Mass, Volume, and Weight Explained
Mass is the amount of matter in an object, volume is the amount of space it occupies, and weight is the force of gravity pulling on its mass. In brief: mass is measured in kilograms (kg), volume in liters (L) or cubic centimeters (cm3), and weight in newtons (N). An astronaut has the same mass on Earth and on the Moon, but weighs about six times less on the Moon.
These three words are used loosely in daily life, but in chemistry and physics each has a precise meaning. This article explains how they differ, how each is measured, how to convert between them, and why a kilogram on a bathroom scale is really a measurement of mass even though the scale responds to weight.
What is mass?
Mass measures how much matter an object contains. It also measures how strongly the object resists a change in its motion, which is called inertia. A heavy truck is harder to start and stop than a bicycle because it has more mass.
The SI unit of mass is the kilogram (kg). In the laboratory, grams (g) and milligrams (mg) are more common: 1 kg = 1,000 g and 1 g = 1,000 mg. The mass of an object does not change when it moves from one place to another, as long as no matter is added or removed.
What is volume?
Volume is the three-dimensional space occupied by matter. Solids with regular shapes have volumes that can be calculated: a box with length 5 cm, width 4 cm and height 3 cm has a volume of 5 × 4 × 3 = 60 cm3. Liquids are measured directly in graduated containers.
The SI unit of volume is the cubic meter (m3), but laboratories use smaller units. One liter (L) equals 1,000 milliliters (mL), and 1 mL is exactly the same volume as 1 cm3. Gases expand to fill their container, so the volume of a gas is the volume of its container.
What is weight?
Weight is a force. It is the pull of gravity on an object and is calculated as weight = mass × gravitational field strength, or W = m × g. On the surface of Earth, g is about 9.8 N/kg (often rounded to 10 N/kg in school problems).
Because g changes with location, weight changes too. Mass is a property of the object itself, while weight depends on where the object is. Weight is measured in newtons (N) with a spring balance (newton meter).
Mass, volume and weight compared
| Quantity | What it measures | SI unit | Common instrument | Changes with location? |
|---|---|---|---|---|
| Mass | Amount of matter | kilogram (kg) | Balance | No |
| Volume | Space occupied | cubic meter (m3) | Measuring cylinder, ruler | No (for solids and liquids) |
| Weight | Force of gravity on mass | newton (N) | Spring balance | Yes |
How are mass and volume measured?
Measuring mass
A balance compares the unknown mass with standard masses or uses an electronic sensor. Because the balance compares masses in the same place, the result does not depend on the local value of g. Always place the object on the pan gently and read the display when it is stable.
Measuring the volume of an irregular solid
An irregular object such as a stone can be measured by water displacement.
- Pour water into a measuring cylinder and read the initial volume, for example 40 mL.
- Lower the stone gently into the water so that it is fully submerged.
- Read the new volume, for example 55 mL.
- Subtract: 55 mL − 40 mL = 15 mL = 15 cm3.
Worked examples
- Weight on Earth. A student has a mass of 50 kg. With g = 9.8 N/kg, the weight is W = 50 × 9.8 = 490 N.
- Unit conversion. A sample has a mass of 2,350 mg. Dividing by 1,000 gives 2.35 g.
- Volume of a box. A container measures 10 cm × 6 cm × 2 cm. Its volume is 10 × 6 × 2 = 120 cm3 = 120 mL.
- Liters and milliliters. A bottle holds 1.5 L. That equals 1.5 × 1,000 = 1,500 mL.
- From weight to mass. A spring balance shows 29.4 N for a bag of flour on Earth. The mass is m = W ÷ g = 29.4 ÷ 9.8 = 3.0 kg.
Weight on different worlds
The next table uses approximate surface values of g and shows the weight of a 60 kg person. The mass stays the same everywhere, and only the weight changes.
| Place | g (N/kg, approx.) | Weight (N, approx.) | Mass (kg) |
|---|---|---|---|
| Earth | 9.8 | 588 | 60 |
| Moon | 1.6 | 96 | 60 |
| Mars | 3.7 | 222 | 60 |
| Jupiter (cloud tops) | 24.8 | 1,488 | 60 |
In deep space far from any large body, weight is nearly zero, yet the astronaut’s mass is unchanged. That is why astronauts still find it hard to push a heavy object into motion.
Which mistakes are common?
Mistake 1: Using weight and mass as synonyms
In everyday speech people say “I weigh 60 kilograms”. Strictly, kilograms measure mass, and the weight would be about 588 N.
Mistake 2: Confusing mass with volume
A large object is not always heavy. A big foam block has a large volume but a small mass, whereas a small lead ball has a small volume but a large mass.
Mistake 3: Forgetting that 1 mL equals 1 cm3
Learners often convert them incorrectly. The two units describe the same volume, so no calculation is required.
Mistake 4: Believing that weightless means massless
An object that seems weightless in orbit still has its full mass. It is in free fall, not free of matter.