Almost every AC sizing conversation starts the same way: a homeowner who’s read the “600 BTU per square metre” rule and worked out their living room needs 12,000 BTU, then asks us to fit that size. Half the time they’re right. Half the time they’re a third out in one direction or the other. Heat-gain calculation isn’t complicated, but a proper one beats every back-of-envelope rule. Here’s how we size for UK homes in 2026, and what actually goes into the number.
1. The proper sizing method
A real heat-gain calc adds up all the heat sources entering the room in summer:
- Solar gain through glazing — depends on window area, orientation (south-west worst), glazing type (single, double, triple, low-E)
- Conduction through walls and ceilings — depends on insulation; modern walls negligible, old solid walls significant
- Occupant heat — 100 W per person seated, 150 W active
- Equipment heat — TV, PC, lighting, kitchen appliances if in-room
- Ventilation/infiltration — heat from outside air leaking in
We sum these for the worst-case design day — typically 30°C dry-bulb outdoor with strong July afternoon sun — and that gives the required cooling capacity. Add a 10% design margin and round to the nearest standard AC size.
2. Why the BTU rule fails
“600 BTU per m²” assumes:
- Average UK glazing ratio (~20% of floor area)
- Average orientation (some south, some north)
- Average insulation (post-2000 build)
- 2–3 occupants
- Modest equipment heat
For a 20 m² average bedroom, the rule says 12,000 BTU (3.5 kW). Real ranges we’ve measured:
- 20 m² north-facing bedroom, double-glazed, modern build, 1 occupant: 1,800 W needed
- 20 m² south-facing bedroom, large glazed gable, loft conversion under flat roof: 4,200 W needed
That’s a 2.3x range for the same square metres. The rule of thumb hits the average and misses the extremes by half.
3. Worked examples — three real Kent rooms
Living room, 24 m², south-facing bay window
- Solar gain (4 m² double-glazed south window, July): ~1,200 W
- Conduction (cavity wall, 2 external walls): ~400 W
- Occupants (3 people typical): ~300 W
- Equipment (TV, lighting): ~200 W
- Infiltration: ~300 W
Total: 2,400 W. With 10% margin: 2,650 W. Round up to nearest standard size: 3.5 kW unit (Mitsubishi MSZ-AP35 or Daikin FTXM35).
Bedroom, 14 m², east-facing
- Solar gain (modest east window): ~400 W
- Conduction: ~200 W
- Occupants (2 at night): ~150 W
- Equipment (negligible): ~50 W
- Infiltration: ~200 W
Total: 1,000 W. With margin: 1,100 W. Nearest standard: 2.5 kW unit (Mitsubishi MSZ-AP25). Bigger than needed, but the smallest practical size most ranges go down to.
Open-plan kitchen-diner, 38 m², west-facing glazing
- Solar gain (5 m² west glazing, late afternoon worst): ~1,500 W
- Conduction (3 external walls): ~600 W
- Occupants (4 at dinner): ~400 W
- Equipment (kitchen heat — oven, hob residual): ~600 W
- Infiltration: ~400 W
Total: 3,500 W. With margin: 3,850 W. Nearest standard: 5.0 kW unit (Mitsubishi MSZ-AP50). Could split into two 3.5 kW units mounted on different walls for better distribution.
4. The orientation rules of thumb
The single biggest UK variable is the direction the windows face. From worst to best for summer overheating:
- West-facing — worst. Late-afternoon sun (3pm–7pm) when ambient temperature is already at the day’s peak.
- South-facing — midday sun, but the room has been warming since morning.
- South-west — combines the worst of both. Add ~30% to baseline sizing.
- East-facing — morning sun, cooler when ambient still low.
- North-facing — almost no solar gain. Smallest AC needed.
5. Multi-split vs single-split economics
A multi-split outdoor unit serves 2–5 indoor units through a single refrigerant circuit. The decision tree:
- 1 indoor unit: single split. Always.
- 2 indoor units close together: two single splits often cheaper than one multi.
- 2 indoor units on different elevations: multi-split aesthetically better (one outdoor unit, not two).
- 3+ indoor units: multi-split usually wins on capital cost and aesthetics.
- 5+ indoor units: consider VRF (Variable Refrigerant Flow) — full commercial-grade system.
6. Common over-sizing mistakes
Three patterns we routinely have to talk customers out of:
- “Future-proofing” — buying a bigger unit because someone might extend the room. AC inverters work best near design load. Future extension means future AC sizing review, not pre-emptive over-spec.
- “To cool the whole upstairs” — one big unit in the landing doesn’t cool bedrooms behind closed doors. AC needs an indoor unit per zone, period.
- “Worst-case heatwave” — sizing for the 2°C-above-design-day extreme is bad design. Modern AC has 30% over-capacity at low fan speed; you have headroom built in.
7. Inverter vs on/off
Modern domestic AC is universally inverter. Inverter compressors modulate output continuously between 20% and 100% capacity. The benefits relevant to sizing:
- Better at handling oversized units — runs longer at lower output instead of short-cycling
- Better dehumidification — longer run times pull more moisture from the air
- Quieter — low-fan-speed operation is barely audible
- Higher SEER ratings — 7–9 typical vs 3–4 for old on/off units
8. How we size at survey
Our survey for AC sizing takes 20–30 minutes per room:
- Measure room dimensions and floor area
- Identify all external walls and glazed openings; measure glass area; note orientation
- Note insulation level (build year is a good proxy; pre-1985 usually solid wall, post-1985 usually cavity)
- Discuss typical occupancy and heat-generating equipment
- Run the calc on a Mitsubishi or Daikin sizing tool (industry-standard)
- Recommend make/model with the right cooling capacity and SEER rating
- Confirm sound rating against room use (bedroom = quieter spec)
Free survey across our Kent coverage area — written quote within 48 hours, sized properly to the room rather than to a marketing chart. See our air conditioning service page for the design and install process, and our AC planning permission guide if you’re in a conservation area.