The drag force on a particle in a fluid-multiparticle interaction system may be expressed as the product of the drag force on an unhindered particle, subject to the same volumetric flux of fluid, and a voidage function. It is demonstrated that for a wide variety of both fixed-bed and suspended-particle systems, the voidage function may be expressed as epsilon(-beta), where the exponent beta is dependent on the particle Reynolds number but independent of other system variables.