Charles's Law states that the Volume (V) of a gas is directly proportional to the temperature (T). This law is valid as long as the pressure and the amount of gas are constant. The temperature must be an absolute temperature:
\[{V \over T} = k \hskip12pt{\rm (constant)}\]
The constant, \(k\), will depend on the number of moles and the pressure. As long as those two state functions are constant, \(k\) will be a constant and Charles's Law will hold. Below is a plot of temperature vs volume for an ideal gas. Note that the line is linear which is consistent with a direct proportionality.
Most Charles's Law problems have an initial set of conditions (V1 and T1) and then a final set of conditions (V2 and T2). BOTH conditions must satisfy Charles's Law and therefore:
\[{V_1 \over T_1} = {V_2 \over T_2}\]
Any units will work here for volume but the temperature must be absolute (Kelvin) .