How heavy is Earth?

Weight is the force of gravity pressing a mass down on whatever supports it. Nothing supports the Earth. It falls around the Sun once a year, moving sideways fast enough to keep missing it. So the honest answer to “how much does Earth weigh?” is its mass: NASA’s Earth fact sheet gives 5.9722 × 10²⁴ kg, which this page rounds to 5.97 × 10²⁴ kg.

Pounds work as a unit of mass too, so the same amount of matter is about 13.17 septillion lb, written out as 13,170,000,000,000,000,000,000,000 lb. Gravity also runs both ways. A 70 kg (154 lb) person standing on the ground is pulled down with about 690 newtons, and pulls the whole planet up toward them with exactly the same force.

You measure how hard it pulls. Tracking satellites such as LAGEOS-1 with laser pulses shows how fast they fall around Earth, and that gives the product of Earth’s mass and the gravitational constant, called GM. NASA’s Jet Propulsion Laboratory lists it as 398,600.435507 km³/s², known to about 2 parts in a billion.

Dividing by G gives the mass: 3.986004 × 10¹⁴ m³/s² ÷ 6.67430 × 10⁻¹¹ m³/(kg·s²) = 5.9722 × 10²⁴ kg. The weak link is G itself, which is known only to about 2 parts in 100,000. That leaves Earth’s mass uncertain by about 6 × 10²⁰ kg, so only the first four or five digits are solid.

Isaac Newton guessed that Earth was five or six times as dense as water, which turned out to be close. The first real measurement came in 1774 at Schiehallion, a mountain in Scotland: astronomers measured how far the mountain’s gravity pulled a plumb line off vertical. Charles Hutton worked out an average density for Earth of about 4.5 g/cm³, about 20% too low.

In 1797 and 1798 Henry Cavendish used a torsion balance designed by John Michell: a 1.8 m (6 ft) rod carrying two 0.73 kg (1.61 lb) lead balls, hung from a wire beside two 158 kg (348 lb) lead balls. The big balls pulled the small ones with a force of about 1.74 × 10⁻⁷ newtons, the weight of 0.0177 mg, and twisted the rod by about 0.16 degrees. From that he got a density of 5.448 times water, about 1% below today’s 5.51. Cavendish never wrote down G himself; later scientists turned his result into a value for it.

Most of it is deep down. The iron and nickel core fills about 15% of Earth’s volume but holds more than 30% of its mass, and the rocky mantle holds close to 70%. The oceans and other water come to about 1.4 × 10²¹ kg, roughly 0.02% of the total, and the whole atmosphere to 5.1 × 10¹⁸ kg, less than one millionth.

All that iron makes Earth the densest planet in the Solar System, at 5,513 kg/m³ on average. Its mass also changes slightly. Earth gains about 37,000 to 78,000 tonnes of space dust and meteorites a year but loses about 95,000 tonnes of hydrogen and 1,600 tonnes of helium to space, a net loss of roughly 55,000 tonnes (121 million lb) a year. That is far too small to notice against the uncertainty in the total.