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*.

From Cauchy's theorem it follows that for a 2D convex body, the Feret diameter averaged over all directions (〈F〉) is equal to the ratio of the object perimeter (P) and pi, i.e., 〈F〉 = P/π. There is no such relation between 〈F〉 and P for a concave object.

Feret diameter is used in the analysis of particle size and its distribution, e.g. in a powder or a polycrystalline solid; Alternative measures include Martin diameter, Krumbein diameter and Heywood diameter. The term first became common in scientific literature in the 1970s.

It is also used in biology as a method to analyze the size of cells in tissue sections.