Last updated: 2026-06-30

Thermal Conductivity

Thermal conductivity (symbol k or λ) measures how readily a material transfers heat by conduction. The SI unit is watts per metre-kelvin [W/(m·K)]: a slab of material 1 metre thick with a 1 K temperature difference across it transfers k watts of heat per square metre. Higher k means faster heat transfer; lower k means better insulation.

Units covered

W/(m·K) — watts per metre-kelvin is the SI unit and the standard for modern engineering and building physics. All major standards (ISO, EN, ASHRAE) now specify thermal conductivity in W/(m·K).

BTU/(hr·ft·°F) — BTU per hour-foot-degree Fahrenheit is the imperial unit, still common in US HVAC, construction, and energy codes. One BTU/(hr·ft·°F) equals 1.730735 W/(m·K).

cal/(s·cm·°C) — calories per second-centimetre-°C is the CGS unit. It appears in older materials science and physics literature published before SI adoption. One cal/(s·cm·°C) equals 418.4 W/(m·K) (using the thermochemical calorie, 4.184 J).

Key conversion table

Material	W/(m·K)	BTU/(hr·ft·°F)
Copper	401	231.7
Aluminium	237	137.0
Steel (carbon)	50	28.9
Concrete	1.7	0.98
Glass	1.0	0.578
Water (20 °C)	0.598	0.346
Wood (pine)	0.15	0.087
Mineral wool	0.040	0.023
Expanded polystyrene	0.038	0.022
Still air	0.026	0.015

Conversion factors

From	To W/(m·K)	To BTU/(hr·ft·°F)	To cal/(s·cm·°C)
1 W/(m·K)	1	0.577789	0.002390
1 BTU/(hr·ft·°F)	1.730735	1	0.004136
1 cal/(s·cm·°C)	418.4	241.9	1

k-value and R-value

Thermal conductivity (k) and thermal resistance (R-value) are inversely related through material thickness:

R = d / k

where d is thickness in metres and k is in W/(m·K), giving R in m²·K/W. A 100 mm mineral-wool batt with k = 0.040 W/(m·K) gives R = 0.1 / 0.040 = 2.5 m²·K/W. US building codes use imperial R-values in hr·ft²·°F/BTU, which are 5.678 times larger than SI R-values.

Usage

Enter a thermal conductivity value in any field. The other units update automatically. Enter 0.04 in W/(m·K) to see typical insulation values; enter 401 to check copper's conductivity in BTU or CGS units.

Frequently Asked Questions (FAQ)

How do you convert W/(m·K) to BTU/(hr·ft·°F)?

Multiply the value in W/(m·K) by 0.5777893 to get BTU/(hr·ft·°F). Conversely, multiply BTU/(hr·ft·°F) by 1.730735 to get W/(m·K). For example, copper has a thermal conductivity of about 401 W/(m·K), which equals 401 × 0.5778 ≈ 231.7 BTU/(hr·ft·°F).

What is the thermal conductivity of common materials?

Thermal conductivity varies widely: copper ≈ 401 W/(m·K), aluminium ≈ 237 W/(m·K), steel ≈ 50 W/(m·K), glass ≈ 1 W/(m·K), concrete ≈ 1.7 W/(m·K), wood ≈ 0.12–0.17 W/(m·K), mineral wool insulation ≈ 0.04 W/(m·K), and still air ≈ 0.026 W/(m·K). Higher k means faster heat transfer.

What is the k-value and how does it relate to R-value?

The k-value (thermal conductivity) measures how readily a material conducts heat — lower is better for insulation. R-value (thermal resistance) is the inverse: R = thickness / k. A 100 mm layer of mineral wool with k = 0.04 W/(m·K) has R = 0.1 / 0.04 = 2.5 m²·K/W. Building codes specify minimum R-values, so knowing k lets you calculate the required insulation thickness.

What is cal/(s·cm·°C) and when is it used?

cal/(s·cm·°C) — calories per second per centimetre per degree Celsius — is the CGS unit of thermal conductivity. One cal/(s·cm·°C) equals 418.4 W/(m·K). It appears in older physics and chemistry literature, particularly in materials data tables published before SI adoption. For modern engineering use, W/(m·K) is preferred.