How many BTU do I need for a 4m by 4m room?

A 4m by 4m room is 16m². At the calculator's 500 BTU/m² starting point that is 8,000 BTU before adjustments. A commonly available 9,000 BTU unit is a sensible starting band, with more capacity potentially needed for strong sun, high ceilings or poor insulation.

What room size will a 9,000 BTU air conditioner cool?

As a starting guide, around 14–18m² with a normal ceiling and average heat gain. A sunny loft, kitchen or heavily glazed room may need more capacity, while a shaded, well-insulated bedroom may need less.

What room size will a 12,000 BTU air conditioner cool?

Roughly 18–24m² as an initial range under average conditions. Room volume, windows, insulation, orientation and occupancy can change the result, so calculate the heat-load factors before buying.

What size air conditioner do I need? UK BTU room-size guide

By AirconCalculator Team · Published 29 May 2026 · Last updated 28 August 2026

For a UK room with a standard 2.4 m ceiling and average heat gain, start at roughly 500 BTU per square metre. That makes a 16m² room about 8,000 BTU before adjustments, so a commonly available 9,000 BTU model is a sensible starting band.

For a tailored result, enter the room into the air conditioner BTU calculator. The table below is for quick comparison, not a substitute for a full installer heat-load calculation.

BTU room-size chart

Air conditioner capacity	Typical starting room area	Cooling capacity
5,000 BTU	Up to about 10m²	1.47 kW
7,000 BTU	About 10–14m²	2.05 kW
9,000 BTU	About 14–18m²	2.64 kW
12,000 BTU	About 18–24m²	3.52 kW
14,000 BTU	About 24–28m²	4.10 kW
18,000 BTU	About 28–36m²	5.28 kW

The ranges use the site’s documented 500 BTU/m² base assumption. Increase the estimate for high ceilings, direct afternoon sun, extensive glazing, weak insulation, kitchens and unusually high occupancy.

How to calculate the starting size

Measure room length and width in metres.
Multiply them to get floor area in square metres.
Multiply area by 500 BTU.
Adjust for room height and heat gains.
Select a commercially available capacity band at or just above the result.

For a 4 m × 5 m living room:

4 × 5 = 20m²

20 × 500 = 10,000 BTU starting load

A 12,000 BTU unit may therefore be the nearest common size, but an average, shaded and well-insulated room could sit closer to 10,000 BTU. Use the calculator rather than automatically rounding up by a large amount.

What room size suits 9,000 BTU?

A 9,000 BTU air conditioner is a reasonable starting point for approximately 14–18m². That often includes a medium bedroom, home office or compact living room.

It may not be enough for a 16m² room when:

the ceiling is substantially higher than 2.4 m;
large south- or west-facing windows receive direct sun;
the room is a loft conversion beneath a hot roof;
cooking or computer equipment adds heat;
doors are left open to halls or adjacent rooms; or
a portable unit’s exhaust opening is poorly sealed.

What room size suits 12,000 BTU?

A 12,000 BTU unit is a starting match for roughly 18–24m², such as a larger bedroom or modest living room. “12,000 BTU room size in metres” refers to floor area in square metres, not a single room dimension: 4 m × 5 m is 20m², for example.

An open-plan layout is harder to treat as a simple rectangle. Air movement between zones, stairs and open doors can make the effective cooling area much larger.

Adjustments that increase the cooling load

Ceiling height

The calculator scales the base result in proportion to ceiling height. A 3 m ceiling contains 25% more air volume than a 2.4 m ceiling for the same floor area, before considering other heat gains.

Sun and glazing

Direct sun through glass can dominate the load. The calculator adds 15% for a sunny aspect and a further uplift for extensive glazing. External shading, blinds and curtains can reduce heat gain but should not be used to justify a severely undersized unit.

Kitchens and equipment

Ovens, hobs, desktop computers, servers and other appliances release heat. The calculator adds a kitchen allowance and lets you include heat-producing equipment.

Occupants

People produce sensible and latent heat. A bedroom for one person and a meeting room with six people should not be sized identically even when their dimensions match.

Insulation and air leakage

Poor insulation, single glazing and uncontrolled draughts increase the amount of heat entering. Sealing the window around a portable unit’s exhaust hose is particularly important.

Why not buy the biggest unit available?

An undersized unit may run continuously and still fail to cool the room. Oversizing is not an ideal cure: it increases purchase cost and can cause short cycling, uneven temperature and weaker humidity removal. The US Department of Energy likewise warns that oversized room air conditioners can use more energy and control humidity poorly.

Aim for a defensible estimate and the nearest suitable product band. For a fixed split system or an awkward open-plan space, ask a qualified installer to calculate the design cooling load.

BTU and electricity use are not the same figure

BTU/h measures cooling output. It does not say how much electricity the appliance draws. Divide BTU/h by approximately 3,412 to convert cooling output to kW, then use EER or the manufacturer’s stated input power to estimate electricity consumption.

Once you have shortlisted a capacity, enter its input watts into the running cost calculator to budget for actual use.