A new empirical relation is proposed describing spectral variation of mode-field radius MFR as inferred from measurements in the far-field of the fiber. It is shown that using this relation, it is possible to estimate the cut-off wavelength of the fiber. The proposed technique is successfully tested through measurements made on two standard step index c single-mode fibers, as well as on an erbium doped fiber EDF having falling within its strong absorption band around c 980 nm. This empirical formula is more accurate than the widely used Marcuse’s formula to describe spectral dependence of MFR determined through measurements made in the fiber’s far-field. The proposed technique is especially suited for estimation of l of doped fibers in which l falls within an absorption band.