Mass vs Weight: What Is the Difference?

Two different quantities, often confused

In everyday speech people say "my weight is 70 kilograms" and nobody is confused. In physics the two words name different things. Mass is a property of the object itself: how much matter it holds and how strongly it resists changes in its motion. Weight is a force, the pull that a planet or moon exerts on that mass.

NIST lists the kilogram as the SI base unit of mass, alongside the metre, second, mole, ampere, kelvin and candela. A force is a derived quantity, and the SI unit of force is the newton. That is why a physicist who says weight means a quantity in newtons, and why a recipe or shipping label that says "weight" in kilograms is really reporting mass.

The relationship between them

Weight equals mass multiplied by the local strength of gravity. Near the surface of the Earth, gravity is about 9.81 newtons for every kilogram, so a 1 kg object weighs about 9.81 N. Doubling the mass doubles the weight at the same place.

Worked example: the same person in three places

Take a person whose mass is 70 kg. On Earth, with gravity of about 9.81 N per kilogram, their weight is 70 × 9.81, or roughly 687 N. On the Moon, where surface gravity is about one-sixth of Earth's, about 1.62 N per kilogram, their weight is about 113 N. In orbit far from any large body the pull is small, though in a spacecraft in orbit the astronaut feels weightless because they are in free fall, not because gravity has vanished. Throughout, the person's mass stays 70 kg.

Why the bathroom scale causes the confusion

A bathroom scale typically measures a force, the push of your feet on the platform, and then displays the result as a mass in kilograms or pounds by assuming standard gravity. That works well anywhere on Earth because gravity varies by only a small fraction of a percent between locations. The same scale carried to the Moon would read about one-sixth as much while your mass stayed the same.

A kitchen balance with a pan and counterweights behaves differently. It compares your sample against reference masses, so gravity affects both sides equally and cancels out. A balance would give the same reading on the Moon, while a spring scale would not.

Pounds, kilogram-force and other units

Everyday units add to the muddle. The pound is used for both mass and force in different contexts, and the kilogram-force, defined as exactly 9.80665 newtons, is a non-SI unit sometimes used for force. In engineering, writing "kg" for a force is a shorthand that works only because of standard gravity. When a calculation needs a force, such as the load on a shelf or the tension in a cable, convert mass to newtons first by multiplying by 9.81.

Where the distinction really matters

For cooking, shopping, and body measurements, treating weight and mass as interchangeable causes no harm. It matters in physics classes, in engineering calculations where forces are required, and in spaceflight, where gravity differs from place to place. Paper manufacturers, for example, report grammage in grams per square metre, which is a measure of mass per area, as explained in What Does Paper GSM Mean.