Feret diameter

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The Feret diameter, or Feret’s diameter, is a simple way to measure an object’s size along a chosen direction. It is the distance between two parallel lines (or planes, for 3D) that just touch the object on opposite sides. In microscopy, this often involves projections of a 3D object onto a 2D plane, where the Feret diameter is the distance between two parallel tangential lines.

For a 2D convex shape, the average Feret diameter over all directions equals the perimeter divided by pi (F̄ = P/π). This relation does not hold for concave shapes.

Feret diameter is used to analyze particle sizes and their distribution in powders or polycrystalline materials. Other size measures include the Martin, Krumbein, and Heywood diameters. The method became widely used in the 1970s and is named after L. R. Feret, who introduced it in the 1930s. It is also used in biology to estimate the size of cells in tissue sections.