Bond length is defined as the equilibrium distance between the nuclei of two covalently bonded atoms in a molecule.
Each atom of the bonded pair contributes to the bond length.
Bond length depends upon the size of atoms and multiplicity of bonds. It increases with increase in size of atom and decreases with increase in multiplicity of bond.
e.g. C – C single bond is longer than C ≡ C triple bond.
Bond lengths are measured by X-ray and electron diffraction techniques.