Scientists measure temperature in degrees Celsius (°C) and also in kelvin (K). The kelvin scale starts at absolute zero, so 0 K is the lowest temperature possible. Because absolute zero is -273 °C, 0 K is the same as -273 °C.
A change of 1 °C is the same size as a change of 1 K, so the two scales are just shifted by 273. To convert from Celsius to kelvin, add 273. To convert from kelvin to Celsius, subtract 273.
For example, water freezes at 0 °C, which is 0 + 273 = 273 K. Water boils at 100 °C, which is 373 K. A room at 20 °C is 20 + 273 = 293 K. A temperature of 300 K is 300 - 273 = 27 °C. A kelvin temperature is never negative.