General Gas Law

Boyle's Law shows that the pressure exerted by a gas sample and its volume are inversely proportional (if one goes up, the other goes down).
P = constant × 1
V
PV = constant 1*
P1V1 = P2V2

Thus if the pressure or volume of a gas sample is changed, provided one of the new values is known, the other can be calculated.
 
The two volumes must have the same units.
The two pressures must have the same units
 
 
Charles' Law shows that the volume of an ideal gas sample is directly proportional to its temperature in Kelvin.
 
V = constant × T
V = constant 2*
T
V1 = V2
T1 T2

Thus if the volume or temperature of a gas sample is changed, provided one of the new values is known, the other can be calculated.
 
The two volumes must have the same units.
The temperatures MUST be in Kelvin.
 
 
The General Gas Law  combines Boyle's and Charles' Laws.
 
PV = constant 3*
T
P1V1 = P2V2
T1 T2

Thus if the initial volume, pressure and temperature of a gas sample are known as well as two of pressure, volume and temperature under new conditions, the third can be calculated.

*If the pressure, temperature or volume for a particular sample of gas is changed, there is no change in constant 1 (or constant 2 or constant 3).  This means for example that for example:

P1V1 = constant 1 = P2V2  
Thus P1V1 = P2V2