Earth radius (denoted as R๐จ or ${\displaystyle R_{E}}$) is the distance from the center of Earth to a point on or near its surface. Approximating the figure of Earth by an Earth spheroid, the radius ranges from a maximum of nearly 6,378 km (3,963 mi) (equatorial radius, denoted a) to a minimum of nearly 6,357 km (3,950 mi) (polar radius, denoted b).

Cross section of Earth's Interior
General information
Unit systemastronomy, geophysics
Unit ofdistance
SymbolR๐จโorโ${\displaystyle R_{E}}$, ${\displaystyle {\mathcal {R}}_{\mathrm {eE} }^{\mathrm {N} }}$
Conversions
1 R๐จ in ...... is equal to ...
SI base unit   6.3781ร106 m[1]
Metric system   6,357 to 6,378 km
English units   3,950 to 3,963 mi

A nominal Earth radius is sometimes used as a unit of measurement in astronomy and geophysics, which is recommended by the International Astronomical Union to be the equatorial value.[1]

A globally-average value is usually considered to be 6,371 kilometres (3,959 mi) with a 0.3% variability (ยฑ10 km) for the following reasons. The International Union of Geodesy and Geophysics (IUGG) provides three reference values: the mean radius (R1) of three radii measured at two equator points and a pole; the authalic radius, which is the radius of a sphere with the same surface area (R2); and the volumetric radius, which is the radius of a sphere having the same volume as the ellipsoid (R3).[2] All three values are about 6,371 kilometres (3,959 mi).

Other ways to define and measure the Earth's radius involve the radius of curvature. A few definitions yield values outside the range between polar radius and equatorial radius because they include local or geoidal topography or because they depend on abstract geometrical considerations.