What Is Mass?
Definition, SI units, measurement, and how mass differs from weight — explained clearly.
Definition of Mass
Mass is a fundamental property of matter that quantifies the total amount of matter contained in an object. Unlike weight — which is a force that depends on gravity — mass is an intrinsic, invariant property that remains constant regardless of where an object is located in the universe.
In everyday language, mass tells you "how much stuff" is in an object. A bowling ball has more mass than a tennis ball because it contains more matter. The SI unit of mass is the kilogram (kg), which is the globally standardized unit used in science, medicine, industry, and everyday life in most countries.
The kilogram is currently defined by fixing the numerical value of the Planck constant (h = 6.62607015 × 10⁻³⁴ J·s), making it one of the most precisely defined units in measurement history — a definition adopted in 2019 by the International Bureau of Weights and Measures (BIPM).
The SI base unit of mass is the kilogram (kg). In the US and UK, mass is commonly expressed in pounds (lbs) for everyday use. 1 kg = 2.2046 lbs. Use our live converter to convert any mass value.
Mass vs Weight — What's the Difference?
Mass and weight are often confused but are fundamentally different physical quantities.
| Property | Mass | Weight |
|---|---|---|
| Definition | Amount of matter in an object | Gravitational force on an object |
| SI Unit | Kilogram (kg) | Newton (N) |
| Everyday Unit | kg, g, lb, oz | N, kgf, lbf |
| Changes with gravity? | No — constant everywhere | Yes — different on Moon, Mars, etc. |
| On the Moon | Same as on Earth | ~1/6 of Earth weight |
| Measured by | Balance scale or inertia | Spring scale or force gauge |
| Formula | m (scalar quantity) | W = m × g |
All Units of Mass
Complete list of mass measurement units and their relation to the kilogram.
| Unit Name | Symbol | Relation to Kilogram |
|---|---|---|
| Kilogram (SI Base) | kg | 1 kg (base SI unit) |
| Gram | g | 1 g = 0.001 kg |
| Milligram | mg | 1 mg = 0.000001 kg |
| Metric Ton | t | 1 t = 1,000 kg |
| Microgram | µg | 1 µg = 0.000000001 kg |
| Pound | lb | 1 lb = 0.45359237 kg |
| Ounce | oz | 1 oz = 0.0283495 kg |
| Stone | st | 1 st = 6.35029 kg |
| Tonne (long) | LT | 1 LT = 1,016.05 kg |
| US Ton (short) | T | 1 T = 907.185 kg |
Mass in Science & Physics
Force equals mass times acceleration. The greater the mass of an object, the more force required to accelerate it. This is called inertial mass.
Energy equals mass times the speed of light squared. This means mass and energy are interchangeable — mass can be converted to enormous amounts of energy.
Weight (W) is the force of gravity on a mass (m), where g = 9.81 m/s² on Earth's surface. Gravitational mass determines how strongly an object is pulled by gravity.
Newton's law of gravitation: The force between two masses (m₁ and m₂) is proportional to the product of their masses and inversely proportional to the square of the distance (r) between them.
How to Convert Mass Units
The most common mass conversion in everyday life is between kilograms and pounds. The exact conversion factor is established by international agreement:
Use our live kg to lbs converter for any mass value, or check the weight conversion chart for a printable reference table.
Frequently Asked Questions
Common questions about mass, its definition, and units.
What is mass?
Mass is the measure of the amount of matter in an object. It is a fundamental physical quantity that does not change with location. Mass is measured in kilograms (kg) in the international SI system.
What is the definition of mass in science?
In physics, mass is defined as the resistance of an object to acceleration (inertial mass) and also as the strength of its gravitational attraction to other objects (gravitational mass). Einstein's equivalence principle confirms both types of mass are equal.
What is the SI unit of mass?
The SI (International System of Units) unit of mass is the kilogram (kg). It is one of the seven base SI units. Since 2019, the kilogram is defined by fixing the numerical value of the Planck constant to exactly 6.62607015 × 10⁻³⁴ J·s.
What are the units of mass?
Common units of mass include: kilogram (kg), gram (g), milligram (mg), metric ton (t), pound (lb), ounce (oz), stone (st), and microgram (µg). The SI standard unit is the kilogram.
What is mass measured in?
Mass is measured in kilograms (kg) in the metric/SI system, and in pounds (lbs) or ounces (oz) in the imperial/US customary system. Scientists worldwide use kilograms.
How is mass different from weight?
Mass is the amount of matter in an object and is constant regardless of location. Weight is the gravitational force acting on that mass and changes with gravity. On the Moon, your mass stays the same but your weight is about 1/6th of what it is on Earth.
What does mass mean in science?
In science, mass quantifies the amount of matter in an object. It is central to Newton's Second Law of Motion (F = ma), where a greater mass requires more force to accelerate. Mass also appears in Einstein's famous equation E = mc², showing mass and energy are equivalent.
What is mass equal to?
Mass is equal to the amount of matter in an object. In Newtonian mechanics: F = ma (force = mass × acceleration). In relativity: E = mc² (energy = mass × speed of light squared). Mass is also related to gravitational attraction between objects.
What is the symbol for mass?
The symbol for mass in physics equations is typically lowercase 'm'. The SI unit symbol for the kilogram is 'kg'. In chemistry, atomic mass uses the unified atomic mass unit 'u' or 'Da' (Dalton).
Is mass the same everywhere in the universe?
Yes. Mass is a fundamental, invariant property of an object that does not change with location. Whether on Earth, the Moon, Mars, or in space, an object's mass remains constant. Only its weight (gravitational force) changes with different gravity levels.