You can use the calculator in this app to calculate the amount of energy (and hence legality) of your air gun. Simply enter the weight of your pellet (you can check the pellet list in the app if you're unsure) and the velocity into the boxes and then click the Calculate button. The calculator will then tell you the energy and if you would require a fire arms certificate (F.A.C.) for an air rifle and air pistol with this energy level.

To calculate the amount of energy your air gun is producing you will require a chronograph. This measures the speed that your pellet is travelling as it leaves the gun.