Smash Factor is the relationship of the clubhead to the golf ball velocity. Put more simply, smash factor is the relationship of the clubhead speed to ball speed.
The USGA has limits on the speed a golf ball can leave a club face. In general, this limit is a ratio of 1.50. This means that 100mph clubhead speed should only generate a 150mph ball speed. This 1.50 ratio is also a good benchmark to measure you own smash factor.
If you’ve got a smash factor over 1.40 you’ve got a decent to very good smash factor. If you’re smash factor is lagging below 1.40 there may be a couple reasons for this inefficient transfer of energy. If you have too much spin, too much of the energy from your clubhead speed is going to be lost to that particular aspect. The same goes with your launch angle.
Maximizing launch angle and spin rate numbers can help in achieving a more efficient smash factor.