What this calculator does
Thermal conductivity measures how readily a material conducts heat through itself: a high value means heat passes through quickly (metals, for example), while a low value means the material resists heat flow and insulates well (foam, wool, still air). This calculator works out thermal conductivity from a straightforward heat-transfer formula, the same one used to derive published k-values for common materials.
The formula is k = (Q × L) ÷ (A × ΔT × t), where Q is the heat energy that passed through the material, L is its thickness, A is the cross-sectional area it passed through, ΔT is the temperature difference across it, and t is the time over which the heat transfer happened. Every one of those five figures is a free input here, so the thermal conductivity formula can be checked against a measured heat flow rather than looked up from a table.
The formula
Multiply the heat energy transferred (Q) by the material thickness (L), then divide by the cross-sectional area (A) multiplied by the temperature difference (ΔT) and the time (t). The result is thermal conductivity in watts per metre-kelvin (W/m·K), the standard unit used for published material values such as 0.6 W/m·K for water or roughly 200 W/m·K for aluminium.
| Term | Meaning |
|---|---|
| k | Thermal conductivity, in watts per metre-kelvin (W/m·K). |
| Q | Heat energy transferred through the material, in joules. |
| L | Thickness of the material in the direction of heat flow, in metres. |
| A | Cross-sectional area the heat passes through, in square metres. |
| ΔT | Temperature difference between the two sides of the material, in degrees Celsius (equivalent to kelvin for a difference). |
| t | Time over which the heat energy was transferred, in seconds. |
The inputs explained
| Field | What to enter |
|---|---|
| Heat energy transferred (Q) (J) | The total heat energy that passed through the material over the measured period, in joules. |
| Material thickness (L) (m) | The thickness of the material sample, measured along the direction the heat is travelling, in metres. |
| Cross-sectional area (A) (m²) | The cross-sectional area of the material the heat passed through, in square metres. |
| Temperature difference (ΔT) (°C) | The temperature difference maintained between the two faces of the material, in degrees Celsius. |
| Time (t) (s) | The time over which the heat energy Q was transferred, in seconds. |
When to use it
Working out k from a lab measurement
A guarded hot-plate or similar heat-flow experiment measures Q, L, A, ΔT and t directly; plugging those five figures in gives the material's thermal conductivity without needing a published lookup table.
Checking an insulation material against its datasheet
If a manufacturer quotes a k-value, running a sample through this formula from a real heat-loss measurement checks whether the material performs close to that stated figure once installed.
Comparing candidate materials for a building or enclosure
Given the same test setup (area, temperature difference, time), a lower resulting k means less heat is getting through for the same conditions, which is the property that matters when choosing an insulating material.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How thermal conductivity changes with material thickness
The same heat transfer test, run across a range of material thicknesses.
| Thickness (L) | Thermal conductivity (k) |
|---|---|
| 0.01 m | 0.0208 W/m·K |
| 0.02 m | 0.0417 W/m·K |
| 0.05 m | 0.1042 W/m·K |
| 0.1 m | 0.2083 W/m·K |
| 0.15 m | 0.3125 W/m·K |
| 0.2 m | 0.4167 W/m·K |
How thermal conductivity changes with temperature difference
The same heat transfer, across a range of temperature differences.
| Temperature difference (ΔT) | Thermal conductivity (k) |
|---|---|
| 5 °C | 0.4167 W/m·K |
| 10 °C | 0.2083 W/m·K |
| 20 °C | 0.1042 W/m·K |
| 30 °C | 0.0694 W/m·K |
| 40 °C | 0.0521 W/m·K |
| 50 °C | 0.0417 W/m·K |
Questions
What units does this calculator use?
Joules for heat energy, metres for thickness, square metres for area, degrees Celsius for the temperature difference, and seconds for time. The result comes out in watts per metre-kelvin (W/m·K), the standard SI unit for thermal conductivity.
Can I use degrees Fahrenheit for the temperature difference?
No, convert to Celsius or kelvin first. A temperature difference in Celsius and kelvin is numerically identical, but a Fahrenheit difference is not, since the two scales have different-sized degrees.
What is a typical thermal conductivity value?
Still air is around 0.024 W/m·K, wood is roughly 0.1 to 0.2 W/m·K, water is about 0.6 W/m·K, glass is around 1 W/m·K, and metals range from about 15 W/m·K for stainless steel up to 400 W/m·K for copper. This calculator works out k from your own measurement rather than assuming any of these.
Why does the calculator also show the heat transfer rate?
Heat transfer rate (Q divided by t) is the wattage of heat flow itself, shown alongside k because it is a useful intermediate figure: thermal conductivity is really just that rate scaled by the geometry (thickness and area) and the temperature difference driving it.
For a related property that describes how quickly a material equalises temperature rather than how much heat it conducts, see the thermal diffusivity calculator.