Measure the served space
Use internal length, width and ceiling height for the room the appliance will actually cool. Open doors, double-height areas and connected kitchens can turn an apparently small room into a much larger load.
Transparent calculator
How many BTUs do I need?
Start with room volume, then adjust for use, insulation, sun, glazing, occupants and heat-producing devices.
What common room inputs return
These examples use the same transparent CoolCompare formula as the calculator. Sun, glazing, insulation, people and equipment can move a real room into a different band.
| Room example | Conditions | Result |
|---|---|---|
| 3 × 3 m bedroom | 2.4 m ceiling; average insulation and sun | 2,500–3,500 BTU/h |
| 4 × 4 m living room | 2.4 m ceiling; average insulation and sun; two occupants | 5,500–7,000 BTU/h |
| 5 × 4 m home office | 2.4 m ceiling; high sun; two heat-producing devices | 8,000–10,000 BTU/h |
How this estimate works
The baseline uses room volume, then applies documented room-use, insulation, sun, glazing and open-plan factors. Occupants and high-heat devices add a modest allowance. The result is deliberately shown as a range.
Use a capacity range, then test the difficult conditions
Room volume is a transparent starting point, but glazing, orientation, occupancy and heat-producing equipment can move a real room away from the baseline.
Record heat gains
Sun through large glazing, poor roof insulation, people, computers and cooking add different loads. Enter what is typical at the time cooling is needed rather than choosing every high adjustment as a precaution.
Check delivery conditions
Published capacity assumes stated test conditions. A portable exhaust leak can return heat and outside air to the room, while a split system still needs a professional design for distribution, pipework and simultaneous use.
Limit: The range is not a professional cooling-load calculation or a guarantee of temperature, humidity or run time. Very unusual rooms and multi-room systems need a competent survey.
BTU sizing questions
How many BTUs do I need for my room?
Start with room volume, then adjust for room use, insulation, sun, glazing, occupants and heat-producing equipment. Use the resulting range as a shortlist, not a guaranteed design load.
Is a bigger BTU air conditioner always better?
No. An undersized unit may struggle, but an unnecessarily large portable model can add power input, size, weight and sound. Match capacity to the room conditions.
Is BTU/h the same as electrical watts?
No. BTU/h describes cooling output. Electrical input watts are a separate specification used to calculate running cost.
Continue with evidence
Your next checks
Move from the current answer to sizing, evidence and a clear commercial next step.
PARENT TOPICCooling calculatorsMove between capacity, cost and installation planning.DECISION TOOLCheck whether AC is the right routeThe result can recommend shading or airflow before a purchase.DECISION TOOLModel the electricity costUse a suitable product's verified input and your own tariff.GUIDERead the full sizing methodUnderstand baseline volume, adjustments and professional-design limits.GUIDECheck the heat-rejection routeCapacity cannot compensate for an unsuitable exhaust setup.NEXT ACTIONFilter models by the resulting rangeCompare only products with source-backed cooling capacity.