What Size Portable Air Conditioner Do I Need? (BTU Guide UK)
MeacoCool MC Series Pro 9,000 BTU
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A quick formula and room-by-room sizing table for choosing the right BTU in the UK.
As a rule of thumb, a UK bedroom needs 340 BTU per square metre. A 12 m² bedroom = ~4,000 BTU minimum; choose the next standard size up (7,000 BTU). A 18 m² living room = ~6,100 BTU minimum; choose 9,000 BTU. Sizing up is always safer than sizing down.
For a full room-by-room breakdown including adjustments for ceiling height, sun exposure, and occupancy, see the BTU room sizing guide.
1. The quick BTU formula for the UK
Start with 340 BTU per m² of floor area, then apply adjustments:
- +10% if the room is south- or west-facing
- +10% per extra occupant beyond one person
- +15% if the room is a kitchen or adjacent to one
- +20% for a conservatory
- +15% if ceiling height is above 2.4 m
- -10% if the room is north-facing or well-shaded
Example: 15 m² south-facing bedroom:
15 × 340 = 5,100 BTU × 1.10 (south-facing) = 5,610 BTU → choose 7,000 BTU (next standard size up).
15 × 340 = 5,100 BTU × 1.10 (south-facing) = 5,610 BTU → choose 7,000 BTU (next standard size up).
2. Portable AC BTU sizing table
Look at the kit — then buy it
Pro Breeze 12,000 BTU 4-in-1 Smart AC
AuraHome Portable AC 9,000 BTU — 4-in-1 Cool/Dehumidify/Fan
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| Room type | Room size (m²) | Min BTU | Recommended BTU |
|---|---|---|---|
| Small bedroom | 9–12 m² | 5,000 | 7,000 |
| Standard bedroom | 12–16 m² | 6,500 | 9,000 |
| Master bedroom | 16–20 m² | 7,500 | 10,000–12,000 |
| Small living room | 18–25 m² | 9,000 | 12,000 |
| Large living room | 25–35 m² | 12,000 | 14,000 |
| Conservatory | 12–20 m² | 10,000 | 14,000 |
| Home office | 8–12 m² | 5,000 | 7,000 |
Conservatory figures assume peak summer sun. Always round up to the next BTU tier — never down.
3. The danger of undersizing
An undersized portable AC runs continuously at full power, never reaches the thermostat’s set point, and wears out the compressor faster. It also costs more to run — 100% duty cycle vs a correctly sized unit’s 60–75%. The room never gets as cool as it should, and the unit works harder than it was designed to.
Always round up to the next BTU tier. The price difference between adjacent sizes (e.g. 9,000 BTU vs 10,000 BTU) is usually £30–60, far less than the cost of buying again or running an overworked compressor.
4. Does BTU rating apply differently for portable ACs?
Yes — portable ACs are less efficient than fixed split systems at the same BTU because they vent hot air through an exhaust hose. A 12,000 BTU portable AC delivers roughly 8,000–9,000 BTU of effective cooling (EER rating vs theoretical). Single-hose units also pull cooled room air out through the exhaust, reducing net efficiency further.
Factor this in: buy one BTU tier up from the room sizing formula when choosing a portable AC. A room that a 9,000 BTU split system handles perfectly will need 10,000–12,000 BTU from a portable unit.
5. Verdict: which BTU to buy
For a standard UK bedroom up to 16 m², the 9,000 BTU class hits the sweet spot — quiet enough for sleeping and correctly sized for the room. For living rooms and open-plan spaces, 12,000 BTU. Only go to 14,000+ BTU if you have a conservatory, high-ceilinged room, or south-facing glass wall.
Get a precise BTU figure with the BTU calculator →
See which BTU sizes are in stock right now →
More on air conditioning
Portable AC BTU sizing FAQs
FAQ
What size portable air conditioner do I need — frequently asked questions
- How many BTU do I need for a 12 m² bedroom?
- A 12 m² bedroom needs a minimum of about 4,800 BTU theoretically (12 × 340 × portable AC efficiency factor = ~4,000–5,000 BTU effective). Choose the 7,000 BTU tier — it's the smallest standard size and has headroom for warm nights and south-facing rooms.
- Is 9000 BTU enough for a living room?
- For a small living room up to about 18–20 m², 9,000 BTU is borderline. For a standard UK living room of 20–25 m², you need 12,000 BTU. An undersized unit will run at 100% duty cycle and never quite reach the set temperature.
- What BTU is best for a bedroom portable AC in the UK?
- 9,000 BTU is the most popular choice for UK bedrooms and the right size for rooms up to 16–18 m². It's quieter than 12,000 BTU units (fewer watts to shift), cools a bedroom in 20–30 minutes, and costs ~24p/hr to run.
- Do I need a bigger portable AC than a fixed system for the same room?
- Yes — by one BTU tier. Portable ACs are less efficient than fixed split systems because they vent through a hose (losing some cooled air). A room that a 9,000 BTU split system handles perfectly may need 10,000–12,000 BTU from a portable AC.
Sources & cited data
- CIBSE — Cooling design guidance for UK buildings — https://www.cibse.org/
- Energy Saving Trust — Heating and cooling your home — https://energysavingtrust.org.uk/
BTU figures are guide recommendations based on typical UK residential construction. Actual requirements vary with insulation quality, sun exposure, occupancy, and appliance heat load. When in doubt, choose the next BTU tier up.
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