Free calculator
AC tonnage calculator
Work out the air conditioner a room needs, in tons and BTU, from its size, the floor it is on, the sun, the people and the city, and what an inverter or non-inverter AC costs to run a month.
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Calculator
Size your AC
Give the room's size and where it is, then how long the AC runs. The answer follows as you type.
The AC for this room
- Cooling load
- 15,680 BTUan hour, 1.31 tons
- The AC gives
- 18,000 BTU15% over the load
- Units a month
- 2881.2 an hour while it runs
- Cost a month
- Rs 16,704at Rs 58 a unit
How the load adds up
- 224 sq ft under the roof70 BTU an hour a sq ft
- 15,680
- Cooling load, BTU an hour
- 15,680
Running it
- Inverter 1.5 tondraws this while it runs
- 1.2 kW
- Units a month8 h a day for 30 days
- 288
- Cost an hour
- Rs 70
- Cost a month
- Rs 16,704
A non-inverter 1.5 ton would use 480 units a month, Rs 27,840: Rs 11,136 more.
A rule of thumb, not a heat load survey. 12,000 BTU an hour is a ton. Running costs from K-Electric's figures for what ACs draw and the home tariff, as of September 2026.
Reference
Which AC for which room
The AC to buy for common room sizes, and the cooling load it is chosen for.
| Room | sq ft | Under another floor | Top floor | Top floor, west or south sun |
|---|---|---|---|---|
| 10 × 10 ft | 100 | 1 ton (4,500 BTU) | 1 ton (7,000 BTU) | 1 ton (7,700 BTU) |
| 10 × 12 ft | 120 | 1 ton (5,400 BTU) | 1 ton (8,400 BTU) | 1 ton (9,240 BTU) |
| 12 × 12 ft | 144 | 1 ton (6,480 BTU) | 1 ton (10,080 BTU) | 1 ton (11,088 BTU) |
| 12 × 14 ft | 168 | 1 ton (7,560 BTU) | 1 ton (11,760 BTU) | 1.5 ton (12,936 BTU) |
| 12 × 16 ft | 192 | 1 ton (8,640 BTU) | 1.5 ton (13,440 BTU) | 1.5 ton (14,784 BTU) |
| 14 × 16 ft | 224 | 1 ton (10,080 BTU) | 1.5 ton (15,680 BTU) | 1.5 ton (17,248 BTU) |
| 16 × 16 ft | 256 | 1 ton (11,520 BTU) | 1.5 ton (17,920 BTU) | 2 ton (19,712 BTU) |
| 16 × 20 ft | 320 | 1.5 ton (14,400 BTU) | 2 ton (22,400 BTU) | 2.5 ton (24,640 BTU) |
| 20 × 20 ft | 400 | 1.5 ton (18,000 BTU) | 2.5 ton (28,000 BTU) | 4 ton (30,800 BTU) |
| 20 × 24 ft | 480 | 2 ton (21,600 BTU) | 4 ton (33,600 BTU) | 4 ton (36,960 BTU) |
A large window, more people, a humid city or a higher ceiling changes these: the calculator takes them all in. Loads are rounded up to the next size sold.
Reference
AC units per hour and per month
What each size draws while it runs, and what a month of use comes to.
| AC | Units an hour, inverter | Units an hour, non-inverter | A month, inverter | A month, non-inverter |
|---|---|---|---|---|
| 1 ton | 0.9 | 1.5 | 216 units, Rs 12,528 | 360 units, Rs 20,880 |
| 1.5 ton | 1.2 | 2 | 288 units, Rs 16,704 | 480 units, Rs 27,840 |
| 2 ton | 1.5 | 2.5 | 360 units, Rs 20,880 | 600 units, Rs 34,800 |
| 2.5 ton | 1.8 | 3 | 432 units, Rs 25,056 | 720 units, Rs 41,760 |
| 4 ton | 2.7 | 4.5 | 648 units, Rs 37,584 | 1,080 units, Rs 62,640 |
Draws from K-Electric's figures for 1, 1.5 and 2 ton units, carried on to 2.5 and 4 ton. Rs 58 a unit is the whole bill over the units for a home using 500 units a month on the home tariff, as of September 2026.
How it is worked out
From room size to tons
The rule is the one the US EPA's ENERGY STAR guide uses for room air conditioners: capacity by floor area, a tenth less for a heavily shaded room, a tenth more for a very sunny one, 600 BTU an hour for each person beyond two, and 4,000 BTU for a kitchen. Its base, about 20 BTU an hour a sq ft for an 8 ft ceiling in a US summer, is far too little for Pakistan.
So the calculator works its base from the heat that comes into a Pakistani room on a 45°C afternoon: through two outside walls of 9 in brick, a 4 × 4 ft window, the hot air that leaks in, and two people and the lights. For a 12 × 12 ft room with a 10 ft ceiling that is about 7,000 BTU an hour, 45 to 50 BTU a sq ft, and the calculator takes 45 for a room under another floor. On a top floor the concrete roof bakes all day and adds about half as much again: 70 BTU a sq ft. A ground floor, on earth and shaded by the boundary wall, takes 40.
From there a living room takes a tenth more, an office 15%, a shop a quarter more; each foot of ceiling above 10 ft adds 5%; a humid city like Karachi takes 5% less than Lahore or Multan and a milder one like Islamabad 15% less; and each large window or glass door adds 1,500 BTU. A ton is 12,000 BTU an hour, and the load is rounded up to the next size sold, because an AC rated at 35°C outside gives less than its rating in a Pakistani June.
For the example, a 14 × 16 ft bedroom on the top floor in Lahore: 224 sq ft at 70 BTU is 15,680 BTU an hour, 1.31 tons, so a 1.5 ton AC. An inverter 1.5 ton draws about 1.2 kW while it runs, so 8 hours a night for 30 days is 288 units, Rs 16,704 at Rs 58 a unit; a non-inverter draws 2 kW, 480 units and Rs 27,840.
The draws are K-Electric's own figures for 1, 1.5 and 2 ton units, carried on in a straight line to 2.5 and 4 ton, and the price of a unit is the whole bill over the units for a home using 500 units a month on the home tariff (rates as of September 2026). This is a rule of thumb, not a heat load survey: for a room over 1,000 sq ft, a shop front or a wall of glass, have an HVAC engineer work out the load.
Terms
AC words explained
- Ton
- A ton of cooling is 12,000 BTU an hour, the heat that melts a ton of ice in a day. It is what the AC takes out of the room, not what it draws from the mains: a 1.5 ton AC cools at about 5.3 kW and draws 1.2 to 2 kW.
- BTU
- British thermal unit, the heat that warms a pound of water by one degree Fahrenheit. AC capacity and cooling loads are given in BTU an hour.
- Inverter AC
- An AC whose compressor slows down once the room is cool instead of stopping and starting, so it draws about 40% less over a day. A non-inverter AC runs its compressor flat out or not at all.
- Cooling load
- The heat that has to be taken out of a room each hour to hold it cool: through its walls, roof and windows, from outside air, and from the people and lights in it.
Questions
AC tonnage questions
Still have a question?
Ask our team during Pakistan business hours, in English or Urdu.
+92 333 2466662Chat on WhatsAppWhich AC size do I need for a 12 × 12 room?
A 1 ton AC in most cases. A 12 × 12 ft bedroom with a 10 ft ceiling under another floor needs about 6,480 BTU an hour and on a top floor about 10,080 BTU, both within a 1 ton AC's 12,000 BTU. On a top floor with a west wall in the sun and a large window it comes to 12,738 BTU, and a 1.5 ton AC is the one to buy.
How big a room does a 1.5 ton AC cool?
A bedroom of up to about 400 sq ft under another floor, or 257 sq ft on a top floor under the roof, with some sun, two people and a 10 ft ceiling in a hot city. A 1 ton AC covers about 266 sq ft and 171 sq ft, and a 2 ton about 533 sq ft and 342 sq ft. Strong sun, more people or large windows bring these down.
How much electricity does an inverter AC use per hour?
About 0.9 units an hour for a 1 ton inverter AC, 1.2 for 1.5 ton and 1.5 for 2 ton while it runs in summer, against 1.5, 2 and 2.5 for non-inverter ACs of the same sizes. These are K-Electric's figures; a unit is a kWh.
How many units does an AC use in a month?
A 1.5 ton inverter AC run 8 hours a day for 30 days uses about 288 units, and a non-inverter one 480. Multiply the units an hour by the hours a day and the days; the calculator does it for any size.
What does it cost to run an AC for a month?
At Rs 58 a unit, the all-in price for a home using 500 units a month (September 2026), a 1.5 ton inverter AC for 8 hours a day costs about Rs 16,704 a month and a non-inverter one Rs 27,840. The units an AC adds cost more than the average, because they push the bill into dearer slabs: added to a home already using 250 units on 4 kW, its 288 units take the bill from Rs 13,131 to Rs 32,405.
What does a ton mean for an AC?
A ton of cooling is 12,000 BTU an hour, the heat a ton of ice takes to melt in a day. A 1.5 ton AC takes 18,000 BTU of heat out of the room each hour. The ton says how much it cools, not how much electricity it uses.
Is a bigger AC better?
Only up to the next size above the load. A much bigger AC cools the air quickly and switches off before it has taken the damp out, so the room feels clammy, and it costs more to buy. One that is too small runs flat out all afternoon and still does not cool the room on the hottest days.
Why does a top floor room need a bigger AC?
Because the concrete roof above it bakes in the sun all day and passes the heat down well into the night. The calculator takes 70 BTU a sq ft under the roof against 45 under another floor. Shading the roof or giving it a heat-reflecting coat or insulation brings the load down.
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