A simple nondestructive method is described for determining the spot size omega(0) (width of the fundamental mode)-one of the most important quantities to characterize monomode optical fibers. The theory involves the definition of effective values of core radius and normalized frequency. Thereby, graded-index fibers can be replaced by equivalent step-index fibers. The experiment consists in illuminating the fiber perpendicularly to its axis and determining the position of the first minimum in the diffraction pattern. In this way, just one measurement yields omega(0) regardless of the index profile and for any operating wavelength, provided the cutoff wavelength is known.