An in-situ triaxial shearing of a quartz sand sample with X-ray micro-tomography scanning is carried out. A grid based method is presented to experimentally quantify the strain field of the sand based on its particle kinematics throughout the shear. The method incorporates the effects of particle rotations into the strain calculation. It is an extension of a grid-based method from the calculation of 2D strain fields of idealized granular materials in a DEM study to the experimental quantification of 3D strain fields of natural sands. Meanwhile, a widely used mesh-based method is also applied to the sand to acquire the strain results based on its particle displacements. The accuracies of both methods are evaluated by comparing the calculated local volumetric strain results with those from a voxel-based calculation. Both strain calculation methods are found to provide results that capture the overall trend of strain localization throughout the shearing, which follows the same trend as the evolution of particle kinematics. However, the results from the two methods exhibit different degrees of strain localization. Specifically, the strain results from the mesh-based method have a lower degree of strain localization than the voxel-based method, which is mainly due to the lack of accounting for particle rotations in the calculation. While the grid-based method provides basically consistent strain results with the voxel-based calculation when sufficient grids are used in the calculation. The study highlights the important role of particle rotation in the strain localization of sands. (C) 2018 Elsevier B.V. All rights reserved.