Energy
AC Tonnage Calculator
What size air conditioner a room needs, in tons, BTU/hr and kW. The rate per square foot is shown and adjustable, because Indian and American rules of thumb differ by a factor of four and a calculator that hides which one it used is wrong for half its readers.
The room
- The per-area rates assume ten feet. A taller room is more air to cool and scales the load in proportion.
- The base rate already allows for two. Each person beyond that adds about 600 BTU/hr.
The building and the climate
Indian and Gulf retail guidance puts one ton on about 120 square feet. American whole-home guidance puts it on 400 to 600. That is a factor of four, and neither is wrong — it is single-glazed uninsulated construction at a 45 °C design ambient against an insulated house at 32 °C, plus the difference between sizing one room and averaging a whole dwelling. Pick the one that describes your building.
Seeded from the choice above and yours to change. If a local supplier or an engineer has given you a figure, use theirs — this is the number the whole estimate turns on, so it is shown rather than buried.
The size to buy
Recommended
1.5 ton
- Calculated need
- 1.25 ton
- From the floor area
- 15000 BTU/h
- Total cooling load
- 15000 BTU/h
- In kilowatts of cooling
- 4.4 kW
How to use it
- Enter the floor area in square feet or metres, and the ceiling height if it is not the usual ten feet.
- Choose the construction and climate that describes your building — this is the setting the estimate turns on.
- Adjust the rate if a supplier or engineer has given you a different figure.
- Tick the sun, roof and kitchen boxes that apply, and set how many people use the room.
- Buy the recommended size, which is rounded up to the half ton machines are sold in.
How it works
Floor area times the cooling rate gives the base load, scaled by ceiling height against a nominal ten feet.
Strong sun and an exposed roof each add ten per cent, multiplying rather than adding, because they act on the surface taking the heat.
Occupants past the first two add 600 BTU/hr each, and a kitchen adds 4 000 for its equipment.
The total in BTU/hr divided by 12 000 gives tons, which is then rounded up to the next half ton.
Everything runs in your browser and nothing is sent anywhere.
Common questions
Why do Indian and American charts disagree so much?
Because they describe different buildings in different climates. An uninsulated single-glazed room in a 45 °C summer loses the fight far faster than an insulated house at 32 °C, and Indian guidance sizes one room while American guidance averages a whole dwelling with ducting. One ton per 120 square feet and one ton per 500 are both honest advice — for different situations. That is why the rate is a control here rather than a constant.
What is a ton, and why that word?
It is the cooling a ton of ice does as it melts over a day, which is where the name comes from and why it has nothing to do with weight. It is defined as 12 000 BTU per hour, which is 3.52 kW of cooling. A 1.5 ton machine removes 18 000 BTU/hr.
Should I round up or down?
Up, and the tool does. An undersized machine runs continuously and still never reaches the set point, which costs more than the larger unit would have. Badly oversized is its own problem — it cools fast, switches off before it has dehumidified, and leaves the room cold and clammy — but the sizes actually sold come in half-ton steps, so rounding up to the next one rarely overshoots by enough to matter.
Does it account for the running cost?
No. Tonnage is capacity; what it costs to run depends on the star rating, the ISEER, how many hours it runs and your tariff. The appliance running cost calculator handles that side once you know the size.
Appliance running cost →Why does the top floor matter?
Because a roof in direct sun is the largest single heat gain most rooms have, and there is nothing above it to take the load first. A room on an intermediate floor sits under a conditioned space and gains far less. The same logic applies to a west wall in the afternoon, which is the other toggle.