To get the temperature in kelvin, just add 273 to the degrees in celcius. Therefore, 20C would be 293 degrees kelvin. (20+273=293)
To get the temperature in celcius from kelvin, just subtract 273 degrees from the degrees in kelvin. Therefore, 278K would be 5C. (278-273=5)
Absolute zero is 0 degrees kelvin or -273C.