Units and SI factors
All units are referenced to the SI base W/(m·K) (Watt per metre kelvin).
| Unit | Name | Factor to SI | 1 unit = ? SI |
|---|
Thermal conductivity in engineering practice
Thermal conductivity describes how readily a material conducts heat, and its SI unit is the watt per metre kelvin — W/(m·K). It is the key property behind insulation selection, heat-sink design and building energy codes. In US building and HVAC practice the same property is often quoted in BTU per hour per foot per degree Fahrenheit — BTU/(hr·ft·°F), while older material-science literature uses the cgs unit cal/(s·cm·°C).
Typical values and where units appear
- Copper ≈ 400, aluminium ≈ 237, steel ≈ 50, water ≈ 0.6, air ≈ 0.026, mineral-wool insulation ≈ 0.03–0.04 (all W/(m·K)).
- W/(m·K) — material datasheets, heat-sink calculations and European building codes.
- BTU/(hr·ft·°F) — US insulation and HVAC specifications.
- cal/(s·cm·°C) — legacy chemical and thermal-reference tables.
Conversion gotchas
The Fahrenheit scale is 1.8× the kelvin and Celsius scale, and a foot is 12 inches, so the BTU factor is not a round number: 1 W/(m·K) ≈ 0.5778 BTU/(hr·ft·°F). Watch the W/(cm·K) unit too — it is exactly 100× W/(m·K). When reading insulation “R-values”, remember they are the imperial inverse of conductivity per unit thickness, while European U-values use W/(m²·K).