A gas does not have a definite volume. By its definition, a gas is a fluid state of matter that fits to the shape and volume of its container.
The volume of a gas is dependent on mass, pressure and temperature. Boyle's Law states that at a fixed mass and temperature, the pressure and the volume of the gas are inversely related. This relationship is written in the equation PV = k, where P is the pressure, V is the volume and k is a constant value. If the pressure rises, the volume decreases in order to keep the value constant.
Charles's Law also explains a gas volume relationship, this time to temperature. It says that when the pressure is constant, the volume of the gas and temperature are directly related. The equation is written V/T = k, where V is the volume, T is temperature and k is a constant value. If the temperature rises, the volume also rises.