Define bond length.

Bond length: Bond length is defined as the equilibrium internuclear separation distance of the bonded atoms in a molecule. Each atom of the bonded pair contributes to the bond length. 

In the case of a covalent bond, the contribution from each atom is called covalent radius of that atom.
The bond length in a covalent molecule AB
R = r _A + r _B
where R is the bond length; r_Aand r_B are the covalent radii of atoms A and B respectively.