Measuring Volumes by Counting Unit Cubes

People have been measuring volume for thousands of years. Every time someone needed to know how much grain could fit inside a basket, or how much water a jug could hold, they were thinking about volume. Let's take a quick trip through history to see how measuring volume has changed over time!

Throughout history, people always asked the same big question: "How much space does this object take up?" That question is exactly what volume answers. Today, you'll learn the clearest and simplest way to find volume — by counting unit cubes.

Before we start counting cubes, let's make sure we understand four super-important ideas. These are the building blocks (pun intended!) for everything else in this lesson.

Let's look at a rectangular box (also called a rectangular prism) and count the unit cubes inside it. This is the most common way to measure volume by counting cubes. Notice how the cubes fill up every bit of space inside the shape — no gaps and no overlaps.

In the diagram above, you can see a box that is 4 units long, 3 units wide, and 2 units tall. The golden highlighted cube in the corner shows you what one unit cube looks like. If you count every single cube inside the box, you get 24 unit cubes. That means the volume is 24 cubic units!

Here's a helpful way to think about it. The bottom layer of the box has 4 × 3 = 12 cubes. Since the box is 2 layers tall, the total is 12 × 2 = 24 cubes. You can always find the volume by counting one layer and then multiplying by the number of layers.

Counting every single cube one by one works fine for small shapes. But what if a box has hundreds of cubes? That would take forever! Luckily, there's a shortcut — a formula that lets you multiply instead of count.

This formula tells you: multiply the length by the width by the height. The answer you get equals the number of unit cubes that fit inside the shape. Let's see why this makes sense.

When you multiply length × width, you find how many cubes fit in a single flat layer on the bottom. Then multiplying by the height tells you how many layers are stacked on top of each other. It's like knowing how many cookies are on one tray and then multiplying by the number of trays!

Notice that when all three measurements are in centimeters, the answer is in cubic centimeters (cm³). If the measurements were in inches, the answer would be in cubic inches (in³). And if they were in feet, the answer would be in cubic feet (ft³). Always use the same unit for all three measurements!

Whether you use tiny cubes or big cubes, the idea of volume stays the same — you're counting how many unit cubes fit inside a shape. The only thing that changes is the size of each cube. Let's look at the most common units and some fun improvised ones too.

The unit you choose depends on what you're measuring. You'd use cubic centimeters to measure small things like a juice box. You'd use cubic inches for things like a shoebox. And cubic feet would be great for measuring bigger things like a closet or a swimming pool.

With improvised units, you can explore volume even without a ruler. Just grab a bunch of identical small objects (like sugar cubes), pack them tightly into a box, and count them. Your answer might be "the box holds 48 sugar cubes." It's not an official measurement, but it still tells you the volume!

Let's solve a complete problem together so you can see exactly how it works.

You now know two ways to find volume: counting every unit cube one by one, or using the formula V = l × w × h. Let's compare them so you know when each method is the best choice.

Both methods give you the same answer for rectangular prisms. Counting cubes helps you understand volume, and the formula helps you calculate volume quickly. Together, they make you a volume expert!

Now that you can count unit cubes and use V = l × w × h, you're ready for some exciting next steps in math. Here's a peek at what's ahead!

In later grades, you'll learn to find the volume of shapes that aren't boxes — like cylinders (soup cans), triangular prisms (toblerone boxes), and even spheres (basketballs). The cool thing is that the idea of "how much space does this take up?" never changes. You're building the foundation for all of that right now!

You might also discover that volume connects to other subjects. In science, you'll measure the volume of liquids in milliliters, and 1 mL is the same as 1 cm³! In real life, knowing volume helps you figure out if your stuff will fit in a suitcase, how much soil you need for a garden box, or how much water a pool can hold.

Time to try it yourself! Start with the easier ones and work your way up. Click "Show Answer" when you're ready to check your work.