Heating and Cooling Load Calculator
Roughly what capacity a room needs to heat or cool — and why oversizing is a real mistake.
A rough sizing rule multiplies floor area by a factor for the climate and the insulation.
How the heating and cooling load calculator works
A rough sizing rule multiplies floor area by a factor for the climate and the insulation. It is genuinely rough: a proper heat-load calculation accounts for glazing, orientation, ceiling height, air changes and occupancy, and can differ from this by a third either way.
The error worth avoiding is oversizing. An oversized air conditioner cools the air quickly, satisfies the thermostat and switches off before it has removed much humidity, leaving a room that is cold and clammy and cycling constantly. Short cycling also wears the compressor. Undersizing is usually the safer error.
Use this to sanity-check a quote, not to replace one.
Formula: capacity ≈ floor area × factor for climate and insulation × height adjustment
Worked examples
| Inputs | Estimated capacity | Note |
|---|---|---|
| A 30 m² living room | 4.05 kW | about 4 kW |
| Poorly insulated with west sun | 6.83 kW | much larger |
| A small well-insulated bedroom | 1.12 kW | about 1.1 kW |
FAQFrequently asked questions
What size air conditioner do I need?
Roughly 135 W per square metre in a temperate climate with average insulation, adjusted for ceiling height, glazing and orientation.
Is bigger safer?
No. An oversized unit satisfies the thermostat before removing humidity, leaving the room cold and clammy, and short-cycles the compressor.
How accurate is this?
Within about 15% at best. A proper heat-load calculation accounts for glazing, air changes and occupancy, and is worth getting for anything expensive.
What is a BTU rating?
BTU per hour, the US convention for the same capacity. One kilowatt is 3,412 BTU/h.
Does this cover heating too?
Roughly. Heating loads are usually similar or a little higher in cold climates, but heat pumps lose capacity as the outside temperature drops.
Where these figures come from
- International Energy Agency — Energy Efficiency — appliance consumption and standby power context
- Ofgem — the UK energy regulator
Last checked: September 2026. Appliance power figures are typical ratings; check the label on your own equipment, which is authoritative for it.