What temperature scale is used in gas law calculations?
You should always use the Kelvin scale to measure temperatures of gases for use with the gas laws. Note that the ° symbol should not be used with Kelvin temperatures.
To convert from Celcius to Kelvin you should add 273.1
0° = 273.15K (0 + 273.15)
To convert from Kelvin to Celcius you substract 273.15
273.15K = 0°C (273.15 - 273.15 = 0)
Why is this important?
Imagine you have a gas at 1°C and it is heated to 2°C. Just using Celcius temperatures you have doubled the temperature when in reality the amount of additional kinetic energy of a gas at 2°C is only slightly higher than a gas at 1°C.
Also, imagine working a problem where you would need to divide by a temperature of 0°C. Wouldn't work! You have the same issue at 0K, but since the material would not be a gas at 0K it is not an issue...