What is Charles' law formula?

The physical law that the volume of a fixed mass of gas held at a constant pressure varies directly with the absolute temperature.

This law states that the volume of a given amount of gas ,at a constant pressure gas varies with its temperature

or simply we can say that the volume of gas changes with its temperature. At a higher temperature , a gas will take up more volume ( expands) , at a lower temperature, a gas will take up lesser volume or it contracts.

Let us suppose that we have a given amount of gas enclose in a balloon, the temperature of the gas is ##T_1## (Kelvin) , and it takes up the volume ##V_1## (Liter). If the Temperature is changed to a new value called ##T_2## , then the volume changes to ##V_2##.

as per the law the ratio of Volume / temperature stays the same, if one increases , the other decreases but the ratio does not change.

##V_1## / ##T_1## = k ......(a)

## V_2## / ##T_2## = k ......(b)

equating the two equations (a) and (b)

##V_1## / ##T_1## = ##V_2## /##T_2##

or

##V_1## . ##T_2## = ##V_2## . ##T_1##