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 | 75,000 BTU | 4 ton and above (multiple units) |
| Hot | 33-40°C | 56,200 BTU | 4 ton and above (multiple units) |
| Warm | 27-33°C | 37,500 BTU | 3 ton |
| Temperate | 20-27°C | 26,200 BTU | 2.5 ton |
| Cool | <20°C | 18,800 BTU | 1.5 ton |
The figures above assume standard conditions: average insulation, 2 occupants, 9-foot ceiling. Add to the baseline for:
A 30x25 room covers 750 square feet and is typically a very large open-plan space or small commercial room. 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 750 sq ft that gives 18,800 BTU (1.5 ton). In India or the Gulf the correct figure is 75 BTU/sq ft, giving 56,200 BTU (4 ton and above (multiple units)). Using the US rule undersizes the AC by roughly three times.
For a 30x25 room (750 sq ft) in a hot climate (India, Gulf, South-East Asia), you need 56,200 BTU which is 4 ton and above (multiple units). In a very hot climate above 40 degrees Celsius, you need 75,000 BTU (4 ton and above (multiple units)). In a cool temperate climate, 18,800 BTU (1.5 ton) is sufficient.
A 750 sq ft room needs between 18,800 BTU (cool climate) and 75,000 BTU (very hot). For India and Gulf states use 56,200 BTU (4 ton and above (multiple units)) as your baseline, then add 10 percent for kitchens, west-facing windows or poor insulation.
4 ton and above (multiple units) is the correct size for a 30x25 room (750 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 750 sq ft room needs 4 ton and above (multiple units) in a hot climate but only 1.5 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 56,200 BTU (4 ton and above (multiple units)). 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.