Opening subject page...
Loading your content
Learn how to find out how long something is by using smaller things lined up in a row!
Have you ever wanted to know how long your desk is? Or how tall your friend is? People have always wanted to measure things! A very long time ago, there were no rulers. So people used parts of their bodies — like their feet or their hands — to measure. That is where the word "foot" comes from!
Let's look at how people learned to measure over time.
The big question is: How can we use little things to find out how long a big thing is? That is exactly what this lesson is about!
Before we start measuring, let's learn four important rules. These rules will help you measure the right way every time!
Let's look at a picture. Here is a pencil. We want to know how long it is. We will use small cubes to measure it. Watch how we lay the cubes end to end along the pencil.
Do you see what happened? We put 6 cubes in a row under the pencil. Each cube touches the next one — no gaps and no overlaps! They go in a straight line from one end of the pencil to the other end. So the pencil is 6 cubes long.
The cube is our length unit. A length unit is the small thing we use to measure. We can say: "The pencil is 6 length units long."
Here are the steps you follow every time you measure something. Let's learn them one at a time!
Choose a small object. It could be a paper clip, a cube, a crayon, or even your thumb! Make sure you have many copies of the same object.
Put the first small object at one end of the thing you are measuring. Put the next one right next to it. Keep going until you reach the other end.
Count how many small objects you used. That number is your measurement!
Notice that the answer is always a whole number. That means a counting number like 1, 2, 3, 4, 5, and so on. We do not use pieces of numbers (like 2 and a half). We use the whole number of objects we can fit!
Here is something really interesting. If you measure the same thing with different small objects, you will get different numbers! That is because some objects are bigger than others. Let's see this in a picture.
Look! The same crayon is 3 big cubes long. But it is also 6 small cubes long. The crayon did not change — only the size of the cube changed!
Here is the rule: when you use bigger objects to measure, you need fewer of them. When you use smaller objects, you need more of them.
| THING WE MEASURED | LENGTH UNIT | HOW MANY? |
|---|---|---|
| Crayon | Big cube | 3 |
| Crayon | Small cube | 6 |
| Book | Paper clip | 5 |
| Book | Eraser | 3 |
Let's try a full example from start to finish. We want to measure how long a ribbon is using paper clips.
Even grown-ups can make mistakes when measuring! Let's learn what to watch out for so you always get the right answer.
| RIGHT WAY ✅ | WRONG WAY ❌ | WHY IT MATTERS |
|---|---|---|
| All objects are the same size | Mix of big and small objects | You cannot count them fairly if they are different sizes |
| Objects touch end to end | Gaps between objects | Gaps make you miss some length, so your number is too small |
| Objects do not overlap | Objects stacked or overlapping | Overlapping makes your number too small because you are double-counting space |
| Straight line | Wiggly or curvy line | A curvy path is longer than a straight one, so your number would be too big |
| Start at one end | Start in the middle | You will miss part of the thing you are measuring |
Right now you are measuring with objects like paper clips and cubes. That is wonderful! But soon you will learn to measure with rulers. A ruler has little marks called inches or centimeters. Those marks are just like tiny cubes all in a row — but they are already printed on the ruler for you!
Everything you are learning today is the same idea. Whether you use paper clips, cubes, or a ruler, you are always asking: How many of the small thing fits along the big thing?
| WHAT YOU DO NOW | WHAT COMES LATER |
|---|---|
| Use paper clips, cubes, or other objects | Use rulers with inches and centimeters |
| Count the objects yourself | Read the number on the ruler |
| Answer is a whole number | You might get half-inches and more |
| Measure short things | Measure long things like rooms and hallways |
You are building the foundation right now. Once you understand end to end and counting, everything else will be easy! 💪
Try these problems! Think carefully, then click "Show Answer" to check.
Today you learned how to find the length of an object by using smaller objects called length units. You lay many copies of the same small object end to end in a straight line with no gaps and no overlaps. Then you count how many you used. That number is a whole number that tells you how long the object is.
You also discovered that using a bigger length unit gives you a smaller number, and using a smaller length unit gives you a bigger number. This same idea is used by rulers and tape measures — you are building the skills that will help you measure everything around you!