Estimate the air-conditioner BTU needed to cool a room.
Calculated instantly in your browser.
How do you calculate the BTU needed to cool a room?
Base ≈ 20 BTU per square foot, ±10% for very sunny or shaded rooms, +600 BTU per occupant above two, and +4,000 BTU for a kitchen. A unit too small struggles to cool; too large cycles off before removing humidity. A 300 sq ft room needs about 6,000 BTU.
Understanding your result
A unit that is too small struggles to cool, while one that is too large cools quickly but cycles off before removing humidity. These are general guidelines — high ceilings, poor insulation or a hot climate need more capacity.
Formula and method
Base ≈ 20 BTU per square foot, ±10% for very sunny or shaded rooms, +600 BTU per occupant above two, and +4,000 BTU for a kitchen.
Assumptions and limitations
These figures are rough planning guidelines based on floor area with simple adjustments, not a formal load calculation. They assume standard ceiling height and average insulation, and do not account for window area, orientation, humidity, climate zone or building airtightness. For sizing a permanent installation, have a professional Manual J or equivalent heat-load survey performed.
Worked example
A 300 sq ft room needs about 6,000 BTU — or roughly 10,000 BTU if it is a kitchen.
How to use this tool
- Enter the room area.
- Set sun exposure, occupants and whether it is a kitchen.
- Read the recommended BTU.
Common mistakes to avoid
- Ignoring sun exposure and heat-producing appliances.
- Oversizing, which leaves the room humid.
About the BTU Calculator
The BTU Calculator estimates the air-conditioner cooling capacity a room needs, based on its floor area with adjustments for sun exposure, the number of occupants and whether it is a kitchen.
Who should use this tool
Homeowners and renters choosing the right size of air conditioner.
Benefits
- BTU from room area and conditions.
- Adjusts for sun, occupants and kitchens.
- Shows the equivalent in tons and kW.
- Square feet or square metres.
Practical use cases
- Sizing a window or split AC unit.
- Checking if an existing unit is big enough.
- Comparing rooms with different sun exposure.
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Frequently asked questions
How many BTU do I need per square foot?
About 20 BTU per square foot as a starting point, adjusted up for sun, occupants, kitchens, high ceilings or poor insulation.
What is a ton of cooling?
One ton equals 12,000 BTU per hour. The calculator shows the tons equivalent alongside the BTU figure.
Does a higher ceiling change the BTU I need?
Yes. The area-based estimate assumes a standard ceiling of roughly 2.4 metres or eight feet. A room with a higher ceiling holds more air volume to cool, so it needs proportionally more capacity. As a guide, scale the estimate up in line with the extra ceiling height.