What size air conditioning unit do I need? (UK room-size guide)
Undersize your AC and it'll run flat-out and never quite get comfortable. Oversize it and it'll short-cycle, dehumidify poorly, and cost more to buy. Here's the honest sizing guide we use on real Midlands homes.
Room size to unit size — quick reference
Air conditioning capacity is measured in kilowatts (kW) or BTUs (British Thermal Units). 1 kW ≈ 3,412 BTU. As a rule of thumb for UK homes:
| Room size | Capacity needed | Typical unit |
|---|---|---|
| Up to 15 m² (small bedroom) | 2.0 – 2.5 kW | 7,000 – 9,000 BTU |
| 15 – 25 m² (double bedroom, small lounge) | 2.5 – 3.5 kW | 9,000 – 12,000 BTU |
| 25 – 40 m² (family lounge, kitchen-diner) | 3.5 – 5.0 kW | 12,000 – 18,000 BTU |
| 40 – 60 m² (open-plan, large lounge) | 5.0 – 7.1 kW | 18,000 – 24,000 BTU |
| 60 m²+ | 7.1 kW+ | 24,000 BTU+ |
When to bump up a size
These factors mean you should pick the next size up:
- South- or west-facing room with large windows (afternoon sun load)
- Loft conversion or top-floor bedroom (roof heat)
- Ceilings over 2.6m tall
- Poor insulation, single glazing, or a conservatory
- Kitchen with cooking appliances contributing heat
- More than 2 adults regularly in the room
- Tech-heavy home office (gaming PC, multiple monitors)
Single-split vs multi-split — sizing the whole home
If you're cooling multiple rooms, you have two options:
Single-split: one indoor unit + one outdoor unit per room. Simpler, cheaper per room, easiest to service. Best when the rooms are far apart or you only really need cooling in 1–2 spaces.
Multi-split: one bigger outdoor unit serving 2–5 indoor units. Neater exterior, less wall penetration, better long-term value if you're cooling 3+ rooms. The outdoor unit is sized to about 80–90% of the combined indoor capacity (they're rarely all running flat out at once).
Common sizing mistakes
- Buying "portable" AC units and expecting bedroom-level performance — they're single-hose, half as efficient, and noisy
- Sizing a lounge unit by floor area alone and ignoring huge south-facing patio doors
- Trying to cool a whole house with one big unit in the hallway — air doesn't turn corners well
- Undersizing a home office because it's small, forgetting the tech load
Let us size it for you
We do a proper heat-load calculation on every survey — free, no obligation. Send us a photo and your rough room dimensions on WhatsApp and we'll come back with the right unit size the same day.
FAQ
Can one outdoor unit cool my whole house?
Yes — a multi-split system runs one outdoor condenser with up to five indoor units. Most 3–4 bedroom Midlands homes work well on a single outdoor unit.
Are bigger units always better?
No. An oversized unit short-cycles (turns on and off frequently), which dehumidifies poorly, wastes electricity, and wears the compressor faster. Correct sizing matters.
How loud is a properly sized unit?
In quiet mode, most modern indoor units are 19–22 dB(A) — quieter than a whisper. If yours is loud, it's usually undersized and running at max fan speed constantly.
Ready for a real price on your home?
Free home survey, fixed written quote, F-Gas certified installers across the West Midlands. 60-second form or WhatsApp us a photo.