⚡ Quick summary
Mass is a measure of quantity of matter—that is, how much substance something is made of. Weight is a measure of gravitational force—how hard gravity is pulling on something. Mass and weight are closely related in how they are measured in science and are often measured outside of science using the same units, such as kilograms or pounds.
The words mass and weight are used both in physics and outside of it to describe how large (mass) or heavy (weight) something is. While these words are often used interchangeably outside of scientific contexts, they do refer to different things.
In this article, we will define the words mass and weight, explain how they are used in science to measure different quantities, and discuss why they are often used interchangeably outside of scientific contexts.
weight vs. mass
In science, mass is “the amount of matter as determined from its weight or from Newton’s second law of motion.” In comparison, weight is “the force that gravitation exerts upon a body, equal to the mass of the body times the local acceleration of gravity.”
In simpler terms, mass is a measure of how much matter (substance) something is made of, while weight is a measure of how hard gravity is pulling on something. Scientists measure mass in grams, while weight as gravitational force is measured in Newtons.
Outside of scientific contexts, weight and mass are often both measured and expressed using the same units, such as grams, kilograms, ounces, or pounds. In fitness and nutrition, for example, a person’s body mass and body weight are typically discussed as being the same thing and measured using the same units.
The difference between mass and weight is most easily seen under abnormal conditions, such as in outer space. On Earth, a person’s mass and weight are typically described as being equal. If a person goes into outer space or experiences conditions where gravity is different from Earth’s gravity, their mass will stay the same, but their weight will go down or up depending on the differences in gravity. For example, an astronaut has the same mass on the Earth and the moon, but their weight will be lower on the moon because the moon’s lower gravity will be pulling on them less. There isn’t less of the astronaut but rather less pull by gravity.
Mass is used in chemistry and physics because gravity often changes under abnormal conditions. Mass is a more useful and consistent measurement when discussing subatomic particles or objects in outer space. An object’s mass will typically remain constant regardless of the environment. This is why chemists and physicists will more often discuss body mass, which isn’t affected by gravity, rather than body weight, which is affected by gravity, when discussing astronauts and celestial objects.
