What Size AC Do I Need? (BTU & Tons)
Enter your conditioned area and climate band to get a rule-of-thumb cooling load in BTU/h, the nearest half-ton size, and the airflow it implies. This is a starting estimate — a professional Manual J is the real answer before you buy.
Calculator
A 1,500 sq ft home in this climate band needs about ≈ 38,100 BTU ≈ 3.0 tons of cooling — roughly 1,200 CFM of airflow. This is a starting estimate; confirm with a Manual J.
Cooling load is roughly proportional to the floor area you are conditioning, but two homes of the same size in different climates need very different capacity. This calculator uses the classic area × climate-band rule of thumb and then layers on the adjustments that matter most for a house: heavy sun exposure, extra occupants, a kitchen inside the zone, and tall ceilings. The result is expressed three ways — total BTU/h, nominal tons (1 ton = 12,000 BTU/h), and target airflow at the standard 400 CFM/ton — so you can sanity-check a contractor’s proposal.
Rules of thumb are deliberately conservative and generic. They do not know your window area, insulation, air-sealing, duct losses or local design temperature. Use this number to get in the right ballpark and to spot a wildly oversized quote — not as the final specification.
Formula
The cooling load is built up in steps:
cooling_BTU = area × climate_band\n × sun_factor (1.0 average, 1.10 heavy sun)\n × ceiling / 8 (only when ceiling > 8 ft)\n + 600 × max(0, occupants − 2)\n + kitchen_BTU (0 or 4,000)\ntons = round(cooling_BTU / 12,000 to the nearest 0.5)\nairflow_CFM = tons × 400
The climate band (about 20–35 BTU per sq ft) is a labeled IECC-zone typical, not a measured value. Basis: 1 ton = 12,000 BTU/h; ~400 CFM/ton airflow. See Sources.
Worked example
A 1,500 sq ft home in a moderate climate (25 BTU/sq ft), average sun, three occupants, no kitchen load and an 8 ft ceiling:
base = 1,500 × 25 = 37,500 BTU/h\nsun × 1.0 = 37,500\nceiling (8 ft, no change) = 37,500\npeople + 600 × (3 − 2) = 38,100 BTU/h\ntons = 38,100 / 12,000 ≈ 3.2 → 3.0 tons\nairflow = 3.0 × 400 = 1,200 CFM
So a 1,500 sq ft house lands near 3 tons and about 1,200 CFM of airflow. Push the climate band to “hot” (35 BTU/sq ft) and the same house jumps past 4 tons — which is exactly why the climate input matters more than any single adjustment.
Right-sizing in practice
Why bigger is not better. An oversized AC cools the air fast but short-cycles: it shuts off before it has run long enough to wring humidity out of the air, so the house feels cool and clammy. It also wastes energy and wears the compressor. Right-sizing (or even slightly under-sizing to a Manual J) gives longer, gentler runtimes and better dehumidification.
What this tool ignores. Window count and orientation, insulation R-values, infiltration, duct location and leakage, and your local 1% design temperature all move the real load. A professional Manual J models them room by room; a Manual S then matches equipment to that load. Treat the number here as a planning estimate only.
Reference table
| Climate band | BTU/sq ft | Load for 1,500 sq ft | ≈ Tons |
|---|---|---|---|
| Cool / northern (~20 BTU/sq ft) | 20 | 30,000 BTU/h | 2.5 |
| Moderate / mixed (~25 BTU/sq ft) | 25 | 37,500 BTU/h | 3.0 |
| Warm / southern (~30 BTU/sq ft) | 30 | 45,000 BTU/h | 4.0 |
| Hot / desert (~35 BTU/sq ft) | 35 | 52,500 BTU/h | 4.5 |
Bands are labeled IECC-zone typicals (rule of thumb), before sun, occupancy, kitchen and ceiling adjustments.
Frequently asked questions
What size AC do I need for 1,500 sq ft?
In a moderate climate a 1,500 sq ft home works out to roughly 37,500 BTU/h ≈ 3 tons using the 25 BTU/sq ft rule of thumb. In a hot climate the same house can need 4 tons or more. Always confirm with a Manual J before buying.
How many BTU is one ton of cooling?
One ton of cooling equals 12,000 BTU/h. So 3 tons is 36,000 BTU/h. See the Tons to BTU converter for the full conversion.
Is it better to oversize an air conditioner to be safe?
No. An oversized unit short-cycles, so it never runs long enough to remove humidity. The room feels cool but damp, comfort drops and the compressor wears faster. Match the load, do not pad it.
Does ceiling height change the AC size?
Yes. Cooling load tracks the volume of air, so a ceiling above the 8 ft reference scales the load up proportionally (a 10 ft ceiling adds about 25%). Enter your real ceiling height for a closer estimate.
Can I use this instead of a Manual J load calculation?
No. This is a rule-of-thumb starting estimate. A Manual J accounts for windows, insulation, infiltration and your design temperature room by room. Use this to ballpark and to catch an oversized quote, then have a pro run the real calc.