Air conditioner BTU calculator

Your room

TVs, ovens, computers, etc.

Use local solar and property data

Optional. We use your postcode to estimate July sun on an unshaded vertical window. Buildings and trees are not included, so you remain in control of the sun setting.

Result

Enter your room size to see a recommendation.

How we calculated this

The calculator applies a staged method to estimate the cooling load:

  1. Base load — 500 BTU/m² at a 2.4 m reference ceiling. Taller rooms scale the load linearly by ceiling / 2.4. Internal conservative estimator: base cooling load per m² of floor area at a 2.4 m ceiling for UK domestic rooms.
  2. Optional local context — PVGIS estimates solar radiation on a vertical window for the selected direction. EPC data can suggest an insulation starting point, but neither source measures the room or local shade.
  3. Sun exposure — multiplier applied: shaded 0.9, average 1, sunny 1.15. Internal estimator: solar-gain multiplier by window exposure.
  4. Occupancy — +600 BTU per occupant beyond the first. Internal estimator: per occupant beyond the first.
  5. Kitchen load — +4000 BTU added when room type is kitchen. Internal estimator: additional load allowance for kitchen heat sources.
  6. Appliances — +500 BTU per heat-producing appliance. Internal estimator: per heat-producing appliance (TV, oven, etc.).
  7. Insulation factor — good ×1, average ×1.05, poor ×1.15. Internal estimator: insulation/glazing-quality factor.
  8. Large glazing uplift — ×1.2 when large-glazing is ticked. Internal estimator: uplift for large glazing / conservatory-style rooms.
  9. BTU to kW conversion — divide BTU/h by 3412.14. Physical constant: BTU/h per kW.

Air conditioner room-size chart

Use this table as a first pass for a room with a standard 2.4 m ceiling and average heat gain. The calculator is more useful when the room is sunny, poorly insulated, occupied by several people or contains cooking and computer equipment.

Nominal capacityTypical starting room areaCooling kW
5,000 BTUUp to about 10 m²1.47 kW
7,000 BTUAbout 10–14 m²2.05 kW
9,000 BTUAbout 14–18 m²2.64 kW
12,000 BTUAbout 18–24 m²3.52 kW
14,000 BTUAbout 24–28 m²4.10 kW

These are starting ranges derived from the calculator's 500 BTU/m² base assumption, not guarantees of performance. A 9,000 BTU unit may suit an average 16 m² bedroom but struggle in a similarly sized loft room with strong afternoon sun.

BTU, kW and input power are different

BTU/h and cooling kW describe cooling capacity. They are not the electricity drawn from the socket. To estimate bills, find the unit's input watts or combine its cooling capacity with its EER, then use the running cost calculator.

Want to know what it will cost to run? Try the running cost calculator. Comparing two units? Use the efficiency comparison calculator.