The correct AC size depends primarily on your climate. Use the Hot row for India, Gulf and South-East Asia. Use Very Hot for regions with peak temperatures above 40 degrees Celsius.
| Climate zone | Peak temperature | BTU needed | Recommended size |
|---|---|---|---|
| Very Hot | >40°C | 50,000 BTU | 4 ton and above (multiple units) |
| Hot | 33-40°C | 37,500 BTU | 3 ton |
| Warm | 27-33°C | 25,000 BTU | 2 ton |
| Temperate | 20-27°C | 17,500 BTU | 1.5 ton |
| Cool | <20°C | 12,500 BTU | 1 ton |
The figures above assume standard conditions: average insulation, 2 occupants, 9-foot ceiling. Add to the baseline for:
A 25x20 room covers 500 square feet and is typically a large open-plan living and dining area. Getting the AC size right matters in both directions: too small and it runs continuously without cooling the room; too large and it short-cycles with poor dehumidification.
Why the old "25 BTU per square foot" rule is wrong for hot climates. Most BTU calculators online use 25 BTU/sq ft, the US temperate-climate standard. For 500 sq ft that gives 12,500 BTU (1 ton). In India or the Gulf the correct figure is 75 BTU/sq ft, giving 37,500 BTU (3 ton). Using the US rule undersizes the AC by roughly three times.
For a 25x20 room (500 sq ft) in a hot climate (India, Gulf, South-East Asia), you need 37,500 BTU which is 3 ton. In a very hot climate above 40 degrees Celsius, you need 50,000 BTU (4 ton and above (multiple units)). In a cool temperate climate, 12,500 BTU (1 ton) is sufficient.
A 500 sq ft room needs between 12,500 BTU (cool climate) and 50,000 BTU (very hot). For India and Gulf states use 37,500 BTU (3 ton) as your baseline, then add 10 percent for kitchens, west-facing windows or poor insulation.
3 ton is the correct size for a 25x20 room (500 sq ft) in a hot climate. If your climate peaks above 40 degrees, or the room is on the top floor, faces west or has poor insulation, increase to 4 ton and above (multiple units).
The biggest factor is climate zone: the same 500 sq ft room needs 3 ton in a hot climate but only 1 ton in a cool one. Other factors: top-floor location (add 15 to 20 percent), kitchen (add 10 percent), west-facing windows (add 10 percent), poor insulation (add 15 to 20 percent), and more than 4 or more occupants (add 600 BTU per extra person).
Yes, if the window opening fits the required unit size. In a hot climate you need 37,500 BTU (3 ton). Check that a window AC is available in that capacity. For rooms above 200 sq ft in hot climates, a split AC is usually a better choice.
Recommendations use 100/75/50/35/25 BTU per sq ft for the five climate zones. Actual requirements vary with insulation, solar exposure, ceiling height and occupancy. Confirm sizing with a qualified HVAC technician for large or unusual spaces.