BTU Calculator

Summary

Calculate the BTU heating or cooling requirement for a room based on its dimensions, insulation, windows, occupants, and climate.

How it works

1. Enter the room length and width in feet, or type the total square footage directly.
2. Set the ceiling height — taller ceilings increase the volume of air to condition.
3. Choose an insulation quality that best describes your walls and attic.
4. Enter the total window area and select sun exposure (shaded, average, or sunny).
5. Set the number of regular occupants — each person adds roughly 600 BTU/hr of heat gain.
6. Select a climate zone to apply the correct design temperature difference.
7. Click Calculate to see the recommended BTU/hr for both cooling and heating.

Use cases

• Sizing a window air conditioner or portable AC unit for a bedroom or living room.
• Choosing the right mini-split capacity for a home addition or converted garage.
• Estimating heater size for a basement, workshop, or sunroom.
• Comparing how adding insulation or window film reduces required BTU capacity.
• Planning multi-zone ductless systems by calculating each room separately.
• Verifying that an existing HVAC unit is not grossly over- or under-sized.
• Teaching students the fundamentals of residential heat load estimation.
• Generating a quick BTU figure before requesting a contractor quote.

Frequently Asked Questions