Free tool
AC Tonnage Calculator: What Size AC Does Your Room Need?
What tonnage AC does this room need?
Floor area is only the starting point. A west-facing top-floor room can need a full size more than the same room one floor down.
Recommendation
Buy a 1.5 ton AC
Calculated load: about 1.08 ton for 144 sq ft.
1.5 ton is the most-sold size in India because it fits the most common Indian bedroom and living room. Buy an inverter model so it slows down once the room is cool rather than switching on and off.
Assumptions: about 90 BTU/h per square foot for a mid-floor room with a 10 ft ceiling, ordinary afternoon sun and two occupants, which matches the room-size bands Indian AC brands publish. Top floor adds 20%, strong west or south sun 15%, extreme dry heat 10%, and each extra person about 600 BTU/h. Treat it as a starting point — a room with unsealed doors or very large glass areas can need more.
| Room size | Mid floor, some sun | Top floor with afternoon sun |
|---|---|---|
| 100 sq ft (10 × 10) | 1 ton | 1 ton |
| 120 sq ft (10 × 12) | 1 ton | 1.5 ton |
| 150 sq ft (10 × 15) | 1.5 ton | 1.5 ton |
| 180 sq ft (12 × 15) | 1.5 ton | 2 ton |
| 200 sq ft (10 × 20) | 1.5 ton | 2 ton |
| 250 sq ft (12.5 × 20) | 2 ton | Two units |
| 300 sq ft (15 × 20) | 2 ton + 1 ton, or two 1.5 ton | Two units |
Indian AC sizing advice looks strangely generous if you have read an American guide, and there is a good reason for it. A US rule of thumb allows 20-25 BTU per hour for each square foot. Indian brands publish room bands that work out to roughly 90 BTU per square foot, because an Indian room starts from 40°C or more outside, sits behind brick walls that soak up heat all day and release it all night, and usually has single-glazed windows. The calculator above uses the Indian figure, so its answers match what the brands themselves recommend.
The two adjustments that matter most are the ones buyers skip. A top-floor flat takes the full afternoon heat of the roof slab, which can add a fifth to the load, and a west-facing room with large windows adds another sixth. Put both together and a 150 sq ft bedroom that would be comfortable with 1.5 ton on the third floor of a five-storey building genuinely needs 2 ton on the fifth. Ceiling height is the third: you are cooling air, not floor, and a 12 ft ceiling adds 20%.
When the calculator lands between two sizes, prefer an inverter AC at the smaller size over a fixed-speed AC at the larger one, unless the room is a top-floor west-facing room in a 45°C city. An inverter compressor can briefly run above its nominal capacity to pull a hot room down, and then slows to hold temperature quietly — which covers most borderline rooms. For a hall above about 300 sq ft, split the load across two units rather than buying one very large one: the room cools evenly, and when one unit fails in May you still have the other.