Earth Circumference Calculator
How far would you walk to circle the entire planet? This calculator answers that with the classic formula C = 2πr. Choose one of three standard Earth radii — the IUGG mean radius (6,371.0088 km), the WGS84 equatorial radius (6,378.137 km) or the WGS84 polar radius (6,356.752 km) — or type in any custom radius, and the tool returns the circumference in kilometres and miles instantly.
The results are spherical approximations, which is why the three models disagree slightly: the famous 40,075 km figure is the equatorial circumference, while the mean-radius sphere gives about 40,030 km. For reference, education and trip planning these differences don't matter; for geodesy, surveyors use ellipsoidal models instead.
Formula: C = 2πr with r = 6,371.009 km (spherical-Earth approximation; the real Earth is an oblate spheroid, so equatorial and polar circumferences differ slightly).
How to use this tool
Pick the mean, equatorial or polar radius from the dropdown. Mean is the best all-round choice; equatorial reproduces the famous 40,075 km figure.
Select “Custom radius…” and type any positive value in kilometres — handy for other planets, e.g. 3,389.5 km for Mars.
The circumference appears in kilometres and miles, with the exact unrounded value shown underneath each figure.
What people use this tool for
Teachers show how C = 2πr scales with radius, then compare the equatorial and polar results to explain why Earth is an oblate spheroid.
Travellers use the 40,075 km equatorial figure to sanity-check circumnavigation itineraries against great-circle route totals.
Amateur astronomers enter other planets' radii as custom values to compare their circumferences with Earth's at a glance.
How it's calculated
Circumference C = 2πr, treating Earth as a sphere. Constants: IUGG arithmetic mean radius 6,371.0088 km, WGS84 semi-major axis (equatorial) 6,378.137 km, WGS84 semi-minor axis (polar) 6,356.752 km. Miles use the international mile, 1 km = 0.621371 mi. The real Earth is an oblate spheroid, so these spherical values are approximations good to about 0.1%.
Related tools and resources
Frequently asked questions
Why do the mean, equatorial and polar options give different answers?
Earth bulges at the equator: the equatorial radius (6,378.137 km) is about 21 km larger than the polar radius (6,356.752 km). Since C = 2πr, that difference carries straight through — roughly 40,075 km for the equatorial model versus about 39,941 km for the polar one.
Who first measured Earth's circumference?
Eratosthenes of Cyrene, around 240 BCE. He compared the Sun's angle at noon in Syene and Alexandria, estimated the angle between the cities, and scaled it up to a full circle — landing within a few percent of the true value with nothing but sticks and shadows.
Is 40,075 km the exact circumference?
No. It is 2π × 6,378.137 km rounded to the nearest kilometre — the circumference of a sphere with the WGS84 equatorial radius. The mean-radius sphere gives about 40,030 km, and the true oblate Earth has no single circumference at all.
Can I calculate the circumference of other planets?
Yes. Choose “Custom radius…” and enter the planet's mean radius in kilometres — 3,389.5 km for Mars gives about 21,297 km, for example. The same C = 2πr formula works for any sphere.
Data sources & methodology
- Wikipedia — Earth radius — IUGG mean radius 6,371.0088 km and the distinction between equatorial, polar and mean radii.
- WGS84 (NGA) — World Geodetic System 1984 defining document; source of the 6,378.137 km / 6,356.752 km ellipsoid constants.
- IUGG — International Union of Geodesy and Geophysics, which standardises Earth's mean radius values.