Identify the atoms involved in the bond
Determine the bond order
Use the periodic trend or covalent radii of the atoms
Look up the bond length in a reference table or database
Measure the internuclear distance from experimental data such as X-ray diffraction, electron diffraction, or spectroscopy
For a molecule, use the average of the atomic positions from its structure
For a theoretical estimate, add the covalent radii of the two atoms
Adjust for bond type, since single, double, and triple bonds have different lengths
Compare with similar known compounds if exact data is unavailable