Heat Loss Calculator (U × A × ΔT by Surface)
Work out a room or house's design heat loss surface by surface: walls, ceiling, floor and windows from their R-values or U-factors, plus air infiltration, at your indoor and outdoor design temperatures. Returns BTU per hour with watts, kilowatts and each surface's share.
Last updated: October 2026
Design Heat Loss
25,124 BTU/h
Formula below · 3 sources (energystar.gov, engineeringtoolbox.com, acca.org) · Updated Oct 2026
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About this calculator
Heat flows out of a building through every surface that separates warm air from cold, at a rate set by the surface's area, how well it insulates and the temperature difference across it. For each surface the conduction loss is Q = U × A × ΔT, where U is the overall heat transfer coefficient in BTU per hour per square foot per °F, A is the area in square feet and ΔT is the indoor design temperature minus the outdoor design temperature. Insulation is usually quoted as an R-value, the reciprocal of U: a wall rated R-13 has U = 1/13 = 0.077 BTU/h·ft²·°F, and an R-38 attic has U = 0.026. Windows are quoted as U-factors directly (about 1.0 for single glazing, 0.5 for ordinary double glazing, 0.3 for low-e double glazing).
Infiltration
Outside air leaking in through cracks, doors and vents has to be heated as well. With the leakage expressed as air changes per hour (ACH), the volume of outside air per hour is ACH × floor area × ceiling height, and heating it takes 0.018 BTU per cubic foot per °F (air's heat capacity, 0.24 BTU/lb·°F × 0.075 lb/ft³). The infiltration term is therefore 0.018 × ACH × volume × ΔT. Leaky older houses run at 1 ACH or more under winter conditions; a tight new house is nearer 0.25-0.35.
What the total is for
The sum of the surface losses and infiltration is the design heat loss, the heat a furnace, boiler or heat pump must deliver on the coldest design hour. Use it with the furnace size calculator (which converts output to the nameplate input via AFUE) or the heat load calculator for a quick whole-house estimate. The method is the one behind ACCA Manual J; the full procedure adds duct losses, solar and internal gains, and treats slabs and basements by perimeter rather than area, so for a slab-on-grade house leave the floor at 0 and expect the true loss to be a little higher.
SI units
Multiply BTU/h by 0.293 for watts. An imperial R-value times 0.1761 is the metric RSI (m²·K/W), and an imperial U-factor times 5.678 is W/m²·K.
Typical insulation values for the inputs (imperial units)
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| Element | Typical R-value | U-factor (1 ÷ R) |
|---|---|---|
| 2×4 wall, fibreglass batts | R-13 | 0.077 |
| 2×6 wall, fibreglass batts | R-19 to R-21 | 0.053 to 0.048 |
| Attic, blown insulation | R-38 to R-60 | 0.026 to 0.017 |
| Floor over crawlspace or garage | R-19 to R-30 | 0.053 to 0.033 |
| Single-glazed window | R-1 | 1.0 |
| Double-glazed window | R-2 | 0.5 |
| Low-e double-glazed window | R-3.3 | 0.3 |
| Triple-glazed window | R-5 | 0.2 |
How to use
Example: a single-storey 1,500 sq ft house with 1,200 sq ft of net wall at R-13, an R-38 attic, a floor over a crawlspace at R-19, 200 sq ft of double-glazed windows and doors (U 0.5), 8 ft ceilings, 0.5 ACH, kept at 70°F when it is 10°F outside (ΔT = 60°F). Walls: 1,200 ÷ 13 × 60 = 5,538 BTU/h. Ceiling: 1,500 ÷ 38 × 60 = 2,368. Floor: 1,500 ÷ 19 × 60 = 4,737. Windows: 200 × 0.5 × 60 = 6,000. Infiltration: 0.018 × 0.5 × (1,500 × 8) × 60 = 6,480. Total design heat loss = 25,124 BTU/h, or 7.36 kW, about 16.7 BTU/h per square foot: the windows and the air leakage, not the walls, are the biggest items. These are the defaults. A second example: 800 sq ft of R-19 wall, a 1,000 sq ft R-49 ceiling, no floor loss, 150 sq ft of low-e glass (U 0.3), 9 ft ceilings, 0.35 ACH, 70°F inside and 0°F outside gives 2,947 + 1,429 + 0 + 3,150 + 3,969 = 11,495 BTU/h.
Frequently asked questions
What is the heat loss formula?
For each surface, heat loss = U × A × ΔT: the U-factor (BTU/h per square foot per °F) times the area (square feet) times the indoor-minus-outdoor temperature difference (°F). U is 1 divided by the R-value. Add the surfaces together, then add infiltration, 0.018 × air changes per hour × building volume (cubic feet) × ΔT. The result is in BTU per hour; multiply by 0.293 for watts.
How do I convert an R-value to a U-value?
U = 1 ÷ R. An R-13 wall has U = 0.077, R-19 is 0.053, R-38 is 0.026 and R-49 is 0.020 BTU/h·ft²·°F. Going the other way, a window with U-factor 0.5 is R-2. To convert imperial R to the metric RSI (m²·K/W), multiply by 0.1761; to convert an imperial U-factor to W/m²·K, multiply by 5.678.
Which outdoor temperature should I use?
The heating design temperature for your location, usually the 99% winter value from ASHRAE climate data: the temperature that is exceeded 99% of the hours in a typical year, so the system falls short for about 88 hours a year at most. It is not the record low. Using a record low oversizes the equipment, which then short-cycles and runs less efficiently on ordinary winter days.
What R-values are typical?
A 2×4 wall with fibreglass batts is about R-13 (R-15 with high-density batts), a 2×6 wall R-19 to R-21, an insulated attic R-38 to R-60 depending on climate, and a floor over a crawlspace or garage R-19 to R-30. Windows are rated by U-factor: roughly 1.0 for single glazing, 0.5 for standard double glazing, 0.3 for low-e double glazing and 0.2 for triple glazing. The R-value of a wall is the whole assembly, so framing, sheathing and air films shift the number a little either way.
Why is infiltration such a large share?
Because the whole house volume is exchanged with outdoor air several times a night. At 0.5 ACH, the 12,000 cubic foot house in the example brings in 6,000 cubic feet of 10°F air every hour, and heating that to 70°F takes 6,480 BTU/h, more than the walls. Air sealing (weatherstripping, caulking, sealing attic penetrations) is often the cheapest way to cut heat loss; a blower-door test measures the actual leakage.
Is this a Manual J calculation?
It is the core of one: ACCA Manual J uses the same U × A × ΔT and infiltration terms, surface by surface. The full procedure also counts duct losses, internal and solar gains (which matter for the cooling load), basement walls below grade and slab edges by perimeter, and applies tested infiltration rates. Use this calculator to understand where the heat goes and to size roughly; have a Manual J run before buying equipment.
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Sources & references
- The Engineering ToolBox — Heat transmission loss through building elements (H = U A dt)
- The Engineering ToolBox — Heat loss from buildings: transmission, ventilation and infiltration
- ACCA Manual J — Residential Load Calculation
- ENERGY STAR — Seal and insulate: methodology and recommended insulation levels
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