How to calculate heat loss in a house

1. What heat loss actually measures

Every building slowly leaks heat — through walls, windows, the ceiling and gaps that let cold air in. Heat loss is the rate at which a building loses thermal energy to the outdoors, measured in watts (W) or BTU per hour (BTU/hr). It is the key number for sizing a furnace, boiler or heat pump: the heating system has to supply at least as much heat as the building loses during the coldest conditions it is designed for.

Heat moves through a solid material by conduction: the rate depends on how good the insulation is (its R-value), the area of the surface and the difference in temperature across it. Uncontrolled air leakage — infiltration — adds a separate heat-loss path that bypasses the insulation entirely. A complete heat-loss estimate covers both.

2. The core formula: Q = U × A × ΔT

For any opaque surface (wall, ceiling, floor), heat loss by conduction is:

U-value is the reciprocal of R-value. In the SI (metric) system, U [W/m²·K] = 5.678 ÷ R-value (imperial ft²·°F·h/BTU). So a wall with an imperial R-13 has a U-value of 5.678 ÷ 13 ≈ 0.437 W/m²·K. The same relationship in metric works directly: U [W/m²·K] = 1 ÷ RSI [m²·K/W].

For windows and glazed areas, manufacturers publish the U-factor directly (BTU/h·ft²·°F in imperial, or W/m²·K in metric). To convert an imperial window U-factor to SI: U_SI = U_imperial × 5.678. Typical double-pane windows have a U-factor of about 0.30–0.48 BTU/h·ft²·°F (1.7–2.7 W/m²·K).

3. Worked example: a two-storey house

Take a 140 m² two-storey house with 2.4 m ceilings, average insulation (R-13 walls, R-30 attic, U-0.35 windows), 20% of gross wall area in windows, a 30°C indoor–outdoor temperature difference and an ACH of 0.6.

In BTU/hr: 5,993 × 3.412 ≈ 20,400 BTU/hr. With a 20% safety factor, the suggested equipment size is about 7.5 kW / 25,000 BTU/hr.

Notice that infiltration contributes about 33% of the total — and windows, at 32 m², contribute about 32% despite making up only 20% of the wall area, because their U-value is roughly four times higher than the insulated wall's. These are the two paths most worth targeting in an energy retrofit.

4. The infiltration formula

Air leakage (infiltration) bypasses the insulation entirely — cold outside air replaces warm inside air, carrying heat with it. The ASHRAE sensible infiltration formula is:

The ACH rate (air changes per hour) describes how many times per hour the entire air volume of the house is replaced by outside air. Modern well-sealed homes may achieve 0.35 ACH or less; older drafty homes can run above 1.0. A blower-door test is the accurate way to measure it; the typical assumption for planning is 0.5–0.6 ACH.

In imperial units the same formula is Q_inf (BTU/hr) = 0.018 × ACH × V (ft³) × ΔT (°F). Both versions give the same result — just different unit sets.

5. Geometry: estimating the envelope areas

To apply the formula you need the area of each surface. For a quick estimate:

• Ceiling area ≈ conditioned floor area (one storey) or floor area ÷ storeys (the attic sits above the top floor only).
• Gross wall area = perimeter × (stories × ceiling height). For a square floor plan, perimeter = 4 × √(footprint), where footprint = total conditioned floor area ÷ storeys. For a known footprint, measure the perimeter directly.
• Window area = gross wall area × window fraction. Typical new construction is 15–25% of gross wall area. Measure individual windows for accuracy.
• Net wall area = gross wall area − window area (doors can be treated as windows or as insulated walls depending on door type).
• Volume = total conditioned floor area × ceiling height (equivalent to footprint × ceiling height × storeys).

The heat loss calculator on this site uses a square-floor-plan approximation for the perimeter, which is accurate enough for planning and matches what a typical sizing rule of thumb requires. For an irregular or complex floor plan, add the actual wall segments.

6. From heat loss to furnace or heat pump size

The calculated heat loss tells you the rate at which the house loses heat at the design temperature difference. A furnace, boiler or heat pump has to supply at least that much heat output to maintain comfort. In practice, HVAC engineers apply a safety factor — typically 15–25% — to account for unusually cold snaps, duct losses and start-up loads:

The calculator uses 1.2 (20% safety factor) and rounds to the nearest 5,000 BTU/hr (or 0.5 kW) as a rule of thumb. For heat pumps, also account for the COP: a 12 kW heat pump running at COP 3 draws only 4 kW of electricity to deliver 12 kW of heat. Oversizing a unit wastes capital cost and causes short-cycling — do not simply double the calculated load to “be safe.”

For an installed system, a licensed HVAC contractor performs a Manual J load calculation (ACCA Manual J, 8th edition), which uses local climate data (design temperatures from ASHRAE Fundamentals), window orientation and shading, thermal mass and duct losses — all factors that a simplified formula does not capture. The calculation here is a planning tool, not a design specification.

For a related sizing tool, try the BTU calculator (rule-of-thumb sizing for a room or zone), or the R-value calculator to check the thermal resistance of a wall or roof assembly before running the heat-loss calculation. More HVAC tools are on the HVAC hub.

Common questions

How do you calculate heat loss in a house?

Heat loss through each part of the building envelope is Q = U × A × ΔT, where U is thermal conductance (1 ÷ R-value), A is the surface area and ΔT is the indoor–outdoor temperature difference. Calculate that separately for walls, ceiling, windows and floor, then add infiltration heat loss (0.33 × ACH × volume × ΔT). Sum all four and you have the total heat loss. Use the heat loss calculator to run these numbers without doing the arithmetic by hand.

What is the formula for heat loss through a wall?

Q (watts) = U × A × ΔT, where U = 5.678 ÷ R-value (converting imperial R to SI U-value), A is the net wall area in m², and ΔT is the indoor–outdoor difference in kelvin (same magnitude as °C). For example, a 20 m² wall with R-13 insulation and a 20°C temperature difference: U = 5.678 ÷ 13 ≈ 0.437 W/m²·K, so Q = 0.437 × 20 × 20 ≈ 175 W.

What is a typical heat loss for a house?

A typical 150 m² (1,600 sq ft) two-storey house with average insulation and a 20–30°C (35–55°F) temperature difference loses roughly 4–9 kW (14,000–30,000 BTU/hr). Older or poorly insulated houses can reach 10–15 kW; well-insulated newer homes may need only 3–5 kW. The dominant variables are floor area, climate (temperature difference) and envelope insulation quality.

How does R-value affect heat loss?

Since U = 1 ÷ R, doubling the R-value halves the U-value and halves the conductive heat loss through that assembly. Going from R-13 to R-26 walls cuts wall conduction losses in half — but because windows, ceiling and infiltration are separate heat-loss paths, the saving on the whole-house total is proportionally smaller. You can see this in the breakdown the heat loss calculator provides.

What size furnace or heat pump do I need?

A rough starting point is total heat loss × 1.2 (a 20% safety factor), rounded up to the next available equipment size. So 10 kW of calculated heat loss points to a 12–12.5 kW unit. For an accurate sizing a licensed HVAC contractor performs a Manual J load calculation, which accounts for local climate data, window orientation, shading and duct losses.