Find out what size air conditioning or heating a room actually needs. Use Quick mode for an instant estimate from room dimensions and usage, or Detailed mode for a full room heat gain and heat loss calculation covering fabric, glazing, solar gain, occupancy, equipment and ventilation, following CIBSE and ASHRAE methods.

Why unit sizing matters

An undersized air conditioning unit runs continuously, never reaches setpoint on the hottest days and wears out early. An oversized unit is not the safe option people assume: it short cycles, dehumidifies poorly, costs more to buy and often ends up noisier than a correctly sized smaller unit modulating gently. Correct sizing starts with an honest load calculation, not a rule of thumb from a brochure.

Quick mode and Detailed mode

Quick mode gives a fast indicative load from room size, usage type and glazing exposure. It is deliberately conservative and is intended for early conversations: is this a 2.5 kW room or a 7 kW room?

Detailed mode runs a proper steady state room calculation in the style of CIBSE and ASHRAE methods:

• Fabric gains and losses through walls, roof, floor and glazing using U values, with typical UK construction presets or your own figures
• Solar gain through glazing by orientation and shading
• Internal gains from occupants, lighting and equipment, with sensible defaults for offices, server rooms, retail and residential use
• Ventilation and infiltration loads from fresh air rates or air change assumptions
• Winter heat loss calculated alongside cooling load, so heat pump systems can be checked for both duties from one set of inputs

The result is a sensible design load with the main contributors broken out, so you can see whether the answer is being driven by glazing, occupancy or equipment and challenge the inputs that matter.

From load to unit selection

The load tells you what the room needs; the right unit depends on distribution, noise limits, pipe runs and aesthetics. Once you have a figure, use the "Get unit advice" button in the tool and our technical team will recommend suitable equipment from our range, including options where the visual design of the installation matters as much as the duty.

Methods and standards

The detailed calculation follows the steady state room heat gain and heat loss approach of CIBSE Guide A and the ASHRAE Handbook, Fundamentals, using UK design conditions. Outputs are indicative design loads for equipment selection and budgeting. They are not a substitute for a full dynamic thermal model where one is required, and final selections should be confirmed by a competent engineer against project specific conditions.

Who this tool is for

Homeowners and developers wanting a sanity check before accepting a quote. Installers producing consistent, defensible sizing on every survey instead of rules of thumb. Architects and designers checking early whether a comfort cooling strategy is viable for a glazed space. Engineers running quick feasibility loads before detailed modelling.

Frequently asked questions

What size air conditioner do I need for my room? It depends on far more than floor area: glazing, orientation, insulation, occupancy and equipment all matter. As a very rough UK starting point, a typical well insulated living room needs around 100 to 150 W per square metre of cooling, but a south facing glazed room can need double that. Use the calculator above for a figure based on your actual room.

Is it better to oversize an air conditioning unit? No. An oversized unit short cycles, switching on and off rapidly instead of modulating. This gives worse temperature control, poor dehumidification, more noise and a shorter compressor life. Size the unit to the calculated load with a modest margin, not double it.

How many BTU is a kW? 1 kW is approximately 3,412 BTU/h. So a 3.5 kW air conditioning unit is roughly 12,000 BTU, often sold as a "12000 BTU" unit. The calculator reports loads in kW with a BTU conversion for comparing against unit brochures.

What is the difference between heat gain and heat loss? Heat gain is the summer cooling load: solar energy, warm outside air, people and equipment adding heat that the system must remove. Heat loss is the winter heating load: heat escaping through the building fabric and ventilation that the system must replace. Reversible heat pump systems should be checked against both, which this calculator does in Detailed mode.

Do I need a full CIBSE calculation for building control? For most residential comfort cooling installations, an indicative load calculation supporting a competent equipment selection is sufficient. Larger commercial projects, landlord requirements or unusual buildings may require full dynamic simulation. This tool provides the former and tells you honestly when a project needs the latter.

Related tools

• [Refrigerant Pipe Sizing Calculator]: size the pipework for the unit you select
• [Duct Sizing Calculator]: size ductwork for ducted systems
• [Part F Ventilation Calculator]: check ventilation requirements alongside the load