Wall Heat Transfer Calculator

What this calculator does

This tool calculates steady-state heat transfer through a plane wall, panel, barrier or insulation assembly. It combines the inside surface resistance, conduction through one or more solid layers, and the outside surface resistance into a single overall U-value.

Use it to estimate heat loss or heat gain, compare insulation arrangements, identify the dominant thermal resistance, and calculate wall-surface and layer-interface temperatures. The tool supports both SI and US customary units and keeps the calculation internally normalized to SI to avoid conversion inconsistencies.

This is intentionally separate from the EngineerHub Heat Exchanger Calculator. It does not use LMTD, effectiveness–NTU, process-stream balances or exchanger geometry.

Inputs

Wall area, A m²

Inside fluid temperature, T₁ °C

Outside fluid temperature, T₂ °C

Surface coefficient basis

Inside surface coefficient, h₁ W/(m²·K)

Outside surface coefficient, h₂ W/(m²·K)

Use effective coefficients when convection and linearized long-wave radiation are combined. Do not add radiation again.

Wall layersheat flow: inside → outside

Material / descriptionThickness (mm)k [W/(m·K)]

Preset conductivities are illustrative screening values. Verify properties for the actual material, density, moisture content and temperature.

Results

Overall U-value

—

Total R-value

—

Heat-transfer rate

—

Heat flux

—

Inside wall surface

—

Outside wall surface

—

Enter valid inputs to calculate heat flow.

Resistance and temperature-drop breakdown

Component	R [m²·K/W]	Share	ΔT [K]

Temperature profile

The profile is linear within each homogeneous layer. Temperature labels are staggered above the plot and connected to their interface points to prevent overlap.

Worked example: insulated external wall

Using the calculator's built-in insulated-wall example with 20 m² of wall area, 21 °C indoors and −5 °C outdoors.

Inside filmh₁ = 8 W/(m²·K)

Outside filmh₂ = 25 W/(m²·K)

Wall area20.0 m²

Temperature difference26 K

Gypsum board12.5 mm · k = 0.17 W/(m·K)R = 0.074 m²·K/W

Mineral wool150 mm · k = 0.040 W/(m·K)R = 3.750 m²·K/W

Brick masonry100 mm · k = 0.80 W/(m·K)R = 0.125 m²·K/W

01

Add all thermal resistances

R″_total = 1/h₁ + Σ(L/k) + 1/h₂ = 4.114 m²·K/W.

02

Calculate the overall U-value

U = 1/R″_total = 0.243 W/(m²·K).

03

Calculate heat flux

q″ = U(T₁ − T₂) = 0.243 × 26 = 6.32 W/m².

04

Calculate total wall heat transfer

Q = q″A = 6.32 × 20 = 126.4 W. Heat flows from inside to outside.

05

Estimate inside and outside wall-surface temperatures

T_surface,1 = 21 − q″/h₁ ≈ 20.21 °C; T_surface,2 = −5 + q″/h₂ ≈ -4.75 °C.

Total R-value4.114 m²·K/W

Overall U-value0.243 W/(m²·K)

Heat flux6.32 W/m²

Heat-transfer rate126.4 W

Inside surface20.21 °C

Outside surface-4.75 °C

Interpretation: the 150 mm mineral-wool layer provides most of the wall's thermal resistance, so most of the temperature drop occurs across that layer. The example is steady-state and one-dimensional; thermal bridges, framing, moisture and contact resistance are not included.

How to use the calculator

Enter the wall area and boundary-fluid temperatures.
Select whether surface coefficients include radiation or represent convection only.
Add layers in order from the inside surface to the outside surface.
Review the resistance shares and surface temperatures to see where the major temperature drops occur.

Heat-flow sign: the calculator reports a positive magnitude and states the actual direction, avoiding ambiguous negative heat-loss values.

What the model does not include

This calculator is for plane walls and insulation assemblies. It does not model cylindrical pipe walls, transient heat storage, multidimensional thermal bridges, moisture migration, condensation, contact resistance, fins or heat-exchanger performance.

Surface temperatures may help with screening, but condensation assessment requires humidity, vapour-pressure and dew-point analysis.