Joule rating is a better way to know how hard a bb is being shot than just feet or meters per second since it takes into account the weight of the BB as well as the speed to calculate the energy the bb exerts. A .32 gram bb and a .20 gram bb being shot at the same velocity hit with very different amounts of energy.
The formula to determine Joules is (joules) = half of the mass, in grams, x the velocity, in meters per second,^2. A good rule of thumb to remember is an airsoft rifle shooting at 330 fps or 100m/s with .20 gram bbs is shooting at about 1 joule and an airsoft rifle shooting at 400 fps or 120 m/s with .20 gram bbs is shooting at about 1.5 joules