Calculator
Square footage is the starting point, not the answer. Sun, insulation, ceiling height and climate move the number a long way.
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| Base load at 20 BTU per sq ft | 30,000 BTU |
| Ceiling height factor | × 1.00 |
| Sun exposure factor | × 1.00 |
| Insulation factor | × 1.00 |
| Climate factor | × 1.00 |
| People and kitchen | + 4,600 BTU |
Use this to sanity check a contractor's proposal, not to order equipment. A proposal much above this figure is worth a question, because an oversized system short cycles and does not dehumidify. The calculation equipment is actually sized on is a Manual J.
Now get it sized properly. A local crew runs the Manual J and quotes against it. Send them this figure and see whether the proposal matches.
Get quotes for this size →The instinct with an air conditioner is that oversizing buys you headroom. It does the opposite, and understanding why is the single most useful thing on this page.
An oversized system cools the air to the thermostat setpoint very quickly and then shuts off. That sounds efficient. The problem is that removing humidity takes time, not power. Moisture condenses on the evaporator coil only while air keeps moving across it, so a system that satisfies the thermostat in eight minutes and stops has not dehumidified the house. You end up with a room that is cold and clammy, and you push the thermostat lower to compensate.
The short run cycles also cost you. Compressors draw their heaviest current at startup, so a system that starts and stops all day uses more electricity and wears out sooner than one that runs long steady cycles. Short cycling is one of the more common reasons a system fails well before the bottom of its expected life.
Undersizing has one honest failure mode, which is that the house never quite gets cool on the very hottest afternoons. Given the choice, that is the cheaper mistake.
Almost every sizing chart online is built on the same national assumption, and it is wrong for a lot of the country by a wide margin. Twenty BTU per square foot is a reasonable starting point for a mixed climate. It is too much for a mild coastal one and not enough for a hot inland one, and the gap between those two is roughly a third of the total load.
This matters concretely in our own service area. Homes in Alameda, San Leandro and along the shoreline sit in a marine climate where summer afternoons in the high seventies are a warm day. The national chart routinely puts those homes a full ton above what they need. Go five or ten miles inland towards Livermore or Walnut Creek and the same square footage genuinely needs more capacity.
If a contractor sizes your system from square footage alone, without asking where you are or looking at your windows, that is worth pushing back on.
It is a rule of thumb, and the honest way to use it is as a sanity check. The calculation the industry actually sizes equipment on is a Manual J load calculation, published by ACCA. It works from your specific wall construction, insulation R-values, window orientation and glazing, air infiltration rate, duct losses and local design temperatures, and it produces a room by room answer rather than one number for the house.
A good contractor performs one before quoting a replacement, and many jurisdictions require it for a permit. If yours does not offer one, ask. The reasonable use of this page is to arrive at that conversation knowing roughly what to expect, so that a proposal a ton and a half above this estimate prompts a question rather than a signature.
One further caveat for ducted systems: your existing ductwork has a capacity of its own. Fitting a larger condenser to ducts that cannot carry the airflow gives you the cost of the bigger unit and none of the benefit.
A ton of cooling is 12,000 BTU per hour. The unit is a genuine historical artefact: it is the rate of cooling you get from a ton of ice melting over 24 hours, and it stuck.
Residential equipment is sold in half ton steps, typically from 1.5 to 5 tons, which is why this calculator rounds. You can read the size off the model number on the outdoor unit, where most manufacturers encode the nominal BTU in hundreds. A number containing 024 is 24,000 BTU, or 2 tons, and 036 is 3 tons. It is the quickest way to find out what you currently have before anyone quotes you a replacement.
Every row below is this calculator's own output, so you can see how the inputs move the answer.
| Home | Climate | BTU | Tons |
|---|---|---|---|
| 1,000 sq ft | Mixed | 24,000 | 2.0 |
| 1,500 sq ft | Mixed | 34,500 | 3.0 |
| 2,000 sq ft | Mixed | 45,000 | 4.0 |
| 1,500 sq ft | Mild coastal | 30,000 | 2.5 |
| 1,500 sq ft, sunny and poorly insulated | Hot | 48,000 | 4.0 |
FAQ
As a starting point, about 30,000 BTU, or 2.5 tons, using the standard 20 BTU per square foot. That figure moves substantially with conditions. The same house in a mild coastal climate with good insulation lands closer to 2 tons, while a sunny, poorly insulated one in a hot inland climate can need 3.5 tons or more.
Twenty BTU per square foot is the usual rule of thumb for a mixed climate. Adjust down towards 17 for a mild coastal climate or a well sealed modern home, and up towards 23 or more for hot climates, heavy sun exposure, or older homes with single pane windows.
Yes, and it is a more expensive mistake than undersizing. An oversized system reaches the thermostat setpoint before it has run long enough to pull humidity out of the air, which leaves the house cold and damp. The resulting short cycles also increase electricity use and wear the compressor faster, because startup is the hardest moment in a compressor's duty cycle.
No. A Manual J is the ACCA load calculation contractors size real equipment on, and it accounts for your wall construction, insulation values, window orientation, air infiltration, duct losses and local design temperatures, room by room. This page is a rule of thumb for sanity checking a quote. Ask your contractor for a Manual J before a replacement, since many jurisdictions require one for the permit anyway.
Look at the model number on the data plate of the outdoor unit. Most manufacturers encode the nominal capacity in hundreds of BTU, so a model number containing 024 is 24,000 BTU or 2 tons, and one containing 036 is 3 tons. That tells you what was fitted, though not whether it was sized correctly to begin with.
Yes, because you are cooling a volume of air rather than a floor area. Nine foot ceilings add roughly ten percent to the load and vaulted or ten foot ceilings around twenty percent. Square footage charts assume eight foot ceilings and quietly undersize homes that do not have them.
Related guide:
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