Earth radius

Earth radius
Equatorial (a), polar (b) and arithmetic mean Earth radii as defined in the 1984 World Geodetic System revision (not to scale)
Other names	terrestrial radius
Common symbols	R🜨, RE, a, b, aE, bE, ReE, RpE
SI unit	meters
In SI base units	m
Behaviour under coord transformation	scalar
Dimension	${\displaystyle {\mathsf {L}}}$
Value	Equatorial radius: a = (6378137.0 m) Polar radius: b = (6356752.3 m)

Nominal Earth radius
Cross section of Earth's Interior
General information
Unit system	astronomy, geophysics
Unit of	distance
Symbol	${\displaystyle {\mathcal {R}}_{\mathrm {E} }^{\mathrm {N} }}$, ${\displaystyle {\mathcal {R}}_{e\mathrm {E} }^{\mathrm {N} }}$, ${\displaystyle {\mathcal {R}}_{p\mathrm {E} }^{\mathrm {N} }}$
Conversions
1 ${\displaystyle {\mathcal {R}}_{\mathrm {E} }^{\mathrm {N} }}$ in ...	... is equal to ...
SI base unit	6.3781×106 m
Metric system	6,357 to 6,378 km
English units	3,950 to 3,963 mi

Earth radius (denoted as R_🜨 or 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 (an oblate ellipsoid), the radius ranges from a maximum (equatorial radius, denoted a) of about 6,378 km (3,963 mi) to a minimum (polar radius, denoted b) of nearly 6,357 km (3,950 mi).

A globally-average value is usually considered to be 6,371 kilometres (3,959 mi) with a 0.3% variability (±10 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). All three values are about 6,371 kilometres (3,959 mi).

Other ways to define and measure the Earth's radius involve either the spheroid's radius of curvature or the actual topography. A few definitions yield values outside the range between the polar radius and equatorial radius because they account for localized effects.

A nominal Earth radius (denoted ${\displaystyle {\mathcal {R}}_{\mathrm {E} }^{\mathrm {N} }}$) is sometimes used as a unit of measurement in astronomy and geophysics, a conversion factor used when expressing planetary properties as multiples or fractions of a constant terrestrial radius; if the choice between equatorial or polar radii is not explicit, the equatorial radius is to be assumed, as recommended by the International Astronomical Union (IAU).