Heating BTU Calculator
Estimate room-by-room heating heat loss using your building-component and airflow data.
Use a winter design temperature appropriate for your location. No climate value is guessed automatically.
Room 1
Floor area: Not entered
Building envelope
Enter each component's area and its whole-assembly U-factor. Enter gross exterior wall area; window and door areas are subtracted automatically.
Exterior walls
Windows
Exterior doors
Ceiling / roof
Floor
Air leakage & ventilation
YOUR ESTIMATE
Estimated total heating heat loss
—BTU/h
— kW
Envelope transmission—
Infiltration—
Mechanical ventilation—
Room-by-room breakdown
This is a component-based heat-loss estimate using the values you entered. It is not an ACCA Manual J calculation, a full equipment-sizing result, or a substitute for professional design.
How to Use the Calculator
- Choose US or Metric units and enter indoor and outdoor design temperatures.
- Add each room and enter the gross exterior wall area plus window and door areas separately.
- Enter the U-factor for each component that has an area. Use whole-assembly values, not insulation-only R-values.
- Enter design infiltration airflow and mechanical outdoor airflow separately. Do not count the same air twice.
- Results update automatically as you enter or change valid inputs.
Assumptions and Limitations
- This tool calculates steady-state sensible heat loss for entered components using Q = U × A × ΔT and an approximate air heat-loss factor.
- U-factors must represent the complete relevant assembly. The calculator does not generate default construction U-factors or infer local design temperatures.
- Gross wall area is reduced by entered window and exterior-door areas. Do not enter net wall area.
- Infiltration airflow must be calculated separately using a suitable method. The calculator does not estimate infiltration from ACH, climate, wind, or building leakage.
- Mechanical ventilation airflow must exclude air already included in infiltration. Ground-contact floors, thermal bridges, ducts, latent loads, and complex boundary conditions may need separate engineering methods.
- This tool is not represented as ACCA Manual J compliant and does not select a furnace or heat pump.
