Formulas
Base load (BTU/h) = floor area (ft²) × load per ft² for the climate
Ceiling factor = 1 + ½ × (height ÷ 8 ft − 1) (only above 8 ft)
Cooling load = base × ceiling factor × sun (0.9 shaded, 1, 1.1 sunny)
+ 600 × (people − 2) + 4,000 if it includes a kitchen
Tons = BTU/h ÷ 12,000 kW = BTU/h × 0.000293
AC size = smallest standard size covering 95% of the tonsLoads per ft² by climate: mild 20, warm 25, hot 30, very hot with little insulation 80 BTU/h (63, 79, 95 and 252 W/m²).
Worked example
A 1,200 ft² house in a warm climate, 8 ft ceilings, four people, average sun:
- Base load = 1,200 × 25 = 30,000 BTU/h
- People over two: (4 − 2) × 600 = 1,200 → 31,200 BTU/h
- 31,200 ÷ 12,000 = 2.6 tons (9.14 kW)
- 95% of 2.6 = 2.47 tons → a 2.5 ton unit
AC Size for a Single Room (two people, 8 ft ceiling)
| Room area | Mild | Warm | Hot | Very hot |
|---|---|---|---|---|
| 100 ft² (9 m²) | 0.5 ton | 0.5 ton | 0.5 ton | 0.75 ton |
| 150 ft² (14 m²) | 0.5 ton | 0.5 ton | 0.5 ton | 1 ton |
| 200 ft² (19 m²) | 0.5 ton | 0.5 ton | 0.5 ton | 1.5 ton |
| 250 ft² (23 m²) | 0.5 ton | 0.5 ton | 0.75 ton | 2 ton |
| 300 ft² (28 m²) | 0.5 ton | 0.75 ton | 0.75 ton | 2 ton |
| 400 ft² (37 m²) | 0.75 ton | 1 ton | 1 ton | 3 ton |
AC Size for a Whole House (four people, 8 ft ceiling)
| Floor area | Mild | Warm | Hot | Very hot |
|---|---|---|---|---|
| 600 ft² (56 m²) | 1.5 ton | 1.5 ton | 2 ton | 4 ton |
| 800 ft² (74 m²) | 1.5 ton | 2 ton | 2 ton | 5.5 tons total |
| 1,000 ft² (93 m²) | 2 ton | 2.5 ton | 2.5 ton | 6.5 tons total |
| 1,200 ft² (111 m²) | 2 ton | 2.5 ton | 3 ton | 8 tons total |
| 1,500 ft² (139 m²) | 2.5 ton | 3.5 ton | 4 ton | 10 tons total |
| 1,800 ft² (167 m²) | 3 ton | 4 ton | 5 ton | 11.5 tons total |
| 2,000 ft² (186 m²) | 3.5 ton | 5 ton | 5 ton | 13 tons total |
| 2,500 ft² (232 m²) | 5 ton | 5.5 tons total | 6.5 tons total | 16 tons total |
| 3,000 ft² (279 m²) | 5 ton | 6.5 tons total | 7.5 tons total | 19.5 tons total |
Above 5 tons, the total is shared between several units or zones.
AC Size by Floor Area
Each page shows one floor area for every climate, and what sun, ceiling height, a kitchen or more people change: 200 sq ft · 400 sq ft · 600 sq ft · 700 sq ft · 800 sq ft · 1,000 sq ft · 1,200 sq ft · 1,300 sq ft · 1,400 sq ft · 1,500 sq ft · 1,700 sq ft · 1,800 sq ft · 2,000 sq ft · 2,500 sq ft · 3,000 sq ft.
Where the Numbers Come From
The sun, people and kitchen adjustments are the ones ENERGY STAR gives for sizing room air conditioners, and its sizing chart works out at roughly 20 BTU/h per ft² for mid-sized rooms. The very hot figure matches the charts Indian AC makers publish for 40 to 48°C summers: about 1 ton for up to 150 ft² and 1.5 ton up to 250 ft². These are rules of thumb for choosing a unit. For a central system or a new building, a heat-gain calculation (ACCA Manual J or similar) accounts for insulation, windows and orientation properly.