Thermal Conductivity Converter

Thermal conductivity (k, sometimes written λ) measures a material's intrinsic ability to conduct heat. It is defined by Fourier's law of heat conduction, Q = -k × A × (dT/dx), where Q is the rate of heat flow, A is the cross-sectional area the heat passes through, and dT/dx is the temperature gradient along the direction of flow. Unlike thermal resistance, which depends on the geometry (thickness, area) of a specific object, thermal conductivity is a property of the material itself, independent of shape — making it the fundamental value used to compare how well different substances transmit heat.

The SI unit is watt per meter-kelvin, W/(m·K). Metals sit at the high end of the scale — copper conducts at roughly 400 W/(m·K) and aluminum at roughly 205 W/(m·K), which is why both are the default choices for heat sinks, cookware, and heat exchangers. Ordinary glass conducts at only about 1 W/(m·K), and common insulating materials such as fiberglass batting sit around 0.04 W/(m·K), while still (non-convecting) air is one of the best readily available insulators at about 0.024 W/(m·K) — the reason double-pane windows and foam insulation work by trapping thin layers of still air.

The watt per centimeter-°C and kilowatt per meter-K units simply rescale the base SI unit for convenience at different magnitudes. W/(cm·°C) is 100 times larger than W/(m·K) numerically (since a centimeter is a hundredth of a meter), appearing in materials science literature for small samples and thin-film measurements. kW/(m·K) is used when discussing bulk industrial heat transfer through very conductive materials or thick metal sections, where writing values in plain watts would produce inconveniently large numbers.

The calorie (IT and th) per second per centimeter per °C units come from the CGS thermal unit system historically used in European and Soviet-bloc scientific and engineering literature, particularly in materials science and metallurgy papers predating widespread SI adoption. The IT (International Table) and th (thermochemical) calorie definitions differ by less than 0.02%, but this converter tracks them separately (418.68 and 418.4 relative to 1 W/(m·K), respectively) so that values transcribed from older technical papers can be converted using the exact calorie definition the source specifies.

The kilocalorie (IT and th) per hour per meter per °C units serve the same historical role at a more human-usable time scale (hours instead of seconds), and are still occasionally found in HVAC and industrial furnace design documentation from countries that used metric-but-not-SI thermal units. 1 W/(m·K) equals roughly 1.163 kcal(IT)/(h·m·°C) or 1.1622 kcal(th)/(h·m·°C) — small differences that matter when validating legacy calculations to modern precision.

The Btu-based units — inch or foot length, second or hour time, per square foot per °F — are the backbone of US insulation and building-envelope engineering. Most importantly, Btu (IT) inch/(hour·sq. foot·°F) is the unit in which a material's k-value is combined with its thickness in inches to calculate its R-value (R = thickness ÷ k), the number printed on every roll of insulation sold in the United States. 1 W/(m·K) ≈ 6.9335 Btu (IT)·in/(h·ft²·°F), so a European material datasheet quoted in W/(m·K) can be directly translated into the units a US insulation contractor or building code table expects. The foot-based and second-based Btu variants scale the same physical quantity to different length and time bases used across various US engineering disciplines, from HVAC (hours) to materials testing (seconds).

This thermal conductivity converter supports watt/meter/K [W/(m·K)], watt/centimeter/°C, kilowatt/meter/K [kW/(m·K)], calorie (IT)/second/cm/°C, calorie (th)/second/cm/°C, kilocalorie (IT)/hour/meter/°C, kilocalorie (th)/hour/meter/°C, Btu (IT) inch/second/sq. foot/°F, Btu (th) inch/second/sq. foot/°F, Btu (IT) foot/hour/sq. foot/°F, Btu (th) foot/hour/sq. foot/°F, Btu (IT) inch/hour/sq. foot/°F, and Btu (th) inch/hour/sq. foot/°F. All conversions are instant, free, and precise to 12 significant digits — built for materials selection, insulation design, and cross-referencing SI and US customary engineering data.