Heat Loss Calculator: BTU, kW and Furnace Size
Calculate heat loss through walls, ceiling, windows, doors, and floor using design temperatures and R-values. Infiltration adds to the total. The tool returns BTU, kW, recommended furnace size, and annual heating cost in imperial or metric units.
Heat loss is the rate at which a building loses heat to the outside. It’s measured in BTU per hour or watts, and it depends on the temperature difference, the area and R-value of every surface, and how much air leaks through gaps. This heat loss calculator takes those inputs and returns the total load, plus a recommended furnace size and annual heating cost. Contractors use it to size equipment. Homeowners use it to check whether a quote makes sense. Enter design temperatures, wall and ceiling areas, R-values, window and door details, and infiltration.
How to Use the Heat Loss Calculator
Pick the unit system: imperial (ft², °F, BTU) or metric (m², °C, W).
Enter the indoor and outdoor design temperatures. The tool calculates the difference.
Set a safety margin. Twenty percent is a common default.
Type the net area and R-value for walls, ceiling, windows, doors, and floor.
For slab-on-grade, add the perimeter and F-factor. Set both to zero if not applicable.
Choose an infiltration method: air changes per hour (ACH), airflow rate (CFM), or none.
Enter the conditioned volume if using ACH, or the airflow rate if using CFM.
Pick a fuel type and cost, then add equipment, labor, and additional cost.
The Summary tab leads with total heat loss and recommended furnace size. Detailed Breakdown lists every component’s heat loss and U-value.
How the Heat Loss Calculator Formula Works
The calculator adds conduction losses through every envelope component to the infiltration loss. Conduction uses U-value, area, and temperature difference. Infiltration uses airflow and temperature difference.
Formula: Temperature difference (ΔT) = Indoor temp − Outdoor temp
Formula: U-value = 1 ÷ R-value
Formula: Conduction loss = U × Area × ΔT
Formula: Slab loss = Perimeter × F-factor × ΔT
Formula: Infiltration loss = 1.08 × CFM × ΔT
Formula: Total heat loss = Conduction + Infiltration
Formula: Design load = Total heat loss × (1 + Safety margin ÷ 100)
Formula: Recommended furnace size = Smallest standard size ≥ Design load
The tool converts R-values and F-factors if you work in metric. An R-value of 13 h·ft²·°F/BTU becomes about 2.29 m²·K/W. The 1.08 multiplier in the infiltration formula converts CFM and ΔT to BTU/h for standard air.
Standard furnace sizes run from 40,000 to 250,000 BTU/h. The tool picks the smallest one that meets the design load. It also estimates annual heating cost from the total heat loss, full-load hours, and fuel cost per million BTU.
Worked Example: A 2,000 ft² Home in a Cold Climate
Say you’re calculating heat loss for a 2,000 ft² home. Indoor design temperature is 70°F. Outdoor design temperature is 10°F. Safety margin is 20%. The envelope has 1,200 ft² of walls at R-13, 1,500 ft² of ceiling at R-30, 200 ft² of windows at R-2, 60 ft² of doors at R-3, and 1,500 ft² of floor at R-19. Slab perimeter is 180 ft with an F-factor of 0.6. Infiltration is 0.5 ACH with a volume of 16,000 ft³. Natural gas costs $12 per million BTU. Full-load hours are 1,800.
ΔT: 70 − 10 = 60°F
Walls: U = 1/13 = 0.0769. Q = 0.0769 × 1,200 × 60 = 5,538 BTU/h
Ceiling: U = 1/30 = 0.0333. Q = 0.0333 × 1,500 × 60 = 3,000 BTU/h
Windows: U = 1/2 = 0.5. Q = 0.5 × 200 × 60 = 6,000 BTU/h
Doors: U = 1/3 = 0.333. Q = 0.333 × 60 × 60 = 1,200 BTU/h
Floor: U = 1/19 = 0.0526. Q = 0.0526 × 1,500 × 60 = 4,737 BTU/h
Slab: Q = 180 × 0.6 × 60 = 6,480 BTU/h
Conduction total: 5,538 + 3,000 + 6,000 + 1,200 + 4,737 + 6,480 = 26,955 BTU/h
Infiltration: CFM = 16,000 × 0.5 / 60 = 133.3 CFM. Q = 1.08 × 133.3 × 60 = 8,640 BTU/h
Total heat loss: 26,955 + 8,640 = 35,595 BTU/h
Total in kW: 35,595 × 0.293071 / 1000 = 10.43 kW
Design load: 35,595 × 1.20 = 42,714 BTU/h
Recommended furnace: 50,000 BTU/h
Annual heating cost: 35,595 × 1,800 / 1,000,000 × $12 = $768.85
The 50,000 BTU furnace covers the 42,714 BTU design load. The windows and slab are the biggest single losses, which is typical for an older home with single-pane glass and an uninsulated slab edge.
