Glossary

Kelvin (symbol: K) is the SI base unit of thermodynamic temperature, meaning it’s the “official” unit used in science and engineering to measure temperature on an absolute scale. Unlike Celsius and Fahrenheit, Kelvin does not use a degree sign (it’s K, not °K), and it starts at absolute zero—the theoretical lower limit of temperature where thermal motion is minimized.

A key point is that Kelvin and Celsius have the same size increment: a change of 1 K is the same temperature change as 1 °C. What’s different is the zero point. The Kelvin scale is offset so that 0 K = −273.15 °C, which is why the conversion is K = °C + 273.15 (and °C = K − 273.15). For example, 25 °C = 298.15 K. Because Kelvin is absolute, temperatures in Kelvin are (in normal engineering contexts) non-negative, and that makes it the correct input for many physical equations.

Kelvin is the temperature unit used in a huge number of industrial and chemical calculations because many laws are derived for absolute temperature. Examples include the ideal gas law (PV = nRT), where T must be in Kelvin, and kinetics/thermodynamics relationships such as the Arrhenius equation (reaction rates vs. temperature), equilibrium constants, and properties that depend on absolute temperature (gas density, vapor pressure models, heat-transfer correlations, etc.). Using Celsius directly in these equations can produce nonsense results because Celsius is not absolute.

From a standards standpoint, the kelvin is part of the SI system maintained by international metrology bodies, and modern SI ties temperature measurement to fundamental constants (in practice, temperature is traceable through defined reference methods and calibrations). In real-world labs and plants, you’ll often see Kelvin used alongside standardized temperature scales and calibration practices so instruments (RTDs, thermocouples, IR sensors) can be validated and kept consistent.

Historically and naming-wise, the unit is named after Lord Kelvin (William Thomson), who helped formalize the concept of an absolute temperature scale. In short: Kelvin is “temperature with a true zero,” which is why it’s the default temperature unit for serious engineering, materials, and chemical work.