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Because the Earth is not perfectly spherical, no single value serves as its natural radius. Distances from points on the surface to the center range from 6,353 km to 6,384 km (≈3,947–3,968 mi). Several different ways of modeling the Earth as a sphere each yield a convenient mean radius of 6371 km (≈3,959 mi).

While "radius" normally is a characteristic of perfect spheres, the term as employed in this article more generally means the distance from some "center" of the Earth to a point on the surface or on an idealized surface that models the Earth. It can also mean some kind of average of such distances. It can also mean the radius of a sphere whose curvature matches the curvature of the ellipsoidal model of the Earth at a given point.

This article deals primarily with spherical and ellipsoidal models of the Earth. See Figure of the Earth for a more complete discussion of models.

The first scientific estimation of the radius of the earth was given by Eratosthenes.

Earth radius is also used as a unit of distance, especially in astronomy and geology. It is usually denoted by $R_\oplus$.