How do we know how the strength of fundamental forces compare?

I have read that the strong force is 100 times stronger than the electromagnetic force, 10 000 times stronger than the weak force, and 10^39 times stronger than gravity. I searched for a way to describe the force per boson, but could only find a formula for a force per gluon ($hk$), where $h$ is the reduced planck's constant and $k$ is the wavevector. How do we create a ratio for the strength of fundamental forces without knowing force per boson for the other bosons? Can this known ratio be applied to other bosons, from the known force of a photon, to figure out how strong the force exerted by a graviton is?

• What do you mean by "force per boson"? I assume the forces are defined for a given distance. – pfnuesel Mar 4 '18 at 5:45
• – Rob Mar 4 '18 at 5:51
• @pfnuesel I think it's just a mistaken idea that the difference between one force and another, is that each type of boson delivers a distinct and uniform amount of force. A good answer would explain how coupling constants determine the force strength, and how it is different from the force-per-boson assumption. – Mitchell Porter Mar 4 '18 at 13:09
• this link may be useful hyperphysics.phy-astr.gsu.edu/hbase/Forces/couple.html – anna v Mar 4 '18 at 19:28