The kelvin (symbol K, written without a degree sign) is the SI base unit of temperature. NIST defines it by fixing the Boltzmann constant at 1.380649 × 10⁻²³ J/K. The scale starts at absolute zero, 0 K, so it has no negative values. One kelvin is the same size as one degree Celsius, and 0 °C is 273.15 K.
Formula
K = °C + 273.15
°C = K − 273.15
K = (°F − 32) ÷ 1.8 + 273.15
Example
Room temperature of 25 °C is 25 + 273.15 = 298.15 K. Boiling water at 100 °C is 373.15 K. Normal body temperature, 98.6 °F, is (98.6 − 32) ÷ 1.8 = 37 °C, or 310.15 K. A temperature of 300 K is 300 − 273.15 = 26.85 °C.
NIST gives the triple point of water, where solid, liquid and vapor coexist, as 273.16 K, 0.01 K above 0 °C. For converting among Celsius and Fahrenheit see the Celsius to Fahrenheit guide and Celsius to Kelvin.
Kelvin vs Celsius
Differences are identical in both: a rise of 10 K is a rise of 10 °C, and 18 °F. Only the zero point differs.