What size air conditioner do I need for my room? (BTU worked out)

How to · Updated 2026-11-10

Look up the room's floor area in the ENERGY STAR sizing chart, then adjust for sun, extra people, a kitchen and a tall ceiling. A 22.5 m² living room worked through in BTU/h, kW and tons.

A room air conditioner is sized from the floor area first, using a chart that gives the cooling power in BTU per hour (BTU/h, British thermal units per hour) for each band of room size, and then nudged up or down for sun, people and heat sources. The AC size looks up the band, applies the adjustments, converts the answer to kilowatts and tons, and names the next common unit size to shop for.

What you need

• The floor area of the room, in square feet or square metres. Length times width is enough for a rectangle.
• The ceiling height, if it is taller than an ordinary 8 ft (about 2.44 m) ceiling.
• How sunny the room is. A room with big west- or south-facing windows (north-facing in the southern hemisphere) gains far more heat than one shaded by trees or a building.
• How many people usually use it, and whether it is a kitchen, since cooking adds a lot of heat.
• A rough idea of the insulation: old single glazing and no loft insulation count as poor.

Step by step

The calculation the tool follows is:

Cooling = chart value × ceiling factor × sun factor × insulation factor + 600 × (people − 2) + kitchen extra

• The chart value comes from the ENERGY STAR band the area falls into. For example, 150 to 250 sq ft is 6,000 BTU/h and 250 to 300 sq ft is 7,000 BTU/h.
• The ceiling factor is ceiling height ÷ 8 ft when the ceiling is higher than 8 ft, because there is more air to cool. Otherwise it is 1.
• The sun factor is 0.9 for a heavily shaded room and 1.1 for a very sunny one. The insulation factor is 1.1 for poor and 0.9 for good.
• Each person beyond two adds 600 BTU/h, and a kitchen adds 4,000 BTU/h.

To convert the answer, divide BTU/h by 3,412 for kilowatts and by 12,000 for tons.

With the calculator:

1. In Units, choose feet or metres. The area and height you already typed are converted for you.
2. Type the Room area and the Ceiling height.
3. Set Sun and Insulation from their drop-down lists.
4. Type how many People who usually use the room. The 600 BTU/h figure for each extra person sits in its own box and can be changed.
5. Tick The room is a kitchen if it is one.
6. Read the result: the chart band, the cooling in BTU/h, the same in kW and tons, and the next common unit size. The heating box underneath gives a rough heater size from watts per square metre.

A worked example

A living room is 5 m by 4.5 m with a 2.7 m ceiling. It faces the afternoon sun, insulation is average, and three people usually sit in it in the evening.

1. Area: 5 × 4.5 = 22.5 m², which is 22.5 ÷ 0.0929 = 242 sq ft.
2. Chart band: 150 to 250 sq ft, so 6,000 BTU/h.
3. Ceiling: 2.7 m is 8.86 ft, so the factor is 8.86 ÷ 8 = 1.107. 6,000 × 1.107 = 6,644 BTU/h.
4. Very sunny: × 1.1 = 7,308 BTU/h.
5. One person over two: + 600 = about 7,908 BTU/h.
6. In other units: 7,908 ÷ 3,412 = 2.32 kW, and 7,908 ÷ 12,000 = 0.66 tons.
7. The next common unit size is 8,000 BTU/h.

For winter, at the rough rule of 100 W per square metre, the same room needs about 22.5 × 100 = 2.25 kW of heating.

Once you have picked a unit, the electricity cost calculator shows what it costs to run from its watts and your price per kWh. For a room with large windows or unusual walls, HVAC load builds the load up surface by surface instead of from a chart.

Check your answer

• The 20 BTU per square foot shortcut. 242 sq ft × 20 = 4,844 BTU/h. The chart starts higher than this for small rooms, and our sun, ceiling and people adjustments push the answer up further, so landing between 6,000 and 8,000 is sensible.
• kW sense check. Most window and small split units for a single room are between about 1.5 and 3.5 kW of cooling. If your answer is 20 kW for a bedroom, the area was probably typed in square feet with the units set to metres.
• Go up, not far up. Choose the next size above your figure. A unit much bigger than needed reaches the set temperature quickly and switches off before it has pulled moisture out of the air, so the room feels cool but clammy.

Questions people ask

Is BTU the same as BTU/h? On air conditioner labels, “BTU” almost always means BTU per hour, a rate of cooling. A 12,000 BTU unit removes 12,000 BTU of heat each hour.

What does “one ton” of cooling mean? It is 12,000 BTU/h, the cooling from melting one ton of ice over a day. A “1.5 ton” split unit is 18,000 BTU/h.

Does an open-plan room count as one room? Yes, add up the whole open area. If it joins a kitchen, tick the kitchen box too, and above 2,500 sq ft ask a professional for a full load calculation.