Choosing the right AC size for a room
How to choose an AC in tons for a room from its floor area, what a ton actually means, how a top floor, sun, people and a kitchen add to the load, and what oversizing or undersizing costs you.
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In short
- A common rule of thumb is one ton of cooling for about 120 sq ft of floor at a normal ceiling height.
- A 12 × 12 ft room comes to 1.2 tons, so a 1.5 ton AC is the usual choice.
- Add about half a ton for a top floor under a sunny roof, and more for big west or south-facing windows.
- One ton of cooling means 12,000 BTU per hour, or 3.517 kW of heat removed. It has nothing to do with the AC's weight.
- Sizing is an estimate. A well-insulated room needs less, and a glass-walled room may need much more.
An air conditioner is a purchase you live with for ten years or more, and its size affects comfort and power bills on every hot day. Too small, and it runs flat out all afternoon and still leaves the room warm. Too big, and you pay more up front, and it cycles on and off without drying the air properly. Shops often size an AC from the floor area alone, which is a fair start but ignores the things that make one room much hotter than another. This page explains how the estimate works, what to add for your room, and how to use the result when you buy.
The basic rule
The starting point is the floor area:
tons = floor area in sq ft ÷ 120
This assumes a ceiling of about 10 ft. For a 12 × 12 ft room, the area is 144 sq ft, and 144 ÷ 120 = 1.2 tons. ACs are sold in fixed sizes, so you round up, which gives 1.5 tons.
Taller rooms hold more air, so the calculator scales the load by ceiling height:
base load = area ÷ 120 × ceiling height ÷ 10
Note that this rule of thumb varies between sources. Some use 100 sq ft per ton in very hot places, and others 150 sq ft for a shaded room. That is why the result is a guide rather than an exact engineering figure.
| Room size | Floor area | Base load | Common choice |
|---|---|---|---|
| 10 × 10 ft | 100 sq ft | 0.83 ton | 1 ton |
| 12 × 10 ft | 120 sq ft | 1.00 ton | 1 ton |
| 12 × 12 ft | 144 sq ft | 1.20 ton | 1.5 ton |
| 15 × 12 ft | 180 sq ft | 1.50 ton | 1.5 ton |
| 16 × 14 ft | 224 sq ft | 1.87 ton | 2 ton |
| 20 × 15 ft | 300 sq ft | 2.50 ton | 2.5 ton |
The figures in this table are for a shaded mid-floor room with two people and a 10 ft ceiling. Most of the choices above sit exactly at the limit, so any extra heat pushes them up a size.
What adds to the load
Heat enters a room through the roof, the walls and the windows, and it is produced inside by people and appliances. The calculator adds a fixed amount for each of the main ones:
- Top floor. A concrete roof in the sun stores heat through the day and releases it into the night. This adds 0.5 ton.
- Sunny windows. West or south-facing rooms with large windows take in direct sun. This adds 0.25 ton.
- People. Each person gives off heat. Beyond two, each adds 0.05 ton.
- Open kitchen. Cooking heat flows into the room. This adds 0.5 ton.
Take a 15 × 12 ft bedroom on the top floor. The base is 1.5 tons, and the roof adds 0.5, which makes 2 tons. So this room needs a 2 ton AC, where the same room lower down would manage with 1.5.
What a ton means
The word has a curious origin. It dates back to the days of ice, when a ton of cooling meant the heat absorbed by one ton of ice melting over 24 hours. That works out at 12,000 BTU per hour, or 3.517 kW of heat removed.
| AC size | BTU per hour | Cooling in kW |
|---|---|---|
| 0.75 ton | 9,000 | 2.64 |
| 1 ton | 12,000 | 3.52 |
| 1.5 ton | 18,000 | 5.28 |
| 2 ton | 24,000 | 7.03 |
| 2.5 ton | 30,000 | 8.79 |
Note that this is the heat moved, not the electricity used. A 1.5 ton AC usually draws much less than 5.28 kW, because it pumps heat rather than making cold. The power it draws is on its label, and an efficient model can draw far less than an old one of the same size.
Why bigger is not better
An oversized AC cools the air quickly and then cuts out. A big fixed-speed unit does this in short bursts, and in humid weather it stops before it has removed enough moisture, so the room feels clammy at the set temperature. It also costs more to buy, and it can need heavier wiring.
An undersized AC never reaches the set temperature on the hottest days. It runs at full load for hours, costs more to run than you expected, and wears out sooner.
Inverter ACs reduce the problem of oversizing, because they slow the compressor when the room is cool rather than switching it off. So if a room falls between two sizes, an inverter model of the larger size is usually a sensible choice, as it can run gently most of the time.
Reading the star label and running costs
Size tells you what the AC can cool. The star label tells you how efficiently it does so. Indian ACs carry a BEE star label and an ISEER rating, which reflects typical conditions over a cooling season. The higher the number, the less electricity for the same cooling.
The label also gives an estimated annual use in units, based on a stated number of hours. If a label says 850 units a year and your rate is ₹8 a unit, running it as the label assumes costs about ₹6,800 a year. To estimate your own bill, multiply the power on the label in kW by the hours you really expect to run it, then by your state's rate per unit.
A higher star rating usually costs more to buy and saves on the bill. It tends to pay back faster where the AC runs for many hours a day, such as a bedroom used all night through a long summer.
Getting more from a smaller AC
Before buying a larger size, it is worth cutting the heat coming in.
- Shade the windows. Curtains, blinds or sun film can cut direct sun substantially.
- Paint or insulate the roof. Reflective white coatings on a terrace reduce the heat stored in the slab.
- Seal gaps. Doors and window frames that leak let in hot air all day.
- Use a fan. A ceiling fan on low allows a set point a degree or two higher, with the same comfort. Each degree saves energy.
- Keep filters clean. A dirty filter reduces airflow and cooling. Clean it every couple of weeks in peak season.
- Service before summer. A low refrigerant charge or a blocked coil can make a good AC perform like a smaller one.
Rooms that need a proper heat load
The rule of thumb does not fit every room. Ask an installer or an HVAC engineer for a heat-load calculation if your room has large glass walls, a high or vaulted ceiling, many computers or machines, a server room or a shop with a large open front. The same is true for a whole-house or ducted system.
Altitude and climate make a difference too. Coastal cities have humidity to deal with, while the northern plains have higher peak temperatures. Check the rated ambient temperature, as a unit rated for 52°C is designed to hold its output in a heat wave.
What this calculator assumes
- Base load is the floor area in square feet ÷ 120, scaled by ceiling height from a 10 ft standard, with a minimum height of 8 ft in the sum.
- Up to two people are included, and each extra person adds 0.05 ton.
- A top floor adds 0.5 ton, an open kitchen adds 0.5 ton and sunny windows add 0.25 ton.
- The result rounds up to the next size sold: 0.75, 1, 1.5, 2, 2.5, 3, 4 or 5 tons.
- It is a rule of thumb, not a heat-load survey, and it takes no account of insulation, glass area or local climate.