Size Guide – Which AC Do I Need?
Buying the right size is the difference between "perfectly cool" and "disappointed". Too small and it runs flat-out without ever catching up; too big and it short-cycles, wastes energy, and leaves the room clammy. Here's how to get it right in 2 minutes.
Step 1 — Measure your room
Multiply length × width in metres. A 5m × 4m living room = 20m². That's it — no tape measure heroics needed, pacing it out is close enough.
Step 2 — Match it to a BTU rating
| Room size | Recommended | Typical rooms |
|---|---|---|
| Up to 15m² | 7,000–9,000 BTU | Bedroom, home office, nursery |
| 15–25m² | 9,000–12,000 BTU | Large bedroom, lounge, studio flat |
| 25–35m² | 12,000–14,000 BTU | Living room, open-plan kitchen-diner |
| 35–45m² | 14,000–18,000 BTU | Large open-plan space |
| 45m²+ | Split system | Whole-floor or permanent cooling |
Step 3 — Adjust for your space
Size up one level if any of these apply:
- Ceilings higher than 2.4m (period properties, lofts)
- Big south-facing windows or a conservatory-style sun trap
- Top-floor flat or directly under the roof
- Kitchen (ovens and fridges add heat)
- More than 2–3 people regularly in the room
- Lots of electronics running (gaming setups, home servers)
Size down is fine if the room is well insulated, north-facing, or you only need to cool a sleeping area at night.
Portable vs Window vs Split — which type?
- Portable — best for renters and flexibility. No installation, moves room to room, packs away in winter. Slightly less efficient than fixed units.
- Window — best value per BTU if you have a suitable sash/sliding window. Frees up floor space, quieter inside the room.
- Split system — best performance, quietest, most efficient. Requires professional installation; ideal if you own your home and want a permanent solution.
Quick answers
"I have a 4m × 3m bedroom" → 12m² → a 9,000 BTU portable is perfect.
"Open-plan lounge/kitchen about 30m² with big windows" → sun + kitchen heat → go 14,000 BTU.
"Loft office, gets roasting" → heat rises + roof exposure → size up one level from the table.