Air conditioner size calculator
Enter the room area, the ceiling height and how much sun the room takes. The tool estimates the cooling load in BTU and recommends the smallest unit on sale that covers it. The aim is the right size, not the biggest: an oversized unit cools fast then shuts off before it has pulled any moisture out.
Shaded: an internal or north-facing room. Average: one external wall. Sunny: west or south glazing, or an exposed corner room.
Each person beyond two adds about 600 BTU of body heat.
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Recommended capacity
1.5 ton
A 18,000 BTU unit for a load of 14,000
Breakdown
| Base load | 14,000BTU |
| Ceiling factor | × 1 |
| Total load | 14,000BTU |
| Tons required | 1.17 |
| Unit headroom over the load | + 29% |
This is a load estimate, not a load calculation. A real one measures glazing area and orientation, wall build-up and air infiltration. Use it to pick a size before buying, and go to a mechanical engineer for a whole project or a ducted system.
How it's calculated
The estimate starts from the area multiplied by a load rate per square metre. The rate depends on sun exposure, which is the single biggest lever: a sunny room needs 700 BTU per square metre against 500 for a shaded one, a 40 percent difference on the same floor area.
That figure is then scaled by the ratio of the ceiling height to three metres, because a taller room holds more air to cool. Fifteen percent is added if the room sits directly under the roof, and 600 BTU for each person beyond two.
Finally the load is divided by twelve thousand to give tons, and the smallest unit on sale that meets or exceeds it is chosen. The card shows how much headroom that unit has over the load, so you can see the margin before you buy.
- Base load = area × the exposure rate (500, 600 or 700 BTU/m²)
- Ceiling factor = ceiling height ÷ 3 metres
- The top floor adds 15 percent, and each person beyond two adds 600 BTU
- Tons = total load ÷ 12,000
- The recommendation is the smallest available unit that covers the load, not the largest
Worked example
A 20 m² bedroom with a 3 m ceiling, west-facing and sunny, two occupants, not on the top floor:
- Base load: 20 × 700 = 14,000 BTU
- Ceiling factor: 3 ÷ 3 = 1, so no change
- No roof allowance and no occupant allowance
- Total load: 14,000 BTU
- Tons: 14,000 ÷ 12,000 = 1.17
The smallest unit that covers the load is 1.5 ton, or 18,000 BTU. Do not jump to 2 ton: it will not cool noticeably faster, but it will use more power and leave humidity behind.
FAQ
Why not buy a bigger unit to be safe?
Because a big unit reaches the set temperature quickly and stops, and pulling moisture out needs sustained running. The result is a cold, clammy room, plus frequent cycling that uses more electricity and wears the compressor. The right size beats the bigger size.
What separates shaded, average and sunny?
Shaded is an internal room or a north-facing one that gets no direct sun. Average has a single external wall with an ordinary window. Sunny has west or south glazing, or is a corner room with two exposed walls. The gap between the extremes is 40 percent, so choose honestly.
Is a ton the same as a thousand BTU?
A ton is 12,000 BTU per hour. The name is historical: it is the heat needed to melt a ton of ice in a day. Gulf units are usually sold in tons and other markets in BTU, and the card shows both numbers.
Does this cover kitchens or rooms full of equipment?
No. A kitchen adds a large load from the oven and hob, and so does any room with servers or equipment running continuously. Add an allowance for equipment on top, or get a detailed load calculation if the equipment is central to the room.
Does insulation change the answer?
A great deal. The bands assume a modern building insulated to code. An older uninsulated building can need 20 to 30 percent more, and single glazing instead of double pushes the load up as well. If your building is old, treat the figure as a floor.
Related tools
Sources
✓ Reviewed September 2026- Load rates of 500, 600 and 700 BTU per square metre by sun exposure, bands suited to Gulf summers in a building insulated to code
- The bands assume a three metre ceiling, common in Gulf villas, and the load scales with the ratio of the actual height to that reference
- A 15 percent allowance for a room directly under the roof, the usual figure when the roof is insulated to code
- 600 BTU for each person beyond two, covering the sensible and latent heat a body gives off
- A ton is 12,000 BTU per hour, and single units are sold from 0.75 to 5 tons, so the recommendation is the smallest capacity that covers the load
Assumptions used
- Cooling load per square metre for Gulf summers. Shaded is a north-facing or internal room, average a room with one exposed wall, sunny a room with west or south glazing or an exposed corner.
- The bands assume a 3 m ceiling, common in Gulf villas. A taller room holds more air, so the load scales with the ratio of the two heights.
- A room directly under the roof takes heat through the slab all afternoon. Fifteen percent is the usual allowance when the roof is insulated to code.
- Each person beyond the first two adds about 600 BTU/h of sensible and latent heat.
- Split and window units are sold in these capacities, so the recommendation is the smallest one that meets the load. Oversizing cools fast then short-cycles, which leaves the room cold and damp.