The approach to particle sizing with optical particle counters is often simple interpolation of calibration data. A method is presented that uses the results of Mie-theory-based simulations to describe the signal between calibration points, thus reducing the number of necessary calibration points or increasing the sizing accuracy significantly. Through the use of Mie theory, particles with a refractive index differing from the calibration particles can be measured without an individual calibration. The method can be used with custom research setups or commercially available optical particle counters with various detector designs. If needed, the method can be applied to particle counters for which only the light wavelength used is known. The method is tested using a commercially available optical particle counter with a polystyrene microsphere calibration, measuring polystyrene microspheres as well as THP-1 cells, Chinese hamster ovary cells, and yeast cells. Without material specific calibration, simple interpolation results in about half the actual particle sizes for these biological samples, whereas the presented method yields accurate results.