HVAC Calculators · Calculator
AC Tonnage Calculator: What Size Air Conditioner Do You Need?
Most homes land between 1.5 and 5 tons. Start near 20 BTU/h per square foot, then adjust for climate, insulation and ducts, and treat windows separately: their area, glazing type and orientation decide the solar share of the load. Add people and kitchen heat, divide by 12,000 BTU/h per ton, and round to the nearest available size rather than always rounding up.
AC tonnage calculator
How the calculator works
A ton of air conditioning is the cooling capacity needed to melt one short ton of ice in 24 hours, which works out to 12,000 BTU per hour. The calculator estimates your peak cooling load in BTU/h and divides by 12,000.
Base = Area × 20 × (Ceiling ÷ 8)
Glass share = 0.25 × (Window % ÷ 15) × Window type × Orientation
Load = Base × Climate × Ducts × (0.75 × Insulation + Glass share) + (Occupants − 2) × 600 + Kitchen
Tons = Load ÷ 12,000
The 20 BTU/h per square foot starting point is the common rule of thumb for an average US home with 8-foot ceilings and clear double-pane windows covering about 15% of the floor area. In that reference house roughly a quarter of the summer heat gain arrives through glass and three-quarters through walls, roof and air leakage, so the calculator splits the base load the same way. The insulation choice scales only the opaque 75%. The glass 25% grows in proportion to window area and is scaled by window type (single pane 1.25, clear double 1.0, double low-E 0.75, triple low-E 0.65) and by orientation and shading. Because orientation acts only on the glass share, a sunny exposure is not counted twice.
Each person beyond the first two adds about 600 BTU/h of sensible and latent heat. Cooking heat is added as a fixed amount: about 1,200 BTU/h for a kitchen open to the living area, 800 for one closed off by walls and 300 for a kitchenette that sees little use.
Worked example
The calculator's default house: 2,000 sq ft, mixed climate, average insulation, 8-foot ceilings, clear double-pane windows equal to 15% of the floor area with an average mix of orientations, four occupants, an open-plan kitchen and ducts inside the conditioned space.
- Base = 2,000 × 20 × (8 ÷ 8) = 40,000 BTU/h
- Glass share = 0.25 × (15 ÷ 15) × 1.0 × 1.0 = 0.25; opaque share = 0.75 × 1.0 = 0.75
- Envelope load = 40,000 × 1.0 × 1.0 × (0.75 + 0.25) = 40,000 BTU/h
- Occupants: (4 − 2) × 600 = 1,200 BTU/h
- Open kitchen: + 1,200 BTU/h
- Load = 42,400 BTU/h, or 3.53 tons (21.2 BTU/h per sq ft)
Rounded to the nearest half ton, that is a 3.5-ton system. Now give the same house single-pane windows covering 20% of the floor, mostly facing west with no shade: the glass share becomes 0.25 × (20 ÷ 15) × 1.25 × 1.4 = 0.58, the load rises to about 55,700 BTU/h (4.64 tons) and the calculator jumps to 5 tons. Replace that glass with double-pane low-E and the share drops to 0.35, the load to 46,400 BTU/h (3.87 tons) and the size to 4 tons.
Quick sizing chart by square footage
For a mixed climate with average insulation, 8-foot ceilings and ordinary windows (clear double pane, about 15% of floor area), before occupant and kitchen adders:
| Floor area | Typical load | Common size |
|---|---|---|
| 600–1,000 sq ft | 12,000–20,000 BTU/h | 1.5 tons |
| 1,000–1,400 sq ft | 20,000–28,000 BTU/h | 2 tons |
| 1,400–1,700 sq ft | 28,000–34,000 BTU/h | 2.5 tons |
| 1,700–2,100 sq ft | 34,000–42,000 BTU/h | 3 tons |
| 2,100–2,400 sq ft | 42,000–48,000 BTU/h | 3.5–4 tons |
| 2,400–3,000 sq ft | 48,000–60,000 BTU/h | 4–5 tons |
Residential split systems top out at 5 tons. Larger homes usually get two systems or a zoned variable-speed unit.
Why oversizing is a real problem
A unit that is too big cools the air fast, satisfies the thermostat and shuts off before it has run long enough to pull moisture out of the air. The result is a cold, clammy house, more on/off cycles, extra wear on the compressor and higher bills. Manual S, the ACCA equipment-selection standard, generally allows cooling capacity of up to 115% of the calculated load for standard single-stage equipment. Going much past that gives you short cycling and poor humidity removal.
If you're between sizes, a two-stage or variable-speed system is a better buy than simply going up a size. It runs at low capacity most of the time and still has headroom for the hottest afternoons.
What changes the answer the most
- Climate: design temperatures in Phoenix and Minneapolis are more than 20 °F apart, and humid climates add a large latent (moisture) load.
- Insulation and air leakage: an older leaky house can need 25–40% more capacity than a tight new one of the same size. See the R-value chart.
- Windows: glass gains far more heat per square foot than an insulated wall. Unshaded west-facing glass is the biggest single heat source on a summer afternoon, and the glazing matters as much as the area: a spectrally selective low-E coating blocks much of the sun's infrared while still passing visible light.
- Ducts in the attic: attics reach 130–150 °F, and leaky or poorly insulated ducts there can lose 15–30% of the cooling before it reaches a room.
- Kitchen: a range, oven and dishwasher running at dinner time add heat right when the outdoor load is still high, most noticeably in open-plan layouts.
When you need a full Manual J
Use this tool to sanity-check a quote. Before buying, ask the contractor for a room-by-room Manual J load calculation. Most building codes require one for new installations. It's also how duct sizes (Manual D) are set. If two quotes differ by more than a ton for the same house, ask each contractor to show their load numbers.
Frequently asked questions
How many square feet does a 3-ton AC cool?
In a mixed climate with average insulation, about 1,500–2,100 square feet. In a hot, sunny climate with older insulation the range drops to roughly 1,200–1,600 square feet.
Is it better to round up to the next size?
Not automatically. Round to the nearest size. If the load is clearly between sizes, choose a two-stage or variable-speed unit rather than oversizing a single-stage one.
How many BTU is one ton?
12,000 BTU per hour. A 2.5-ton unit is 30,000 BTU/h and a 4-ton unit is 48,000 BTU/h.
Do better windows let me buy a smaller air conditioner?
Often, if the house has a lot of glass. In the worked example, swapping single-pane for double-pane low-E windows on a sunny west side cuts the estimate from about 4.6 to 3.9 tons. Exterior shading such as awnings or trees has a similar effect on the window share of the load.
Last reviewed 2026-10-28