Boyle's law states that at constant temperature, the volume, V, of a given mass of gas, varies inversely with the pressure, P.

It can be stated as $V=kx\frac{1}{P}\mathrm{or\; PV}=k$
where:
V is the volumeof the gas, P is the Pressure of the gas, k is a constant.

Example:
If P = 220 Torr and V = 100 ml
k = 220 × 100 = 22000 Torr-ml

If P is doubled: P = 440 Torr
$V=kx\frac{1}{P}=\frac{\mathrm{22000}}{\mathrm{440}}=\mathrm{50\; ml}$

The example above proves that volume is inversely proportional to pressure.