In this paper, demagnetization factor of the ring-core fluxgate is examined. Several sensors are analyzed by calculating their demagnetization factors with three well-known formulas in addition to a finite-element method (FEM) analysis software. Calculated values are compared to the measured ones and theoretical values are shown to vary significantly. None of the studied formulas succeed in achieving an error level below 60% in all cases that 28 sensors are tested. Moreover, FEM-analysis is found to provide consistent and satisfactory results in good agreement with the previously reported measurement data as well as with our measurements performed for the two sensors designed. The optimum dimension range for each formula that results in a reasonable demagnetization factor value is discussed and the stability of each formula is analyzed. Our paper points out that a new demagnetization factor formula for the ring-core is essential by taking into account the high error levels caused by the well-known formulas in the literature. Finally, we propose a novel method to calculate the factor accurately for a wide range of dimensions.