Analysis shows that proton donor is more important than proton acceptor in determining hydrogen bond formation and strength, and a clear explanation is given for this. Within one system, hydrogen bond strength, DeltaH, can be calculated by DeltaH = kx - alpha, where k and alpha are constants and x is the ratio of proton affinities of the acceptor [PA(A)] to the donor [PA(D)]. If the proton donor is fixed and the proton acceptor is varied when one system is fit into the linear function, the k and alpha so obtained are functions of donor's proton affinity, PA(D). Therefore, it is possible to obtain a universal equation to calculate hydrogen bond strength in a variety of systems. One example has the form of DeltaH = aPA(D) + dx + cPA(A) + e, where a, c, d, and e are constants. This equation has been tested with more than 300 hydrogen bond pairs with positive charge. The average absolute error for the test is 1.9 kcal/mol. Other forms of the universal equation have also been discussed.