For a typical room in Lahore with another floor above it, a 1 ton AC covers up to about 200 sq ft and 1.5 ton up to about 400 sq ft. Under a sunny roof the same sizes cover only about 140 and 260 sq ft, so top-floor rooms usually need half a ton more. One ton is 12,000 BTU per hour, about 3.5 kW of cooling.
The calculator works this out for your own room: heat through the roof, walls and windows, heat from people and appliances, and hot air leaking in. It uses the average afternoon high of your city's hottest month, plus a margin for heatwaves.
AC size for common room sizes in Lahore
Rooms with a 10 ft ceiling, two outside walls with some sun, 20 sq ft of glass, two people and 150 W of lights and TV. The top-floor column assumes a concrete roof with earth and tiles on it. The figure in brackets is the exact ton value.
| Room (ft) | Area | Top floor | Floor above |
|---|---|---|---|
| 10 × 10 | 100 sq ft | 1 ton (0.86) | 1 ton (0.71) |
| 10 × 12 | 120 sq ft | 1 ton (0.95) | 1 ton (0.78) |
| 12 × 12 | 144 sq ft | 1.5 ton (1.06) | 1 ton (0.84) |
| 12 × 14 | 168 sq ft | 1.5 ton (1.16) | 1 ton (0.91) |
| 14 × 14 | 196 sq ft | 1.5 ton (1.28) | 1 ton (0.98) |
| 14 × 16 | 224 sq ft | 1.5 ton (1.39) | 1.5 ton (1.06) |
| 16 × 18 | 288 sq ft | 2.0 ton (1.65) | 1.5 ton (1.22) |
| 18 × 20 | 360 sq ft | 2.0 ton (1.93) | 1.5 ton (1.39) |
| 20 × 24 | 480 sq ft | 2.5 ton (2.38) | 2.0 ton (1.66) |
The jump between the columns is the roof. A sunny roof slab can bring in as much heat as all the walls and windows together.
AC size for common rooms in Karachi
Same rooms in Karachi. Afternoons are cooler than in Lahore, but the sea air is more humid, and the AC has to remove that moisture too.
| Room (ft) | Area | Top floor | Floor above |
|---|---|---|---|
| 10 × 10 | 100 sq ft | 1 ton (0.77) | 1 ton (0.64) |
| 10 × 12 | 120 sq ft | 1 ton (0.85) | 1 ton (0.70) |
| 12 × 12 | 144 sq ft | 1 ton (0.94) | 1 ton (0.76) |
| 12 × 14 | 168 sq ft | 1 ton (1.03) | 1 ton (0.82) |
| 14 × 14 | 196 sq ft | 1.5 ton (1.13) | 1 ton (0.89) |
| 14 × 16 | 224 sq ft | 1.5 ton (1.23) | 1 ton (0.95) |
| 16 × 18 | 288 sq ft | 1.5 ton (1.46) | 1.5 ton (1.10) |
| 18 × 20 | 360 sq ft | 2.0 ton (1.71) | 1.5 ton (1.26) |
| 20 × 24 | 480 sq ft | 2.5 ton (2.11) | 1.5 ton (1.51) |
For Multan, Sukkur, Hyderabad and other hotter cities, pick the city in the calculator. The same room there can need half a ton more than in Karachi.
Worked example: a 14 x 12 ft top-floor bedroom in Lahore
This is the example loaded in the calculator. The room is 14 x 12 ft with a 10 ft ceiling, a concrete roof with earth and tiles, two outside walls with some sun and 20 sq ft of window glass. Two people sleep in it, with 150 W of lights and a TV.
Lahore's hottest months average 38.9°C in the afternoon. With a 3°C heatwave margin we design for 41.9°C outside and 25°C inside.
| Heat coming in | Watts |
|---|---|
| Roof | 902 |
| Two outside walls | 843 |
| Inner walls next to rooms without AC | 251 |
| Windows | 461 |
| Two people | 230 |
| Lights and TV | 150 |
| Outside air leaking in | 619 |
| Safety margin, 10% | 346 |
| Total | 3,802 |
At 41.9°C an AC gives about 7% less than its rating, so the rated cooling needed is 4,084 W, which is 13,933 BTU per hour or 1.16 ton. The next size up is 1.5 ton.
The same room with a floor above it needs 0.91 ton, so a 1 ton AC is enough. Painting the roof white brings the top-floor room down to 1.10 ton; insulating the roof brings it to 0.95 ton.
How the calculation works
This is a simplified cooling-load estimate of the kind in the ASHRAE Handbook of Fundamentals. Each part is heat per hour on a hot afternoon:
- Roof and walls: area x U-value x temperature difference. A 9 inch plastered brick wall lets through about 1.9 W per sq m for each degree; a bare concrete slab about 3.1. Sun on a flat roof heats it about 12°C above the air on average through the day.
- Windows: sunlight through the glass, up to 300 W per sq m on a west window in the afternoon, plus heat through the glass itself.
- People: 115 W each, the ASHRAE figure for someone seated.
- Kitchen: 4,000 BTU per hour extra, the allowance in the ENERGY STAR sizing guide.
- Outside air: one room's volume of air leaking in each hour, with its heat and, in humid cities, its moisture.
We add 10% for safety. ACs are rated at 35°C outside and lose about 1% of output for each degree hotter, so we size up for that too. Then we divide by 12,000 BTU to get tons.
Once you know the size, see what it costs to run in the AC electricity cost calculator.
This is a planning estimate, not a full engineering design. It assumes ordinary Pakistani construction and typical window glass. Very large windows, glass walls, a room under a tin roof or a kitchen with heavy cooking can need more. For a large hall or office, ask an HVAC engineer to do a full load calculation.
AC size questions
Which ton AC is best for a 12 x 12 room?
In Lahore a 12 x 12 ft room (144 sq ft) with a floor above needs about 0.84 ton, so a 1 ton AC. On the top floor it needs about 1.06 ton, which calls for 1.5 ton unless the roof is insulated or painted white. Hotter cities such as Multan push the top-floor room further toward 1.5 ton.
How many square feet does a 1.5 ton AC cool?
In Lahore, about 200 to 400 sq ft with a floor above and about 140 to 260 sq ft under a sunny roof, for a room with two outside walls and ordinary windows. In Multan the ranges are about 190 to 380 and 120 to 230 sq ft; in Karachi they are a little larger. Shops often quote smaller ranges to be safe.
How many BTU is 1 ton, 1.5 ton and 2 ton?
One ton of cooling is 12,000 BTU per hour, about 3.5 kW. So 1.5 ton is 18,000 BTU and 2 ton is 24,000 BTU. The name comes from the cooling of one ton of ice melting over a day.
Is a bigger AC better?
Not by much. An oversized non-inverter AC cools the air fast, switches off, and leaves the room damp and clammy. An inverter handles extra size better because it slows down. A too-small AC is worse: it runs flat out, never reaches the setting, and costs more to run.
Does a top-floor room need a bigger AC?
Usually yes. The roof slab soaks up sun all day and keeps giving off heat into the evening. In the worked example the roof alone adds 902 W, almost a quarter of the load. Roof insulation or a white reflective coat can bring the size down by half a ton.
Last reviewed 30 September 2026. Results are estimates; official notices always take precedence.