per kelvin
Thermal expansion coefficientexact by definition
The reciprocal kelvin is the SI unit of the thermal expansion coefficient: the fractional change in length, area or volume per kelvin of temperature change. Because it is a fraction divided by a temperature interval, it is dimensionally an inverse temperature, and its factor of 1 is exact. Steel is about 12e-6 per kelvin, copper 16.5e-6, aluminium 23e-6 and PVC 50 to 80e-6.
Watch out: Linear and volumetric coefficients share this unit and differ by a factor of three for an isotropic material, since a solid expands in all three directions at once. Water's volumetric coefficient near 20 °C is about 207e-6 per kelvin, and it goes negative below 4 °C, which is why ice floats and why lakes freeze from the top.
| 1 1/K | 1 1/K |