Air conditioning size calculator
Wondering “what size air con do I need?” Enter your room and get the recommended cooling capacity in kW and BTU — using the same heat-load engine UK installers run on Vento.
A guide estimate of the room's cooling load, using the 0.10–0.15 kW/m² method set out below plus your glazing, orientation and occupancy. A professional survey accounts for glazing area, shading, shading from neighbours and equipment heat a form can't see.
Recommended capacity
3.5 kW
≈ 11,942 BTU/hr
Taking this to an installer
Vento is the software UK installers design and quote on — we don't install ourselves. Take these figures to your local installers: the capacity above, your room size and which way the windows face is most of what they need to quote like for like.
What should it cost to install? What to ask an installerHow to size air conditioning for a room
Correctly sizing an air conditioner means matching the unit's cooling capacity to the room's heat load — the heat it gains from the sun, the building fabric, the people and the equipment inside it. Get it right and the system cools quickly, runs efficiently and lasts. Get it wrong and you either never reach temperature or waste money short-cycling.
The main factors
- • Floor area & ceiling height — the volume of air to be conditioned.
- • Insulation — a solid-wall period property loses far more than a modern build.
- • Glazing & orientation — south- and west-facing glass adds significant solar gain.
- • Occupancy & equipment — people, computers and kit all add heat.
A common rule of thumb is roughly 0.10–0.15 kW per m² for a typical UK room, but that hides a lot — a sunny conservatory and a shaded north-facing bedroom of the same size need very different units. The calculator above starts from that rule and then adjusts for glazing, orientation, occupancy, insulation and ceiling height, so its answer and this rule are the same method rather than two different ones.
Note: "Manual J" is the US term for a room-by-room load calculation. UK searchers usually ask "what size" or "how many BTU". One thing worth knowing: a Manual J figure is normally a heating load, calculated against a winter design temperature. Cooling is a different sum — it is driven by sun, people and equipment rather than by how much heat escapes — so this page sizes for cooling, which is what an air conditioner has to do in August. For the winter heating figure, room by room, use the heat loss calculator.
BTU to kW reference table
A quick guide to common unit sizes and the rooms they typically suit (1 kW ≈ 3,412 BTU). It is built on the same 0.10–0.15 kW/m² method as the calculator, for an ordinary room with a couple of windows — so a heavily glazed, south-facing or poorly insulated room of the same size will land a size up.
| Typical room | kW | BTU/hr |
|---|---|---|
| Small bedroom or office (up to ~15 m²) | 2.0 | ~7,000 |
| Bedroom or small lounge (~15–20 m²) | 2.5 | ~9,000 |
| Living room (~20–30 m²) | 3.5 | ~12,000 |
| Large lounge / open-plan (~30–45 m²) | 5.0 | ~17,000 |
| Large open-plan or shop (~45–60 m²) | 7.1 | ~24,000 |
| Very large space (~60–85 m²) | 10.0 | ~34,000 |
Frequently asked questions
What size air conditioner do I need for a room?
As a rough UK guide, allow around 0.10–0.15 kW of cooling per square metre of floor area for a typical, reasonably insulated room — so a 20 m² living room works out at roughly 2.5–3.0 kW of cooling, which is a 3.5 kW unit, the nearest standard size up. Sunny, poorly insulated or heavily glazed rooms need more. The calculator above works to exactly that method, adjusting for your room dimensions, glazing, orientation, occupancy and insulation.
How many BTU do I need for air conditioning?
BTU is just another way of stating capacity: 1 kW ≈ 3,412 BTU/hr. A 3.5 kW unit is roughly 12,000 BTU. Enter your room details above and the calculator shows both, taken from the same figure so they always agree.
Is it better to oversize or undersize an air conditioner?
Neither. An undersized unit runs flat out and never cools the room; an oversized one short-cycles, wastes energy and controls humidity poorly. The aim is a capacity matched to the room's actual heat load — which is exactly what a proper survey provides.
Does this replace a professional survey?
No — it's a guide estimate. A qualified installer's survey measures glazing area, shading, air-tightness, internal heat gains and pipe routing that a quick form can't capture, and is required to specify and price the installation correctly.
Installers: size every room automatically
Vento runs a full room-by-room heat load as you survey — then designs, quotes and takes payment on the same platform. Try it free for 14 days.
Start your 14-day trial14-day free trial