Desired Inside Temperature - Outside Temperature = Temperature Increase 

1. Measure the space's dimension cubic feet. (Length x Width x Height = Cubic Feet)
2.         Subtract the outside temperature from the desired inside temperature to determine the temperature increase. 
3.         Calculate the required BTs with this formula
4.                 If you have measured temperature in Fahrenheit
5.                         Cubic Feet x Temperature Increase x .133 = Required BTUs)
6.         If you have measured temperature in Celsius
7.                 (Cubic feet x Temperature Increase x .2394 = Required BTUs)

Heat Rise - 'BTU' Requirement Reference Chart