What Is the Weight of the Planet Earth?

Earth's weight: how and when it was determined

The weight of planet Earth was first determined in 1798 by the celebrated English scientist Henry Cavendish (1731–1811). Cavendish applied Newton's law of gravitation and, for the first time, used a deceptively simple instrument — the torsion balance — to weigh the Earth.

Henry Cavendish was the first to determine the weight of planet Earth using a torsion balance. His result fixed a value for the gravitational constant and, through it, the mass of the Earth — a number that modern measurements have refined but never overturned.

The Cavendish experiment and the torsion balance

The Cavendish experiment measured the tiny gravitational attraction between lead spheres in a laboratory, which then allowed scientists to calculate the entire planet's mass. Because the gravitational constant (denoted G) links force, mass, and distance, measuring G with known masses in the lab makes it possible to "weigh" the Earth without ever placing it on a scale.

The methodology of the Cavendish experiment

Cavendish suspended a horizontal rod with two small lead balls at its ends from a thin wire, then brought two large lead spheres close to them. The gravitational pull between the large and small masses twisted the wire by a minute amount, and the angle of twist revealed the force. Knowing the masses, their separation, and the measured force, Cavendish solved Newton's equation for G, and from G he derived the Earth's density and mass. The genius of the setup lay in detecting a force billions of times weaker than ordinary weight using nothing but a delicately balanced beam.

Modern use of the torsion-balance principle in geological prospecting

The principle of these scales forms the basis of the modern gravitational variometer. This instrument has exceptionally high sensitivity and is widely used in geological prospecting, where small variations in gravitational pull reveal dense ore bodies or oil-bearing structures hidden beneath the surface.

Early estimates and Newton's calculations

Isaac Newton had already estimated the Earth's density before Cavendish ever built his balance, guessing it to be roughly five to six times that of water — remarkably close to the true figure of about 5.5. Earlier still, the Schiehallion experiment of 1774, led by Nevil Maskelyne with calculations by Charles Hutton, attempted to weigh the Earth by measuring how a Scottish mountain, Schiehallion, deflected a hanging plumb line. The Schiehallion experiment gave the first serious value for the Earth's mean density, and Cavendish's torsion balance later improved on it dramatically.

The difference between the weight and the mass of the Earth

Mass and weight are not the same thing: mass is the amount of matter in an object and stays the same everywhere, while weight is the gravitational pull acting on that mass and changes with location. The Earth's mass is an invariant quantity — it would be identical whether measured from the Moon, from Mars, or from deep space. What a bathroom scale reads is weight, the force of gravity squeezing a spring or balancing a lever, and that force varies from planet to planet.

This distinction explains why scientists speak of "Earth mass" as a fixed astronomical unit rather than a weight in tonnes. Astronomers even use the Earth mass as a yardstick, comparing exoplanets such as Kepler-442b to it instead of quoting raw kilograms.

The precise value of the Earth's mass and scientific notation

The Earth's mass is approximately 5.972 × 10²⁴ kilograms, written in scientific notation because the figure is far too large to read comfortably in full. In everyday units that is close to 6,000 trillion tonnes — more exactly 5,989,000,000,000,000,000,000 tonnes. The superscript exponent simply counts how many places the decimal point would move: 10²⁴ means a 1 followed by twenty-four zeros.

The related quantity GM, called the geocentric gravitational constant, is actually known far more precisely than the mass itself, because spacecraft tracking measures the product of gravity and mass directly. The mass on its own carries a larger relative uncertainty, since it depends on the value of G — still the least precisely measured of the fundamental constants. If the whole mass of the Earth could be loaded into railway wagons at 50 tonnes each, the resulting train would be eight billion times longer than the distance from the Earth to the Sun. That is how great and how heavy our Earth is!

The dimensions of planet Earth

Earth is a comparatively large celestial body, very slightly flattened at the poles rather than a perfect sphere. The dimensions of planet Earth are as follows.

Radii, diameters and circumferences

• polar radius – 6,357 kilometres,
• equatorial radius – a little over 6,378 kilometres,
• polar diameter – 12,714 kilometres,
• equatorial diameter – 12,757 kilometres,
• meridian length, i.e. the circumference drawn through both poles, 40,004 kilometres;
• equatorial circumference – 40,075 kilometres.

The equatorial bulge — the equator being wider than the pole-to-pole circumference — is the direct result of the Earth's rotation, which throws material outward at the middle.

Earth's surface area and volume

• surface of the entire globe – about 510 million square kilometres;
• of which land is about 148 million square kilometres,
• seas and oceans about 362 million square kilometres;
• volume of the whole Earth is 1,083 billion cubic kilometres,
• density of the Earth is 5.5 times that of water,
• weight of planet Earth is about 6,000 trillion tonnes, more precisely 5,989,000,000,000,000,000,000 tonnes.

Water covers roughly 71 percent of that surface, yet all of it together — every ocean, sea, lake and river — accounts for only a tiny fraction of the planet's total mass, since the oceans are a thin film over an enormous rocky and metallic interior.

Earth's size relative to the other planets

Earth is the largest of the four rocky inner worlds but is dwarfed by the gas giants. Mercury, Venus and Mars are all smaller than Earth, with Mercury barely a third of its diameter, while Jupiter is about eleven times wider and Saturn, Uranus and Neptune all considerably larger as well. Within the solar system the Moon is roughly a quarter of Earth's diameter, an unusually large satellite relative to its planet.

The density of planet Earth

The average density of planet Earth is about 5.5 times that of water and four times the mean density of the Sun. Individual metals lying deep in the Earth's interior have even higher densities — platinum, for instance, is 21 times denser than water. Yet even the density of platinum is not so very great when one considers that some bodies in the universe have densities that seem utterly incredible.

How density varies inside the Earth

The Earth is far denser at its centre than at its surface, which is why the average value of 5.5 is so much higher than the roughly 2.7 of ordinary surface rock. The outer crust is light silicate rock, the mantle beneath is denser, and the core — chiefly iron and nickel — reaches densities of around 13 times that of water under immense pressure. This layered increase with depth is exactly what makes the planet's average density a blend rather than a single uniform figure.

Earth's composition and internal structure

Earth is built in concentric shells: a thin solid crust, a thick rocky mantle, a liquid outer core, and a solid inner core of iron and nickel. By mass the planet is dominated by iron, oxygen, silicon and magnesium, with the heavy metals concentrated toward the centre and the lighter compounds floating above. This separation, called differentiation, happened while the young Earth was molten and the densest material sank inward.

The age and geological history of the Earth

The Earth is about 4.54 billion years old, formed from the same cloud of dust and gas that produced the rest of the solar system. Over that vast span its surface has been reshaped continually by plate tectonics, volcanism and erosion, so almost none of the original crust survives. Surface temperatures across the modern planet span an enormous range, from well below −80 °C in the coldest Antarctic conditions to above +55 °C in the hottest deserts.

Earth's atmosphere and how gravity holds it

Earth keeps its atmosphere because its gravitational pull is strong enough to prevent the gas molecules from escaping into space. A world's ability to retain an atmosphere depends on its mass and temperature: low-mass bodies like the Moon lose their gases entirely, while massive planets hold thick envelopes. Earth's mass sits in the range that retains nitrogen and oxygen comfortably but lets the lightest gases, hydrogen and helium, slowly leak away.

The composition of the atmosphere

Earth's atmosphere is about 78 percent nitrogen and 21 percent oxygen, with the remaining one percent made up of argon, carbon dioxide, water vapour and trace gases. High above the surface lies the ozone layer, which screens out harmful ultraviolet light; if all that ozone were brought down to sea-level pressure it would form a layer only about three millimetres thick, yet that thin shell makes complex life on the surface possible.

Earth's rotation about its axis

Earth spins once on its axis roughly every 24 hours, and this rotation produces day and night as well as the equatorial bulge noted earlier. The spin also drives large-scale wind and ocean patterns and gives the planet its angular momentum. Because the equator races eastward at over 1,600 kilometres per hour while the poles barely turn, the planet's shape is an oblate spheroid rather than a perfect ball.

The density of other planets and stars

Density across the universe ranges from emptier than air to almost unimaginably compact. The density of the companion of Sirius is 40,000 times that of water. If you filled a matchbox with the substance of that star and placed it on a balance pan, you would have to put 15 people on the other pan to bring the scale into equilibrium.

Ultra-dense stars: the companion of Sirius and Kuiner's star

An even more astonishing density belongs to a tiny star discovered by the astronomer Kuiner. The substance of this little star is 10,000 times denser than platinum. A matchbox filled with this material would weigh about 50 tonnes! Try putting that box in your pocket! It should be noted, however, that among the stars there are also some whose average density is 100 times less than that of the air around us. That is how greatly the density of stars can vary.

The law of universal gravitation

Once it became clear that our planet is bounded on all sides in space, the question naturally arose: on what does the Earth actually "rest" in the abyss of space? What force keeps it on its orbit? This question long remained open.

For several centuries no one could resolve it. At last the brilliant mathematician, mechanic, physicist and astronomer, the great Isaac Newton (1643–1727), discovered the law of universal gravitation. Newton proved that all bodies in the space of the universe attract one another (for more detail: The law of falling bodies).

Newton's discovery of the law

The force of attraction between any two bodies is greater the smaller the distance between them and the greater their mass. More precisely, Newton's law reads: every material particle attracts another material particle with a force directly proportional to their masses and inversely proportional to the square of the distance between them.

How a planet's mass relates to distance and orbital period

Scientists weigh any planet by watching how long its moons or spacecraft take to circle it and how far away they orbit. Newton's law, combined with Kepler's relationship between orbital period and distance, means the mass of the central body follows directly from those two measurements. This is how the masses of Mercury, Venus, Mars, Jupiter, Saturn, Uranus and Neptune were all established — and why a fast, close orbit signals a heavy planet while a slow, distant one signals a lighter body. Satellite-tracking missions such as LAGEOS-1 refine the Earth's own gravitational parameter to extraordinary precision using exactly this principle.

Comparing the masses of the planets in the solar system

To keep all the planets on their orbits, the central body of the system — around which all the planets, including our Earth, move — must possess a gigantic mass. Indeed, the mass of the Sun, the central body of the planetary system, is 750 times greater than the mass of all the planets and their satellites combined, and more than 330 thousand times the mass of the Earth. Among the planets themselves Jupiter is by far the heaviest, more than 300 times the mass of Earth, while Mercury and Mars are only fractions of an Earth mass.

The mass of the Earth compared with the mass of the Sun

The Sun holds the planets on their orbits by its attraction. The ratio between the Earth's mass and the Solar mass is roughly one to 333,000, which is why the Sun governs the entire solar system. If there were no attraction between a planet and the Sun, then a planet P moving, for example, in the direction PK would travel in that direction in a straight line and at constant speed (by the law of inertia). In the first second it would move from point P to point K and would finally leave the solar system.

Conversely, if the planet experienced only the attraction toward the Sun, then in the first second it would move from point P to point L. But because this body is simultaneously attracted toward the Sun and is also moving, it will travel along the diagonal PA of the parallelogram built on these two motions.

Consequently, at the end of the first second the planet will be neither at point K nor at point L, but will have moved along the diagonal to point A. Reasoning in the same way, we conclude that in the second second the planet will move to point B, in the third second to point C, and so on.

So this, it turns out, is the force that in the silence of the immeasurable abysses of cosmic space ceaselessly carries our Earth and the other planets around the Sun without the slightest jolt or shudder. Many of you have probably, as children, enjoyed swinging a pebble tied to the end of an elastic band.

While it is whirled, the band stays stretched, but the moment you stop whirling it the band contracts and the pebble draws back toward your hand.

And if the band were suddenly torn from your hand, the pebble and band would instantly fly off. Something similar would happen to the planets if the Sun were suddenly to stop attracting them.

The origin and etymology of the name "Earth"

The name "Earth" descends from old Germanic and Old English words meaning ground or soil, and unlike the other planets it is not named after a Greek or Roman deity. This reflects how ancient the word is — it referred to the solid ground beneath people's feet long before anyone understood that this ground was itself a planet moving through space.

A visual comparison of Earth's weight with other objects

Putting the Earth's mass beside familiar objects makes the scientific notation easier to grasp, since the bare number — about 6,000 trillion tonnes — defeats the imagination. The African bush elephant, the heaviest land animal alive, weighs around six tonnes; you would need roughly a thousand trillion such elephants to balance the planet. The contrast shows why astronomers abandon ordinary weights entirely and adopt the Earth mass as a unit in its own right, comparing distant worlds and exoplanets to our own planet rather than quoting endless rows of zeros.

Telescopes and space missions have given us the imagery behind these comparisons: infrared surveys such as 2MASS, WISE, Spitzer and Herschel, deep-space probes like Voyager 1, and observations of objects from the Flame Nebula and the Helix Nebula to the galaxy NGC 1097, the neighbouring Andromeda galaxy and the Galactic Center — many catalogued and imaged by NASA/JPL-Caltech and explained for the public by outlets such as Inside Science. For more astronomy reading, browse our Astronomy section or the related articles in Nature.