What size AC do I need?
The honest answer: a rule of thumb gets you a starting BTU number, but only a professional load calculation gets you the right one. Here is how both work.
"What size AC do I need?" is the single most searched HVAC question, and it has two answers. The quick answer is a rule of thumb: multiply your floor area by a climate-zone BTU-per-square-foot band, then adjust for sun, occupancy, ceiling height and kitchen heat. The correct answer is a Manual J load calculation performed by a professional, which models your specific insulation, windows, orientation, ductwork and air leakage. This guide explains the rule of thumb so you can sanity-check a quote, and explains why you should still get the load calc.
The rule-of-thumb formula
Cooling load is dominated by the sensible heat your house gains through walls, roof, windows and infiltration. A first-pass estimate scales that with floor area:
cooling BTU/h = area (sq ft) × climate band (BTU/sq ft) × adjustments
The climate band is the key variable. Cooler northern zones need roughly 18–22 BTU per square foot; mixed climates around 25; warm southern climates 30; and hot desert climates 30–35. These are labeled rule-of-thumb estimates by IECC climate zone, not measured loads. Then apply adjustments: multiply by about 1.10 for a room with heavy afternoon sun, add roughly 600 BTU for each occupant above two, add about 4,000 BTU if the space includes a kitchen, and scale by ceiling height divided by eight if your ceilings are taller than the standard eight feet.
A worked example
Take a 1,500 sq ft home in a mixed climate at 25 BTU/sq ft: 1,500 × 25 = 37,500 BTU/h. Divide by 12,000 BTU/h per ton and you get 3.1 tons, which rounds to a 3.0–3.5 ton system. That system moves roughly 1,200–1,400 CFM of air at the standard 400 CFM per ton. The AC-size calculator runs exactly this math with the adjustments built in, and the tons ↔ BTU ↔ kW converter turns the result into any unit you need.
Why bigger is not better
Homeowners instinctively round up "to be safe," but oversizing is the most common and most damaging mistake in residential HVAC. An oversized air conditioner cools the air to the thermostat setpoint quickly and then shuts off — a pattern called short-cycling. Because it runs in short bursts, it never runs long enough to pull humidity out of the air, so the house feels cold and clammy. Short-cycling also wears out the compressor faster, raises energy use and creates uneven temperatures. A right-sized unit runs longer, gentler cycles that dehumidify properly and hold a steady temperature. We cover this in depth in Right-sizing: bigger is not better.
What the rule of thumb misses
Square footage is a proxy, not a measurement. Two identical-size homes can have loads that differ by 40% because of insulation levels, window area and type, orientation to the sun, air-sealing quality, duct leakage and shading. A Manual J calculation, defined by ACCA, models all of these room by room. A companion Manual S then selects equipment whose capacity matches that load at your design temperature. The rule of thumb cannot see any of it — it is a sanity check, not a specification.
Using the estimate wisely
The best use of the rule of thumb is as a red-flag detector. If your 1,500 sq ft mixed-climate home pencils out near 3 tons and a contractor proposes 5 tons, that is a conversation worth having. Ask whether they ran a Manual J, and ask to see it. A good contractor welcomes the question. Conversely, if a quote is far below your estimate, ask what assumptions drove it — perhaps they know your home has been re-insulated or has high-performance windows. The number is a tool for a better conversation, not a substitute for one.
Finding your climate band
The band is the input that most changes the answer, so it is worth getting roughly right. The United States is divided into IECC climate zones numbered 1 (hot, e.g. south Florida) through 8 (subarctic Alaska); higher numbers mean colder winters and, usually, a smaller cooling band. As a rough guide, hot and humid southern zones use about 30–35 BTU per square foot of cooling, mixed climates around 25, and cool northern zones 18–22. You can look up your zone on the IECC climate-zone map, and pull your design temperatures and degree-days from NOAA or a service like degreedays.net — this site does not store weather data, so you enter the band that matches your location. When in doubt, run the calculator at two adjacent bands to see how much the recommended tonnage moves; if it straddles a half-ton boundary, that is exactly the kind of judgment call a Manual J resolves properly.
Remember that the band is a cooling shortcut, not a heating one. Heating loads use their own, larger BTU-per-square-foot bands (roughly 30–60 depending on climate) because winter temperature differences are bigger than summer ones — which is why the furnace-size tool and the BTU-per-square-foot table keep the two separate.
Once you have a target tonnage, you can move on to the decisions that actually control your bills: efficiency (SEER vs SEER2), running cost (what it costs to run an AC), and whether a heat pump makes more sense than separate heating and cooling equipment.