The Celsius scale is based on the freezing and boiling points of water. The kelvin scale is based on absolute zero. Absolute zero is 0 K on the kelvin scale, which is -273 °C on the Celsius scale. Kelvin has the symbol K, and we write it without a degree sign.
A change of 1 degree Celsius is the same size as a change of 1 K. The two scales are just shifted by 273. To convert from Celsius to kelvin, add 273. To convert from kelvin to Celsius, subtract 273.
temperature in K = temperature in °C + 273
temperature in °C = temperature in K - 273
For example, a cold winter day of 5 °C is 278 K, and a temperature of 400 K is 127 °C. Kelvin temperatures can never be negative, because nothing is colder than 0 K.