Heat Loss Calculator
Calculate the exact heat loss for any room or building to properly size your heating system. Enter room dimensions, window area, insulation levels for walls, ceiling, and floor, outdoor design temperature, indoor target temperature, and air infiltration rate. Get detailed breakdown of losses through each building element plus total heat loss in BTU/hr and kW. Includes a recommended heater size with 15% safety margin. Essential for HVAC design, heater selection, and energy audit analysis.
Sum of all window areas in the room
U-value (lower is better insulation)
U-value of walls (lower is better)
U-value of ceiling/roof (lower is better)
U-value of floor over basement/crawlspace
99% winter design temperature for your area
Target indoor temperature
Air infiltration rate — affects heat loss significantly
How do I calculate heat loss for a room?
Heat loss is calculated by summing losses through each building element: walls, windows, ceiling, floor, and air infiltration. Formula: Q = U × A × ΔT for each surface. Q = heat loss in BTU/hr, U = U-value (1/R-value), A = area in sq ft, ΔT = indoor-outdoor temperature difference. For infiltration: Q = 1.08 × CFM × ΔT, where CFM = room volume × ACH ÷ 60. Total heat loss = sum of all surface losses + infiltration loss. This determines your required heater size.
What is a good R-value for walls?
Recommended R-values by climate zone: Zone 1-2 (hot): R-13 to R-19. Zone 3 (warm): R-20 to R-25. Zone 4 (moderate): R-25 to R-30. Zone 5 (cold): R-30 to R-38. Zone 6-7 (very cold): R-38 to R-49. Existing 2×4 walls can achieve R-13 to R-15 with fiberglass. 2×6 walls achieve R-19 to R-25. Spray foam achieves the highest R-value per inch (R-6 to R-7). Adding exterior rigid foam board can boost R-value without removing drywall.
How does air infiltration affect heat loss?
Air infiltration can account for 25-40% of total heat loss in a typical home. Every CFM of cold air entering requires energy to heat. A leaky home (1.0 ACH) loses 2-3× more heat through infiltration than a tight home (0.35 ACH). Sealing air leaks is often the most cost-effective energy improvement. Common leak sources: windows, doors, attic hatches, recessed lights, electrical outlets, plumbing penetrations, and rim joists. Air sealing can reduce heating costs by 20-30%.
What size heater do I need based on heat loss?
Once you know total heat loss in BTU/hr, select a heater that matches within 10-20%. Oversizing causes short-cycling and poor humidity control. Undersizing runs continuously and never reaches set temperature. Electric heaters: 1 kW = 3,412 BTU/hr. Gas/propane: rated in BTU/hr output, not input (account for efficiency). A 90% efficient 50,000 BTU furnace delivers 45,000 BTU/hr. Rule of thumb: 25-30 BTU per sq ft for average insulation, but the heat loss calculation is far more accurate.