that tells you whether your inputs are believable:
- Under 450 — either the house is genuinely leaky with a lot of unshaded glass, or you have stacked several worst cases at once. Check the envelope answer first.
- 500 to 700 — typical of pre-1980 housing stock. The old rule of thumb was not wrong, it was just describing these houses.
- 800 to 900 — code-built housing from the 1990s onward. This is where ACCA's guidance puts a properly sized system.
- 1,000 and above — a tight, well-glazed envelope. Expect a contractor quoting from square footage to propose something considerably larger, and ask for the load calculation in writing.
House shape matters as much as area. A two-story house has roughly 30 percent less exterior wall than a ranch of the same floor area, so it sizes smaller. That is why the stories input changes the answer without touching the square footage.
When an estimate is not enough
The International Residential Code, section M1401.3, requires a load calculation before equipment is selected, and most permitting authorities accept only a room-by-room Manual J with a Manual S equipment match and a Manual D duct design. This page is a block load: one zone, whole house, no room-by-room distribution. It is the right tool for checking a quote, deciding between 2.5 and 3 tons, or working out whether a proposed system is in the right neighbourhood. It is not the document to submit for a permit, and no honest calculator claims otherwise.
For a straight replacement of like with like, some contractors will size from the existing nameplate. That is only safe if the house has not changed. New windows, added insulation, a sealed crawlspace or a finished attic all lower the load, and inheriting the old tonnage then locks in the oversizing for another fifteen years.
What this estimate cannot see
Duct leakage and design, room-by-room distribution, cathedral ceilings with no attic, crawlspace conditions, shading from a neighbour's wall, and the difference between a west-facing bedroom and a north-facing one. A block load also cannot tell you whether the existing return-air path is adequate, which is a common reason a correctly sized system still runs hot upstairs. Those need eyes on the house.
Frequently asked questions
How many tons do I need for a 2,000 square foot house?
It depends on the envelope and the climate, which is the whole point. In this model a 2,000 square foot two-story in climate zone 4 needs about 2.5 tons with average insulation, 2 tons if it is tightly built, and 4 to 5 tons if it is an uninsulated house with a lot of unshaded glass. Any single number quoted for all 2,000 square foot houses is a guess dressed up as an answer.
Is 4 tons too big for 2,000 square feet?
Usually yes. Four tons works out to 500 square feet per ton, which describes pre-1980 housing stock. For a house built to modern code it is roughly 60 percent more capacity than the load requires, and it will short cycle.
What actually happens if my AC is oversized?
The house reaches the setpoint quickly and the compressor shuts off before the coil has run long enough to remove much moisture. You get a cold, clammy house, larger temperature swings between rooms, more compressor starts per hour than the equipment is designed for, and a shorter service life. The energy penalty is smaller than the comfort penalty, which is why the mistake survives so long.
Is tonnage the same as BTU?
One ton equals 12,000 BTU per hour, so they measure the same thing at different scales. A 30,000 BTU per hour load is 2.5 tons. Equipment sold in the US is usually described in tons, while load calculations and window units are usually in BTU.
Can I just match the tonnage of the old unit?
Only if the house has not changed. Windows, insulation, air sealing and a finished attic all reduce the load, and copying the old nameplate locks in whatever oversizing the previous installer chose.
Do I need a Manual J to replace an AC?
The IRC requires a load calculation before equipment selection, and many permit offices ask for it. In practice, ask the contractor for the calculated load in writing and compare it against their proposed tonnage. If the proposal is much larger than the load, ask why.
Does a higher SEER2 unit need a different tonnage?
No. Efficiency and capacity are separate ratings. A 3-ton 14.3 SEER2 unit and a 3-ton 20 SEER2 unit both remove 36,000 BTU per hour at rating conditions; they differ in how much electricity that takes.
Disclaimer: This tool provides estimates for planning purposes only and is not a substitute for a Manual J load calculation or a licensed contractor's sizing. Design temperatures are representative of each IECC climate zone, not of your county.
Related: All HVAC calculators · BTU calculator · Block load calculator · Materials calculators