Thermal conductivity (the k-value) measures how readily a material conducts heat. The SI unit is the watt per metre-kelvin. This converter moves between W/(m·K), mW/(m·K), kW/(m·K), the calorie unit and two BTU units used in US engineering and insulation.
Reviewed: June 20, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: SI heat-transfer definitions, recomputed in code.
How conversion works
Each unit has a fixed factor to W/(m·K): mW 0.001, kW 1000, cal/(s·cm·°C) 418.4, BTU/(hr·ft·°F) 1.730735, and BTU·in/(hr·ft²·°F) 0.144228. The value is first expressed in W/(m·K), then divided by the target factor. Kelvin and Celsius are interchangeable for conductivity because a 1 K difference equals a 1 °C difference.
Worked examples
1 W/(m·K) to BTU/(hr·ft·°F):
1 cal/(s·cm·°C) to W/(m·K):
Fibreglass 0.04 W/(m·K) to insulation unit:
For context, copper at about 400 W/(m·K) is roughly 231 BTU/(hr·ft·°F), while a good insulator like fibreglass at 0.04 W/(m·K) is about 0.023 — a factor of ten thousand between conductor and insulator.
Frequently Asked Questions
SI: W/(m·K). US: BTU/(hr·ft·°F). Older: cal/(s·cm·°C). Insulation: BTU·in/(hr·ft²·°F).
Divide by 1.730735. 1 W/(m·K) ≈ 0.5778 BTU/(hr·ft·°F).
Yes — a 1 K difference equals a 1 °C difference, so they match exactly.
Copper 400, aluminium 200, steel 50, glass 1, water 0.6, fibreglass 0.04 W/(m·K).
BTU·in/(hr·ft²·°F) in the US — 1 of these = 0.144228 W/(m·K).