Feret diameter
![Illustration of horizontal and vertical Feret diameters of a particle, Fh and Fv, respectively.](/Images/godic/202501/13/Feret_diameter1639.png")
![The diameter of an object measured with a caliper is sometimes called the caliper diameter; it is the same as Feret diameter.](/Images/godic/202501/13/DigitalCaliperEuro1639.jpg")
![Feret diameter applied to a projection of a 3D object.](/Images/godic/202501/13/Diamètre_de_feret1639.jpg")
The Feret diameter or Feret's diameter is a measure of an object size along a specified direction. In general, it can be defined as the distance between the two parallel planes restricting the object perpendicular to that direction. It is therefore also called the caliper diameter, referring to the measurement of the object size with a caliper. This measure is used in the analysis of particle sizes, for example in microscopy, where it is applied to projections of a three-dimensional (3D) object on a 2D plane. In such cases, the Feret diameter is defined as the distance between two parallel tangential lines rather than planes.