BTU Calculator (HVAC Sizing)
Cooling and heating BTUs needed for a room.
Sized using the Energy Star rule of thumb: 20 BTU per square foot for cooling, 30 for heating, with adjustments for sun, insulation, and load. A whole-home Manual J takes climate zone and air leakage into account, so this is a rough check, not a contract sizing.
About
Enter room dimensions and conditions. The tool sizes cooling capacity in BTU per hour using the Energy Star method (20 BTU per sq ft, adjusted for sun, insulation, ceilings, occupancy, and kitchen heat). Heating estimate uses 30 BTU per sq ft.
How to use
- Enter room length, width, and ceiling height.
- Pick sun exposure and insulation.
- Add people and kitchen heat.
FAQ
How accurate is this?+
Good enough for a window unit or single-room mini split. Whole-house sizing should use a Manual J that accounts for climate zone, infiltration, and duct losses.
Why not just go bigger?+
An oversized AC short-cycles. It cools the air fast but doesn't run long enough to dehumidify, leaving a clammy room.