# Filling radius

In Riemannian geometry, the **filling radius** of a Riemannian manifold *X* is a metric invariant of *X*. It was originally introduced in 1983 by Mikhail Gromov, who used it to prove his systolic inequality for essential manifolds, vastly generalizing Loewner's torus inequality and Pu's inequality for the real projective plane, and creating systolic geometry in its modern form.

The filling radius of a simple loop *C* in the plane is defined as the largest radius, *R* > 0, of a circle that fits inside *C*: