Mass is the amount of matter in an object, while volume is the amount of space that object occupies. They are different properties: an object can have a large volume but a small mass, or a small volume but a large mass, depending in part on the material it is made from.
What Mass Means
Mass describes how much matter an object contains. In introductory physics, it is also a measure of an object’s inertia: how strongly it resists a change in its motion. An object with more mass is harder to accelerate or slow down when the same force is applied.
The standard unit of mass in the International System of Units is the kilogram. Smaller quantities are commonly expressed in grams, and very large quantities may be expressed in tonnes. In everyday speech, people often call an object’s mass its “weight,” but physics distinguishes the two. Mass is an intrinsic property of an object; weight is the force of gravity acting on it.
For example, a sealed bag of rice has the same mass whether it is on a kitchen counter or carried to a high mountain. Its weight can change slightly because gravitational strength varies by location, but its mass does not change just because it has been moved.
What Volume Means
Volume measures the three-dimensional space occupied by an object or substance. Its standard SI unit is the cubic metre. Smaller volumes are often given in cubic centimetres, millilitres, or litres. A millilitre is equivalent to a cubic centimetre, a useful relationship when comparing measurements in the laboratory.
Volume applies to solids, liquids, and gases. For a regularly shaped solid, it can often be calculated from dimensions. A rectangular box, for instance, has a volume found by multiplying its length by its width and height. For an irregular object, its volume can be found by measuring how much liquid it displaces, provided the object can safely be submerged and does not dissolve or react.
A liquid’s volume is usually read from a graduated container. In careful laboratory work, the reading is taken at eye level from the bottom of the curved surface, called the meniscus, for many common liquids. The container must be appropriate for the precision needed; a kitchen cup and a calibrated laboratory instrument are not interchangeable for every task.
Mass and Volume Side by Side
| Feature | Mass | Volume |
|---|---|---|
| What it describes | Amount of matter; also related to inertia | Space occupied |
| Common SI unit | Kilogram | Cubic metre |
| Other common units | Gram, tonne | Millilitre, litre, cubic centimetre |
| Typical instrument | Balance | Graduated container or dimensional measuring tool |
| Changes with location? | Not simply because gravity changes | Not simply because gravity changes, though temperature and pressure can affect substances |
| Related concept | Weight | Density |
The distinction matters because the units answer different questions. A measurement of five kilograms says how much mass is present; a measurement of five litres says how much space a substance fills. Those figures cannot be compared directly as though they measured the same property.
How Mass and Volume Are Measured
A balance is used to determine mass by comparing an object with known masses or by sensing the force associated with it and accounting for gravity, depending on the instrument. A balance should be set up on a stable surface and zeroed or tared when appropriate. Taring removes the displayed mass of an empty container from the reading, so the instrument reports the contents alone.
Volume measurement depends on the shape and state of the material:
- For a regular solid, measure its dimensions and use the relevant geometric formula.
- For an irregular solid, use liquid displacement when the method is suitable.
- For a liquid, read a graduated vessel at eye level and use the correct scale.
- For a gas, specify the conditions when needed, because gases expand or contract as pressure and temperature change.
These methods have limits. A balance reading can be affected by poor calibration, drafts, vibration, or a container that has not been accounted for. A volume reading can be affected by an unsuitable vessel, trapped air, spills, or reading the scale from above or below. In science, reporting the measurement and its unit is not enough by itself; the method and precision may also matter.
Density Connects the Two
Density is mass per unit volume. It describes how concentrated matter is in a given amount of space. The relationship is written as density equals mass divided by volume. Rearranging it gives mass equals density multiplied by volume, and volume equals mass divided by density.
Density explains why equal volumes of different materials can have different masses. A small metal cube may have more mass than a much larger piece of a low-density foam. Conversely, equal masses of those materials occupy different volumes. Density is a property used to compare materials, but it can vary with conditions; temperature can change the volume of many substances and therefore change their density.
For a pure, uniform substance under specified conditions, density can help identify or characterize a sample. It is not a direct substitute for measuring mass or volume, however. If a sample is a mixture, porous, or unevenly composed, a single density value may not describe every part of it equally well.
Concrete Examples
Mass example: Imagine weighing a closed jar on a balance. The balance gives the jar’s mass, including its contents, unless the empty jar has been tared or measured separately. Moving the jar from a low-altitude location to a different location does not turn its mass into a volume measurement; the reading remains about matter, not the space the jar occupies.
Volume example: Imagine pouring water into a graduated cylinder. The marked scale tells you the volume of water in the cylinder. If you pour the same water into a wide bowl, its shape and depth change, but the amount of water—and therefore its volume under the same conditions—does not automatically change.
These examples also show why containers can confuse measurements. The jar’s outside dimensions describe the space it occupies, while its contents have their own mass and volume. A container’s capacity is the volume it can hold; that is not necessarily the same as the volume of the container’s material.
Common Mistakes and Misconceptions
- Using mass and weight as synonyms in physics: everyday speech often does this, but weight is a gravitational force and mass is not.
- Assuming a larger object must have more mass: size alone does not determine mass. Material and density matter.
- Assuming a litre always has the same mass: a litre is a volume. Its mass depends on the substance and its conditions.
- Mixing units in a calculation: a density calculation requires compatible units. Convert units consistently before dividing mass by volume.
- Confusing capacity with mass: a bottle’s capacity describes how much volume it can hold, not how heavy its contents will be.
A related question is whether an object’s volume can change while its mass stays the same. Yes. Heating can make many materials expand, so their volume increases while their mass remains unchanged if no matter enters or leaves. Compression can reduce the volume of a gas without removing matter. In either case, density changes because the same mass is spread through a different volume.
How to Remember the Difference
Ask what the measurement is counting. If the question is “how much matter is there?” it is asking about mass. If it is “how much space does it take up?” it is asking about volume. The units are a quick clue: kilograms and grams indicate mass, while litres and cubic units indicate volume.
For a problem in a science course, write the given quantities with units before choosing an equation. If the problem gives mass and volume and asks how concentrated the material is, calculate density. If it gives density and one of the other quantities, rearrange the density relationship to find the missing value. A dimensional check can reveal mistakes: a result for volume should have volume units, not kilograms.
Frequently Asked Questions
Is mass the same as volume?
No. Mass measures matter, while volume measures occupied space. They are related through density, but they are not interchangeable.
Can two objects have the same volume but different masses?
Yes. If they are made of materials with different densities, equal volumes can contain different amounts of mass. A compact metal object and a same-sized foam object illustrate the idea.
Can an object have the same mass but a different volume?
Yes. The same amount of a substance can occupy different volumes when conditions such as temperature or pressure change, especially for gases. Its density changes as its volume changes.
Does a litre measure mass or volume?
A litre measures volume. To find the mass of a litre of a substance, you also need to know its density under the relevant conditions.
Why do people say they weigh a certain number of kilograms?
In everyday settings, scales commonly display a mass in kilograms, and “weight” is often used informally for that reading. In physics, weight refers to gravitational force, while kilograms are units of mass.
Which instrument measures mass and which measures volume?
A balance measures mass. Volume may be measured with a graduated vessel, calculated from dimensions, or found by displacement, depending on the object and material.