Abstract. Accurate magnetic field measurements by fluxgate magnetometers on-board spacecraft require ground and regular in-flight calibrations activities. Therewith, the parameters of a coupling matrix and an offset vector are adjusted; they are needed to transform raw magnetometer outputs into calibrated magnetic field measurements. The components of the offset vector are typically determined by analyzing Alfvénic fluctuations in the solar wind, if solar wind measurements are available. These are characterized by changes in the field components, while the magnetic field modulus stays constant. In this paper, the following question is answered: How much solar wind data are sufficient for accurate fluxgate magnetometer offset determinations? It is found that approximately 50 hours of solar wind data are sufficient to achieve offset accuracies of 0.2 nT, and about 20 hours suffice for accuracies of 0.3 nT or better, if the magnetometer offsets do not drift within these time intervals and if the spacecraft fields do not vary at the sensor position. Offset determinations with uncertainties lower than 0.1 nT, however, would require at least hundreds of hours of solar wind data.