Estimate the air conditioning capacity a room needs, based on UK property type, build era and room use. Tick the rooms you want cooled and refine any figure that doesn't match your home.
Applies to all rooms below — override per room if one's different (e.g. a vaulted loft conversion).
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It's a quick-mode rule-of-thumb calculation, not a full CIBSE Guide A or Manual J load survey — it's meant to give a sensible ballpark before a proper site visit, the same way our BTU heating calculator estimates radiator output.
Each room's floor area is multiplied by a base heat-gain figure (watts per square metre) set by the build era you choose, then adjusted for the property's typical exposure to the sun and outside walls, the room's glazing, ceiling height above 2.4m, how many people typically use the room, and whether it has significant appliance or equipment heat (a kitchen or home office, for example). The result is rounded up to the nearest commonly sold unit size — never down, since an undersized unit will run constantly and never reach setpoint on the hottest days.
Older solid-wall properties (pre-1930) typically lose and gain heat faster than a cavity-wall or modern Part L-compliant home, so they need more cooling capacity for the same size room. Detached properties have more exposed walls and windows than a mid-terrace or flat, which also increases solar heat gain. Changing either of these shifts every room's estimate at once.
No. This is an estimate to help you have an informed first conversation with an installer, not a substitute for a proper site survey. Room dimensions, glazing and occupancy defaults are typical UK averages for each room type — use "Refine this room" to enter your own numbers if you know them. At three rooms or more, a multi-split system is usually more cost-effective than separate single units, but the exact outdoor unit should always be confirmed against the manufacturer's combination chart by whoever installs it.