BTU Calculator
Right-Sizing a Single Room
Window units and ductless mini-splits are sold by BTU rating, and buying the wrong number means either a unit that runs constantly without cooling the room or one that short-cycles and leaves it clammy. This calculator uses a straightforward rule-of-thumb method — based on floor area, sun exposure, occupancy, and whether the room is a kitchen — to estimate the BTU output a single room actually needs.
The Formula
Total BTU = Base BTU + (Extra Occupants Beyond 2 × 600) + Kitchen Allowance
The baseline is 20 BTU per square foot at 8-foot ceilings, adjusted by a sun exposure factor (0.9 for shaded rooms up to 1.1 for very sunny ones). Rooms with more than two regular occupants add 600 BTU/hr per extra person, and kitchens get a flat 4,000 BTU/hr allowance for cooking heat.
A Worked Example
| Component | Value |
|---|---|
| Base BTU (250 × 20 × 1.0 × 1.0) | 5,000 BTU/hr |
| Extra Occupant Load | 0 BTU/hr |
| Kitchen Allowance | 0 BTU/hr |
| Total | 5,000 BTU/hr (0.42 tons) |
Where This Calculation Matters
- Window and portable AC shopping — matching a unit's rated BTU output to the room instead of guessing from the box.
- Ductless mini-split sizing — single-zone systems are selected the same way, room by room.
- Sunroom and attic conversions — heavy sun exposure meaningfully raises the BTU requirement over a shaded room of the same size.
- Kitchen additions — cooking equipment adds a real, fixed heat load that's easy to underestimate.
How to Use This Calculator
- Enter the Room Area in square feet.
- Enter the Ceiling Height in feet (defaults to 8).
- Select the Sun Exposure level: Shaded, Moderate, or Very Sunny.
- Enter the Number of Occupants (defaults to 1).
- Select whether the room is a Kitchen, then select Calculate.
Related Calculations
For a whole-building estimate rather than a single room, use the HVAC Load Calculator, or check the airflow a mini-split's BTU rating needs to deliver with the Airflow CFM Calculator.