Charle's law states that the volume of an ideal gas is directly proportional to the absolute temperature at constant pressure. The law also states that the Kelvin temperature and the volume will be in direct proportion when the pressure exerted on a sample of a dry gas is held constant.This law was formulated in the year 1780 by French physicist Jacques Charles. According to this law: V α T V T = Constant

In the S.T.P. system, both pressure and temperature are in their typical form. Pressure and temperature of this system are considered as 760 torr and 273 K correspondingly. Standard temperature is described as zero degrees Celsius ( 0 ° C ), which transposes to 32 degrees Fahrenheit ( 32 ° F ) or 273 . 15 degrees Kelvin ( 273 . 15 ° K ). This is essentially the freezing point of pure water at sea level, in air at standard pressure. Normal Temperature and Pressure is defined as air at 20 ° C ( 293 . 15 K , 68 ° F ) and 1 atm . NTP is 20 ° C temperature and 1 atm pressure generally.