Thermal Conductivity Converter: W/(m·K), BTU, cal

Thermal conductivity (symbol k or λ) quantifies how readily a material transmits heat by conduction. It is defined as the rate of heat flow per unit area per unit temperature gradient: k = Q·L / (A·ΔT), where Q is heat flow rate (watts), L is material thickness (metres), A is area (m²), and ΔT is the temperature difference (kelvin). A high thermal conductivity means heat passes quickly through the material. Copper conducts heat roughly 10,000 times faster than polyurethane foam, illustrating the enormous range across engineering materials.

The SI unit is watt per metre kelvin, written W/(m·K) or W·m⁻¹·K⁻¹. Because a temperature difference of 1 kelvin equals a difference of 1 degree Celsius, W/(m·K) and W/(m·°C) are numerically identical and are used interchangeably. This converter treats them as equal. Other units such as BTU/(hr·ft·°F) and cal/(s·cm·°C) are derived from non-SI systems and are converted using the factors listed in the reference table.

The thermal resistance (R-value) of a material layer equals its thickness divided by its thermal conductivity: R = L / k. A lower k gives a higher R-value for the same thickness, meaning better insulation. Insulation materials such as polyurethane foam (k ≈ 0.026 W/(m·K)) and fiberglass batts (k ≈ 0.04 W/(m·K)) are valued precisely because of their very low thermal conductivity. US building codes express R-value in ft²·°F·hr/BTU, while international standards use m²·K/W. The conversion factor is 1 m²·K/W = 5.678 ft²·°F·hr/BTU.

In metals, thermal conduction is dominated by free electrons. As temperature rises, lattice vibrations increase and scatter electrons more, lowering conductivity. In non-metallic solids and gases, phonon (lattice vibration) transport dominates, and the relationship with temperature is more complex. For gases, conductivity generally increases with temperature. For engineering calculations, material datasheets specify a temperature at which the conductivity is measured, and manufacturers of insulation products typically provide values at a mean temperature representative of typical use conditions.

Diamond has the highest known thermal conductivity among bulk materials, around 2,000 W/(m·K), due to its rigid crystal lattice. Among metals, silver (429 W/(m·K)) and copper (401 W/(m·K)) are the best conductors. At the other extreme, still-air and aerogel materials have conductivities near 0.01 to 0.026 W/(m·K), making them the best practical insulators. Vacuum eliminates conductive and convective heat transfer entirely, which is why vacuum-insulated panels and thermos flasks achieve such high insulating performance.