Calculate heat flux through any material using Fourier's Law. Enter the material's thermal conductivity (λ), temperature difference (ΔT), thickness (Δx), and cross-sectional area (A) to get the heat flux (q) in W/m² and heat flow rate (Q) in Watts. You can also solve for thermal conductivity if you know the heat flux.
Heat Flux (q)
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Heat Flow Rate (Q)
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Thermal Resistance (R)
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Thermal Conductivity (λ)
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R-Value (Imperial)
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Thermal conductivity (λ or k) is a material property that measures how well a material conducts heat. It is defined as the amount of heat energy transferred per unit time per unit area per unit temperature gradient. High thermal conductivity materials (like metals) transfer heat quickly, while low thermal conductivity materials (like foam or wood) act as insulators. It does not depend on the object's shape or size — only the material itself.
Fourier's Law states that the heat flux (q) through a material is proportional to the negative temperature gradient: q = −λ × (ΔT / Δx). Here, λ is the thermal conductivity, ΔT is the temperature difference across the material, and Δx is the thickness. The negative sign indicates heat flows from hot to cold. This law is the foundation for calculating heat transfer through walls, insulation, and other solid materials.
Heat flux (q) is the rate of heat transfer per unit area, expressed in W/m². Heat flow rate (Q) is the total rate of heat transfer through the entire surface, expressed in Watts (W). They are related by Q = q × A, where A is the cross-sectional area. Heat flux is a material and geometry-independent measure, while heat flow rate gives the total power conducted.
Thermal resistance (R) quantifies how much a material resists heat flow. It is calculated as R = Δx / λ, where Δx is the thickness and λ is the thermal conductivity. A higher R-value means better insulation. Thermal resistance is additive for layers in series, making it very useful for analyzing composite walls or multilayer insulation systems.
R-value is a measure of thermal resistance commonly used in the building and construction industry. In SI units, R = Δx / λ (m²·K/W). In imperial units, R-value is expressed in ft²·°F·hr/BTU. A higher R-value indicates better insulating performance. Materials with low thermal conductivity and greater thickness have higher R-values.
Thermal conductivity varies widely: copper is about 385 W/(m·K), steel around 50 W/(m·K), concrete roughly 0.8–1.4 W/(m·K), glass about 1.0 W/(m·K), wood 0.1–0.4 W/(m·K), glass wool insulation around 0.04 W/(m·K), and air approximately 0.025 W/(m·K). Metals conduct heat well, while gases and porous materials are good insulators.
The SI unit for thermal conductivity is W/(m·K) — watts per meter per kelvin. This can also be expressed as kg·m/(s³·K) in base SI units. In imperial systems, BTU·in/(hr·ft²·°F) or BTU/(hr·ft·°F) are common. For this calculator, all inputs use SI units (W/(m·K), meters, and kelvin/°C temperature differences).
Yes. Switch the <strong>Calculate</strong> mode to <strong>Thermal Conductivity</strong>, then enter the measured heat flux, temperature difference, and thickness. The calculator rearranges Fourier's Law to solve λ = q × Δx / ΔT. This is useful when you have experimental heat flux measurements and want to determine the thermal conductivity of an unknown material.