The effect of spatial sampling in the calculation of sound fields generated by piston radiators mounted in an infinite baffle was analyzed, and quantitative analysis was performed for the case of rectangular pistons. The source function is modified with spatial sampling and the efficient sampling is found to be such that the source size divided by sampling interval should be odd integers. The aliasing error in the field calculation is defined as the ratio of the power radiated by the original source to the power radiated by the replicated sources that are generated in the wave‐number domain by the sampling process. The aliasing error decreases as sampling frequency and source size increase. The results can be used as a reference when calculating sound fields by the spatial sampling method in that, once the source size and desired aliasing error are fixed, appropriate sampling frequency can be obtained.