Heat Transfer Coefficient Calculator — U-value & Thermal Resistance
Calculate the overall heat transfer coefficient U (W/m²K) for a flat wall that exchanges heat with fluids on both sides. The calculator combines inside and outside convective film resistances with the wall's conductive resistance, then computes heat flux and total heat rate.
W/m²K
m
W/mK
W/m²K
°C
m²
Combined resistance of inside convection + wall conduction + outside convection
Inside convective resistance: R_i = 1 / h_i
Wall conduction resistance: R_wall = d / k
Outside convective resistance: R_o = 1 / h_o
Total resistance and U-value: U = 1 / R_total
- 1
Inside convective resistance
R_i = 1 ÷ 8 = 0.125 m²K/W - 2
Wall conduction resistance
R_wall = 0.25 ÷ 0.72 = 0.3472 m²K/W - 3
Outside convective resistance
R_o = 1 ÷ 25 = 0.04 m²K/W - 4
Total thermal resistance
R_total = 0.125 + 0.3472 + 0.04 = 0.5122 m²K/W - 5
Overall heat transfer coefficient
U = 1 ÷ 0.5122 = 1.952
How does this calculator work?
Overall U-value: 1/U = 1/h_i + d/k + 1/h_o (m²K/W). h_i and h_o are convective film coefficients (W/m²K); d/k is the conductive wall resistance. Heat transfer rate Q = U × A × ΔT (W). Typical indoor h_i ≈ 8, outdoor h_o ≈ 25 W/m²K (ISO 6946 defaults).
Formula
How this is calculated
When heat flows through a flat wall from a hot fluid on one side to a cold fluid on the other, three resistances act in series: the inside convective film (resistance R_i = 1/h_i), the wall's thermal conduction (R_wall = d/k, where d is thickness and k is conductivity), and the outside convective film (R_o = 1/h_o). The overall heat transfer coefficient U is the reciprocal of the total resistance: 1/U = R_i + R_wall + R_o.
Convective coefficients h (W/m²K) depend strongly on flow conditions. For still indoor air, h_i is typically 5–10 W/m²K; ISO 6946 uses 7.7 (equivalent surface resistance 0.13 m²K/W). For outdoor surfaces in moderate wind, h_o is 15–25 W/m²K; ISO 6946 uses 25 (surface resistance 0.04 m²K/W). Thermal conductivity k (W/mK) is a material constant: brick ≈ 0.72, concrete ≈ 1.75, glass wool insulation ≈ 0.035, softwood timber ≈ 0.14 W/mK.
The bar chart shows which resistance dominates. In a thin, highly-conductive wall (e.g., a steel heat-exchanger plate), convective resistances dominate and increasing flow velocity (higher h) is the main lever for improvement. In a thick insulated wall, wall conduction dominates and adding more insulation is the best strategy.
Frequently asked questions
For buildings: uninsulated brick cavity wall ≈ 1.5 W/m²K, modern well-insulated wall ≈ 0.15–0.30, double-glazed window ≈ 1.0–2.0, single glazing ≈ 5–6 W/m²K. For industrial heat exchangers, U can range from 100–2 000 W/m²K depending on fluids and flow rates.
Simply add more d/k terms for each additional layer: 1/U = 1/h_i + d₁/k₁ + d₂/k₂ + … + 1/h_o. For example, a wall with a brick outer leaf (0.10 m, k = 0.72), glass-wool insulation (0.10 m, k = 0.035), and a plasterboard inner face (0.013 m, k = 0.25) would have R_wall = 0.139 + 2.857 + 0.052 = 3.048 m²K/W — the insulation dominates as expected.
h (also called the film or surface coefficient) is a local property describing convective heat exchange at a single fluid–surface interface. U (the overall coefficient) combines all h values and conductive resistances into a single number for the whole wall assembly. U is the quantity you use with Q = U × A × ΔT to find the total heat transfer rate.
Also known as
TG we-Calculate Editorial Team. (2026). Heat Transfer Coefficient Calculator — U-value & Thermal Resistance [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/heat-transfer-coefficient-calculator
TG we-Calculate Editorial Team. "Heat Transfer Coefficient Calculator — U-value & Thermal Resistance." TG we-Calculate. 2026. https://we-calculate.com/calculator/heat-transfer-coefficient-calculator.
TG we-Calculate Editorial Team, "Heat Transfer Coefficient Calculator — U-value & Thermal Resistance," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/heat-transfer-coefficient-calculator
@misc{wecalculate_heat_transfer_coefficient_calculator, title = {Heat Transfer Coefficient Calculator — U-value & Thermal Resistance}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/heat-transfer-coefficient-calculator}}, year = {2026}, note = {TG we-Calculate} }
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