Aircon BTU calculator

How to use this BTU calculator

Enter the room's length and width in metres, then tick any of the four room conditions that apply to it. The calculator then shows three results.

What you enter:

• Room length in metres.
• Room width in metres.
• Any conditions that apply: double ceiling height, a large window area, windows facing the sun or extra heat sources.

What you get:

• The floor area in square metres.
• The cooling capacity the room needs in kW.
• The same figure in BTU per hour, rounded to the nearest 100.

Use it as a quick aircon size calculator for each room you plan to cool. Nothing you enter is stored or sent.

Inputs: room size and room conditions

Measure the length and width, in metres, of the floor area that the aircon will cool, not the whole flat.

NEA lists four conditions where a larger aircon may be needed. The calculator has a tick box for each:

• Double ceiling height.
• A large window area, such as full-height glass.
• Windows facing the sun (east-west).
• Extra heat sources, such as bar fridges.

Reading the result

The result gives the cooling capacity in kW and in BTU per hour. The BTU figure is rounded to the nearest 100, so read it as "about" that number.

To compare the result with a unit you are looking at, check its Energy Label. NEA's Energy Label shows the aircon's size as its full load cooling capacity in kW, rounded to two decimal places. For split and multi-split aircons, the label is fixed to the outdoor unit, and the model it shows is the outdoor unit's model number.

Units come in set sizes, so ask the supplier which unit covers your figure. For more on room aircon BTU figures and room sizes, see the guide to aircon size for your room.

The Energy Label also rates efficiency in ticks, from 1 (Low) to 5 (Excellent), for casement, window, split type (non-inverter), split type (inverter) and portable units. Portable units are rated only from 1 to 2 ticks. NEA says appliances with more ticks are generally more energy efficient. If you are still choosing a system, see aircon types and inverter vs non-inverter aircon.

How the calculation works

The calculator applies NEA's rule of thumb and NEA's conversion to BTU, and nothing else:

• Cooling capacity in kW = floor area in square metres / 5.
• BTU per hour = kW x 3,412.

NEA's Resource Efficiency Guide for New Home Owners works through an area of 35 square metres: 35 / 5 = 7kW. Converted with the same factor, 7 x 3,412 is about 23,900 BTU/hr. The guide also says to consult your supplier for a detailed assessment.

Here is the same arithmetic for a room that measures 3.5m by 4m.

Several rooms and multi-split systems

Run the calculator once for each room and note each kW figure. NEA says a multi-split system's total capacity is usually the sum of the rooms' capacities. A smaller total may be enough if the rooms are not all cooled at the same time, so consult your aircon supplier for a detailed assessment.

For example, two bedrooms needing 2.4kW and 2.8kW add up to 5.2kW, or about 17,700 BTU/hr.

In an HDB flat, HDB says the owner and installer must check before installation that the total running current of the condensing units does not exceed the approved electrical rating. Flats with a 30 amp main switch need an electrical permit for a new 15A power point or 20A isolator for the aircon. Flats with a 40 amp main switch, or upgraded under MUP or HIP, need no permit.

To see what the job involves, read about aircon installation. The price depends on the job; see the guide to aircon installation cost in HDB flats.

Limits of this estimate

This is a rule of thumb for typical rooms, not a detailed assessment of your home. NEA says to consult your aircon supplier for a detailed assessment, so treat the result as a starting point to discuss with them.

Size matters in both directions. NEA says oversized units cost more to run and may not dehumidify well because they cool too quickly, while undersized units do not cool or dehumidify adequately.

Servicing is a separate cost that depends on the job. For what companies publish, see aircon servicing prices.

Questions