Strain sensor calibration is investigated using extrinsic Fabry-Pérot interferometric (EFPI) fiber optic sensors. The approach is based on the Bessel harmonic components from the nonlinear EFPI output signal under sinusoidal excitation. For known sensor parameters of optical wavelength and gauge length, the local maxima and minima of each Bessel harmonic profile occur for specific strain levels. The theoretical Bessel harmonic profiles are shown and a calibration approach is proposed using local maxima and minima in selected Bessel harmonics. The signal from a colocated companion strain sensor can be calibrated by comparison to salient EFPI reference strains. An experimental implementation is demonstrated for a polyvinylidene fluoride (PVDF) piezoelectric strain sensor that is colocated with an EFPI reference sensor on a cantilever beam apparatus. The experimental strains at the local maxima and minima closely match the strains determined theoretically.