Room Dimensions
Insulation R-Values
Windows
Floor
Note: Crawlspace/basement floor R-value assumes R-19 insulated floor assembly (fiberglass batts in 2×8 joists). An uninsulated floor over a vented crawlspace has an effective R-value of only R-4 to R-7 — if your floor is uninsulated, actual heat loss will be 3-5× higher than shown.
Temperatures
⚠️ Wall Area Note: This calculator distributes exterior walls evenly around the perimeter. For rectangular rooms where exterior walls are mostly on the long or short sides, the result may be off by up to 50%. For best accuracy on non-square rooms, calculate wall areas manually: multiply each exterior wall length × ceiling height.
About the Heat Loss Calculator
Understanding heat loss is essential for sizing heating systems and evaluating insulation effectiveness. This calculator uses the basic conductive heat loss formula to estimate how many BTUs per hour escape through walls, ceilings, and windows based on their R-values and the temperature difference between inside and outside. The result helps you determine the minimum heating capacity needed to maintain a comfortable indoor temperature.
The calculation considers three main paths of heat loss: walls, ceiling, and windows. Each component's contribution depends on its area and R-value. Windows are typically the weakest link in a building envelope, with standard double-pane windows having an R-value of only about 2, compared to R-13 or higher for insulated walls. This is why window area has a disproportionate impact on total heat loss.
This calculator provides a simplified estimate that covers conductive heat loss only. Real-world heat loss also includes air infiltration through gaps and cracks, which can account for 25-40% of total heating load in older homes. For a comprehensive heating system design, consult an HVAC professional who can perform a Manual J calculation that accounts for all heat loss and gain factors including infiltration, solar gain, and internal heat sources.