How do scientists measure mass?

Figuring out the mass of an object seems like it should be simple: just put it on a scale. However, what scientists actually measure depends heavily on the object’s size, location, and state of matter. For everyday items, we use instruments that compare its pull against a known reference, but for something as vast as a planet or a galaxy, the method shifts entirely to observation and calculation based on fundamental laws of physics. ^[5][2]

# Mass Versus Force

Before diving into the how, it is essential to clarify a common point of confusion: the difference between mass and weight. ^[10] Mass is an intrinsic property of matter—it is the measure of how much "stuff" an object contains and its resistance to changes in motion, known as inertia. ^[1][6] Weight, on the other hand, is the force exerted on that mass by gravity. ^[10] This distinction matters greatly in measurement. If you take a bowling ball to the Moon, its mass remains the same, but its weight decreases significantly because the gravitational pull is weaker. ^[6][10] Therefore, scientists aim to measure mass, even when using instruments that appear to register weight. ^[1]

# Lab Measurements

In a controlled laboratory environment, measuring mass typically involves instruments designed to compare an unknown mass against a known, standard mass. ^[1][9]

# Precision Balances

The workhorse for many laboratory settings is the analytical balance or electronic balance. ^[9] These modern instruments have largely replaced older mechanical balances but operate on a similar principle: creating an equilibrium.

In a simple two-pan balance, you place the object of unknown mass on one pan and add standardized weights to the other until the beam balances perfectly. ^[6] The unknown mass is then equal to the total mass of the standard weights used. ^[6]

Modern electronic balances function through electromagnetic force restoration. When an item is placed on the pan, the pan drops slightly, and a sensor detects this movement. An electronic circuit then generates an opposing electromagnetic force—a calibrated current—to bring the pan back to its exact original position. ^[9] The mass is then calculated based on the precise electrical current required to counteract gravity’s pull on the object. ^[9] This method allows for incredibly high precision, often measuring down to micrograms. ^[1]

Consider this: when a chemist uses a sensitive analytical balance to find the mass of a tiny crystal, the measurement itself is fundamentally a comparison against a standardized unit of mass, such as the kilogram, which is currently defined by a specific number of photons emitted during a specific quantum transition, rather than a physical object kept in Paris. ^[7][1] The sheer accuracy required for chemistry and particle physics means that errors of even a few parts per million must be accounted for, something purely mechanical systems struggle to achieve consistently. ^[1][5]

# Measuring Smaller Scales

For the extremely small—atoms and subatomic particles—scientists use methods that rely on interactions rather than direct weighing. Particle accelerators, for instance, measure mass indirectly by observing how charged particles are deflected by magnetic fields. ^[1] The amount of deflection is directly related to the particle’s inertia (mass). ^[1] A lighter particle bends more easily than a heavier one under the same magnetic force. ^[1]

How do scientists measure the size of the Earth?

# Cosmic Measurements

When dealing with objects that exert significant gravitational fields—planets, stars, or entire galaxies—placing them on a scale is obviously impossible. ^[2] Instead, scientists rely on the universal law of gravitation, which provides a pathway to calculate mass based on observable effects. ^[3]

# Gravitational Influence

The core concept here is Newton’s Law of Universal Gravitation, which states that the force of gravity between two objects depends on their masses and the distance between them. ^[3] If you can observe how one object gravitationally affects another, you can determine its mass. ^[2]

For measuring the mass of the Earth, scientists historically used an object of known mass (like a heavy ball) and measured the acceleration due to gravity ($g$) at the surface. ^[3] Knowing the Earth’s radius ($R$) and the Gravitational Constant ($G$), they can then rearrange the gravitational formula ($F = G\frac{Mm}{R^2}$) to solve for the Earth's mass ($M$). ^[3]

For celestial bodies, the most common technique involves observing orbital mechanics. ^[2] If a satellite orbits a planet, or a planet orbits a star, the period and distance of that orbit are mathematically linked to the mass of the central body. ^[2] Kepler's Third Law, refined by Newton, is key here. ^[2]

For example, to find the mass of a star, astronomers track the orbit of a planet or another star around it. ^[2] The orbital period ($T$) and the semi-major axis ($a$) of the orbit allow for a direct calculation of the central star's mass ($M_{star}$):

$$M_{star} + M_{planet} \approx \frac{4\pi^2 a^3}{G T^2}$$

Since the planet's mass is usually negligible compared to the star's mass, the planet's mass ($M_{planet}$) is often ignored, leaving the star's mass as the dominant factor in the equation. ^[2] This same principle is applied to measure the mass of Jupiter by tracking its moons, or to measure the mass of the Milky Way by observing the orbits of stars near the galactic center. ^[2][3]

# Stellar Mass Comparison

The scale of masses involved in astrophysics dwarfs anything we deal with on Earth. For context, the Earth's mass is approximately $5.972 \times 10^{24}$ kilograms. ^[4] Astronomers often use the term Earth mass ($M_\oplus$) as a convenient unit for planetary bodies within our solar system or exoplanetary systems. ^[4]

To give an idea of the difference in measurement approach, compare measuring a grain of sand versus measuring Jupiter:

It is fascinating to realize that while a chemist needs a precisely calibrated electrical signal to find the mass of a speck of dust, an astrophysicist needs a perfect measurement of the time it takes for a comet to complete half an orbit around the Sun to determine the mass of a planet millions of miles away. ^[1][5] The measurement apparatus in space is literally the entire solar system, governed by unchanging physical laws. ^[2]

# Standards and Realization

All these diverse methods—from magnetic deflection to orbital tracking—rely on a consistent definition of mass, rooted in the International System of Units (SI). ^[7] The base unit of mass is the kilogram ($kg$). ^[7] Historically, this unit was defined by a physical artifact, the International Prototype Kilogram (IPK), a cylinder of platinum-iridium alloy. ^[7]

However, science demands stability and reproducibility independent of a physical object that can degrade or be damaged. Since 2019, the definition of the kilogram has changed to be based on a fundamental constant of nature: the Planck constant ($h$). ^[7] This allows for the mass scale to be realized anywhere in the world with high accuracy using specialized equipment, like the Kibble balance (a highly advanced form of the electromagnetic balance mentioned earlier). ^[7] This shift from artifact to constant reinforces the foundation of physical measurement, ensuring that a kilogram measured in a lab today has the exact same mass as one measured in deep space millions of years ago, provided the calculations are based on the fixed value of $h$. ^[1][5]

How do we measure the mass of galaxies?

# Inferences from Non-Gravitational Data

While gravity is the primary way to determine the mass of large, isolated objects, scientists can also infer mass from other properties, particularly in extreme environments. For instance, in studying black holes or neutron stars, mass can sometimes be inferred from the energy emitted or the behavior of accretion disks surrounding them. ^[2] Furthermore, in particle physics, the concept of mass-energy equivalence ($E=mc^2$) allows scientists to calculate the mass of particles that exist only fleetingly by precisely measuring the energy released when they decay. ^[1] This relationship means that measuring energy accurately can, under the right conditions, be equivalent to measuring mass. ^[5]

The breadth of techniques employed—from balancing electromagnetic forces in a vacuum chamber to charting planetary arcs across the night sky—highlights that "measuring mass" is less about a single procedure and more about selecting the most appropriate physical interaction available for the scale of the object in question. ^[5][6]