Settling velocity and aerodynamic equivalent diameter are properties of eolian particles that determine eolian transportation and deposition. This study establishes a relationship between geometric diameter and aerodynamic equivalent diameter. Glass spheres and five natural eolian deposits were dry-sieved to 15 size classes from 10 to 600 μm using an ultrasonic sieve. A vertical settling air tube (VSAT) was used to measure the average settling time and velocity. Terminal settling velocity and aerodynamic diameter were obtained by Stokes' Law and Newton's motion equation. Aerodynamic equivalent diameter varies as a power function of sieve diameter. Aerodynamic diameters are larger than geometric diameters for particles finer than 100 μm and smaller for particles coarser than 100 μm. Air settling decreases the spread of particle sizes as estimated from sieving. A shape factor (ψ) is defined to show the relationship between aerodynamic equivalent diameter and geometric diameter. The measured ψ varies as a natural logarithmic function of particle Reynolds number, not a constant as the existing literature suggested.