FREE TOOL · HVAC ENGINEERS
Calculate the correct BTU and kW capacity for any UK room in 2026 — factoring insulation, ceiling height, climate zone, and window count for accurate quotes.
Each window adds ~1,000 BTU to the heat load.
BTU Required
3,600
BTU/hr
kW Equivalent
1.1
kilowatts
Install costs include supply, refrigerant pipework, electrical connection, drainage, and F-Gas commissioning. Prices are UK 2026 estimates — get 3 quotes from F-Gas certified engineers.
| Capacity | Suitable for | Install cost |
|---|---|---|
| 6,000 BTU / 1.8 kW | up to 75 m² | £500–£900 |
| 9,000 BTU / 2.6 kW | up to 113 m² | £700–£1,100 |
| 12,000 BTU / 3.5 kW | up to 150 m² | £900–£1,400 |
| 18,000 BTU / 5.3 kW | up to 225 m² | £1,200–£1,800 |
| 24,000 BTU / 7.0 kW | up to 300 m² | £1,500–£2,400 |
| 36,000 BTU / 10.5 kW | up to 450 m² | £2,000–£3,200 |
| 48,000 BTU / 14.0 kW | up to 600 m² | £2,800–£4,500 |
Correct HVAC sizing is the single most important factor in system performance. Undersized units run constantly and never reach temperature; oversized units short-cycle, wear out faster, and leave rooms feeling humid.
The starting point is 80 BTU per m² for a well-insulated UK room at standard 2.4m ceiling height. This figure is derived from CIBSE (Chartered Institution of Building Services Engineers) guidance for temperate UK climates. It assumes modern double-glazing and cavity wall insulation — the baseline for most post-2000 UK builds.
Insulation quality dramatically affects heat load. A poorly insulated pre-1970s property (solid walls, no loft insulation) can require 40% more capacity than a modern build (×1.4 multiplier). A Passivhaus-standard new build, by contrast, needs only 80% of the baseline (×0.8). The UK housing stock is predominantly older — the “average” multiplier of ×1.15 reflects the reality of partial cavity fill and single-glazed extensions common in the existing stock.
Every 0.1m of ceiling height above 2.4m adds 8 BTU per m² of floor area — so a Victorian property with 3.0m ceilings needs an additional 48 BTU/m² (6×8). Each standard window adds approximately 1,000 BTU to account for solar gain and conduction heat loss. South-facing rooms with large glazing should be sized at the higher end of the recommended unit range.
The calculator selects the smallest standard unit size that meets or exceeds the calculated heat load. Standard sizes jump in 3,000–6,000 BTU increments; the tool recommends the next size up to avoid undersizing. The install cost range reflects UK 2026 trade prices for supply and full commissioning by an F-Gas certified engineer.
WORKED EXAMPLE
James is an F-Gas engineer quoting to install air conditioning in a 25 m² living room in a 1990s semi in Leeds. The room has average insulation (×1.15), 2.4m ceilings, and 3 windows. He needs to recommend the right unit size and cost.
Base load
25 × 80 = 2,000 BTU
Insulation ×1.15
2,300 BTU
3 windows
+3,000 BTU
Total required
5,300 BTU
Recommended unit
6,000 BTU / 1.8 kW
Supply & install: £500–£900 · F-Gas commissioning included
Always confirm sizing with a site survey. South-facing rooms or rooms with skylights should size up to the next unit. Use the calculator above for your specific room dimensions.
Calculate household electricity consumption and whether a CU upgrade is needed.
Find the right hourly rate for your trade and region.
Calculate total labour cost including NI and travel.
See exactly how much you make on any job after all costs.
Set your sell price and see the true margin on materials.
Estimate roof replacement cost by area, pitch, and material.
Calculate exact litres and tin count for any room.
Common questions about HVAC sizing and installation costs.
The standard rule is 80–100 BTU per m² for a well-insulated UK room at a standard 2.4m ceiling height. You then adjust upward for poor insulation, higher ceilings, more windows, or hotter climates. Oversizing wastes energy and causes short-cycling; undersizing means the unit never reaches temperature on cold days. Always commission an F-Gas certified engineer to confirm sizing before purchasing equipment.
BTU (British Thermal Unit) is the traditional HVAC measurement; kW (kilowatts) is the metric equivalent used increasingly in UK spec sheets. 1 kW ≈ 3,412 BTU. A 12,000 BTU unit is roughly 3.5 kW. Both refer to the heating or cooling capacity of the unit, not the electricity it consumes.
A single split-system air conditioning / heat pump unit costs £700–£2,400 supplied and installed for a typical room. Larger multi-split systems covering multiple rooms range from £2,000–£5,000+. Installation includes refrigerant pipework, electrical connection, drain line, and commissioning. Always use an F-Gas certified engineer. Prices are 10–15% higher in London and the South East.
A standard UK bedroom of 12–16 m² with average insulation and a 2.4m ceiling requires approximately 1,000–1,600 BTU from the base calculation, but accounting for insulation quality and windows, a 9,000 BTU (2.6 kW) unit is typically recommended for a well-insulated double bedroom. Always size up slightly if the room is south-facing or has large glazed areas.
Open-plan areas of 30–50 m² with average UK insulation typically require an 18,000–24,000 BTU (5–7 kW) unit. If the kitchen is part of the open plan, add 4,000 BTU for cooking heat load. Multi-split systems allow one outdoor unit to serve multiple indoor heads across different rooms, which is more efficient than installing separate systems.
No — oversizing is a common and costly mistake. An oversized unit cools or heats too quickly, causing short-cycling (frequent start/stop). This wastes energy, increases wear on the compressor, and leaves rooms feeling clammy because the unit doesn't run long enough to dehumidify the air. Size accurately using a proper heat-load calculation.
Yes — in the UK, anyone handling refrigerants (including installation, maintenance, and decommissioning of split-system HVAC units) must hold F-Gas certification under UK F-Gas Regulation 517/2014. Certified engineers are listed on the REFCOM register. Uncertified installation is illegal and can void equipment warranties and insurance. This applies to both residential and commercial installations.
Air source heat pumps (ASHP) cost £7,000–£15,000 installed for a typical UK home, depending on capacity and complexity. The UK Government's Boiler Upgrade Scheme offers a £7,500 grant to eligible homeowners replacing fossil fuel heating, bringing the net cost to £0–£7,500. Ground source heat pumps cost £15,000–£35,000. Running costs are typically lower than gas boilers, especially on off-peak electricity tariffs.
STOP UNDERCHARGING
You've got the sizing. Now turn it into a professional quote in minutes. Sleepless Tradesman quotes, invoices and plans every job — for free, no card required.
No card · No commitment · Cancel anytime