AC Size Calculator
An oversized AC short-cycles and leaves humidity behind; an undersized one never catches up on the hottest day. This AC size calculator applies the classic tons-per-square-foot rule of thumb for your climate zone, corrects for ceiling height, and rounds to the nearest half ton — the way residential equipment is actually sold. For a room-by-room load figure, cross-check the result with the BTU Calculator.
Conditioned space only — exclude garages and unheated areas.
Rule-of-thumb square feet cooled per ton of capacity.
Adjusted against an 8 ft baseline.
Recommended AC size
2tons
- Calculated load (exact)
- 2tons
- Cooling capacity
- 24,000BTU/hr
- Sizing range (low)When between sizes, compare both with a contractor.
- 1.5tons
- Sizing range (high)
- 2.5tons
How it was calculated
- Raw load: 1000 ft² ÷ 500 × (8 ÷ 8) = 2.00 tons
- Rounded to equipment steps: 2.0 tons = 24000 BTU/hr
- Discussion range: 1.5–2.5 tons
How it works
Tonnage starts as area ÷ a climate divisor — a rule of thumb for square feet per ton: 600 in mild climates, 500 in moderate, 400 in hot. A 2,000 ft² home in a hot climate starts at 5 tons before any adjustment.
Ceiling height adjusts against an 8-foot baseline: tons × (height ÷ 8). Taller rooms hold more air, so a 10-foot ceiling adds 25% to the load.
The result rounds to the nearest 0.5 ton because residential condensers are sold in half-ton steps (1.5, 2, 2.5, 3 tons and up) — a 2.4-ton load becomes a 2.5-ton unit.
BTU/hr = tons × 12,000, and the suggested range is ±0.5 ton around the rounded size. When your load lands between sizes, the range shows both candidates to discuss with a contractor.
Worked examples
Suburban home, moderate climate
A 1,200 ft² home with 8 ft ceilings in a moderate climate: 1,200 ÷ 500 = 2.4 tons, which rounds to 2.5 tons (30,000 BTU/hr). The 2.0–3.0 ton range gives the contractor two sizes to evaluate against ductwork and windows.
Hot climate with high ceilings
A 2,000 ft² home with 10 ft ceilings in a hot climate: 2,000 ÷ 400 × 1.25 = 6.25 tons, rounding to 6.5 tons (78,000 BTU/hr) with a 6.0–7.0 ton range. At this size the answer is usually two smaller systems, not one giant unit.
How to measure
- Use cooled floor area only — skip garages, attics, and unconditioned porches.
- Add up each room separately; L-shaped homes are easier measured as two rectangles.
- Measure ceiling height in the tallest common area; for mixed heights use the average.
- Pick "hot" for desert Southwest and Gulf Coast climates, "mild" for coastal Pacific Northwest and similar.
- Bring both the rounded size and the range to every contractor quote for comparison.
Assumptions & limitations
Planning estimate
- Climate divisors (600/500/400 ft² per ton) are a rule of thumb, not a substitute for measured loads.
- planning estimate only — Manual J load calculation by a professional is required for equipment selection; this tool does not replace it.
- Duct condition, window area, insulation, and shading are not modeled; they can shift real sizing by half a ton or more.
- Oversizing is not harmless: short cycling raises indoor humidity and wears equipment faster.
- Very large results (5+ tons) usually mean multiple systems rather than one oversized unit.
Frequently asked questions
One ton equals 12,000 BTU/hr of cooling — the historical rate at which a ton of ice melts over 24 hours. A "3-ton" central AC therefore removes about 36,000 BTU of heat per hour. It has nothing to do with the weight of the equipment.
Because that is how residential equipment is manufactured and sold: 1.5, 2, 2.5, 3, 3.5, 4, and 5-ton units. There is no 2.4-ton condenser, so the calculator rounds your calculated load to the nearest size you can actually buy.
Usually not. An oversized unit satisfies the thermostat quickly and shuts off before running long enough to wring moisture from the air — leaving the house cold but clammy, with more wear from frequent starts. Slight undersizing with good insulation often feels better.
Yes, modestly. Against the 8-foot baseline, 9-foot ceilings multiply the load by 9 ÷ 8 = 1.125, about a 12% increase. That is rarely a full half-ton on its own, but combined with a hot climate or poor insulation it can tip the rounding.
This tool answers "what size central AC" in tons from whole-home area and climate — a quick equipment-sizing shortcut. The BTU Calculator estimates the actual heating/cooling load in BTU/hr for a specific room, accounting for insulation and sun exposure. Use both: tons for shopping, BTU/hr for precision.