Hydrogen bonds can vary in strength from weak (1–2 kJ/mol) to strong (161.5 kJ/mol in the bifluoride ion, ). Typical enthalpies in vapor include:
*   (161.5 kJ/mol or 38.6 kcal/mol), illustrated uniquely by 
*   (29 kJ/mol or 6.9 kcal/mol), illustrated water-ammonia
*   (21 kJ/mol or 5.0 kcal/mol), illustrated water-water, alcohol-alcohol
*   (13 kJ/mol or 3.1 kcal/mol), illustrated by ammonia-ammonia
*   (8 kJ/mol or 1.9 kcal/mol), illustrated water-amide
*   (18 kJ/mol or 4.3 kcal/mol)
The strength of intermolecular hydrogen bonds is most often evaluated by measurements of equilibria between molecules containing donor and/or acceptor units, most often in solution. The strength of intramolecular hydrogen bonds can be studied with equilibria between conformers with and without hydrogen bonds. The most important method for the identification of hydrogen bonds also in complicated molecules is crystallography, sometimes also NMR-spectroscopy. Structural details, in particular distances between donor and acceptor which are smaller than the sum of the van der Waals radii can be taken as indication of the hydrogen bond strength. One scheme gives the following somewhat arbitrary classification: those that are 15 to 40 kcal/mol, 5 to 15 kcal/mol, and >0 to 5 kcal/mol are considered strong, moderate, and weak, respectively.
Hydrogen bonds involving C-H bonds are both very rare and weak.