What is measuring?
Measuring is finding the size or length of something. When we measure, we are trying to answer a question like, "How long is this book?" or "How tall is my pencil?" We find out by seeing how many units fit along the object.
- You can measure the length of your desk.
- You can measure the height of a water bottle.
- You can measure the distance around a shape.
Length tells us how long something is from one end to the other end.
What is a centimeter?
A centimeter is a unit of length in the metric system. It is a standard unit, which means it is always the same size. We use the abbreviation cm to stand for centimeter. It is a small unit, perfect for measuring things like crayons, erasers, and books. The centimeter is a standard unit used by students and people all over the world.
- The width of a large paperclip is about 1 cm wide.
- A small green pea is about 1 cm wide.
- Your pinky finger might be about 1 cm wide.
Using a standard unit like a centimeter means everyone gets the same measurement. It doesn't matter if you measure a crayon in the United States or in Brazil—one centimeter is always the same length.
Measuring with a centimeter ruler
A centimeter ruler is a tool used to measure length. It has numbers and long lines to mark each centimeter. In first grade, we use a ruler to see how many centimeters long an object is. The numbers on the ruler tell you how many centimeters you have counted so far.
- Step 1: Line up one end of the object with the zero (0) on the ruler. The zero is at the very beginning of the ruler, not the very edge.
- Step 2: Hold the ruler steady and flat against the object.
- Step 3: Look at where the other end of the object lands on the ruler.
- Step 4: Read the number at that spot. That number tells you the length in centimeters.
Always start at the zero line, not the edge of the ruler. The edge of the ruler can be worn down or sometimes not exactly at zero.
The rules of measuring with units
When we measure, we are really laying down many "length units" end to end. A centimeter is our length unit. There are two very important rules to follow so our measurement is correct: no gaps and no overlaps.
- No Gaps: If you use centimeter cubes to measure, you must place them right next to each other with no space in between. Gaps would make your measurement too short.
- No Overlaps: You also cannot let the cubes pile on top of each other. Overlaps would make your measurement too long. They must sit end to end in a straight line.
Measuring with a ruler does this work for you! The lines on the ruler show you where one centimeter ends and the next one begins, with no gaps or overlaps.
Measuring objects longer than the ruler
Sometimes you want to measure something that is longer than your ruler. What do you do? You can measure it in parts. This means you measure a section, make a small mark, and then move the ruler to measure the next part.
- Line up the zero at the start of the book. Measure to the end of the ruler. Let's say your ruler is 15 cm long. Write down 15.
- Make a tiny pencil mark on the book at the 15 cm spot.
- Move the ruler so the zero lines up with your mark.
- Measure from the mark to the end of the book. Let's say it is 7 cm. Write down 7.
- Add the two numbers together: 15 cm + 7 cm = 22 cm. The book is 22 cm long.
Be very careful to line up the zero exactly with your mark. If you are off by even a little, your final answer will not be right.
Finding objects around us in centimeters
One of the best ways to learn about centimeters is to start noticing how long things are. You can become a measurement detective by looking at objects in your classroom or home and guessing how long they are before you measure. This guess is called an estimate.
- A new crayon is about 9 cm long.
- A small paperclip is about 3 cm long.
- A large paperclip is about 5 cm long.
- A glue stick is often about 10 cm tall.
- A adult's thumb is about 5 cm wide.
Estimating first helps you check if your final measurement makes sense. If you estimate a pencil is 8 cm, but you measure and get 19 cm, you know you should measure again!
How to write centimeters
When we write a measurement using centimeters, we write the number first and then the abbreviation for centimeters, which is cm. There is no period after "cm" unless it is the end of a sentence. We do not add an "s" to make it plural. For example, we write "5 cm" for five centimeters.
- The pencil is 12 cm long.
- The book is 24 cm wide.
- The ribbon is 30 cm.
Always write the number first, then a space, and then "cm". For example, write "8 cm" not "8cm" or "cm 8".
Comparing lengths using centimeters
Once you have measured two or more objects in centimeters, you can compare them. You can say which one is longer and which one is shorter. You can also figure out how much longer one object is than another.
- The red crayon is 10 cm long.
- The blue crayon is 7 cm long.
- The red crayon is longer than the blue crayon.
- The blue crayon is shorter than the red crayon.
- How much longer is the red crayon? You can subtract: 10 cm - 7 cm = 3 cm. The red crayon is 3 cm longer.
Subtraction helps you find the difference between two lengths. You can ask, "What is the difference between the length of the red crayon and the blue crayon?" The answer is 3 cm.
Using a centimeter cube to measure
Before using a ruler, it is very helpful to practice measuring with centimeter cubes. These are small blocks that are exactly 1 cm long on each side. By lining them up, you can see how measurement works with no gaps or overlaps. In fact, the numbers on a ruler work just like lining up centimeter cubes!
- First, measure a crayon by lining up centimeter cubes. If it takes 8 cubes, the crayon is 8 cm long.
- Now, look at a ruler. The first number "1" is at the end of the first centimeter cube. The number "8" is where 8 cubes lined up would end. The ruler is like a line of centimeter cubes glued together!
When you understand how cubes measure, you understand how a ruler measures. It is the same idea, but the ruler is faster to use.
Common Core alignment: CCSS.MATH.CONTENT.1.MD.A.2 - Express the length of an object as a whole number of length units, by laying multiple copies of a shorter object (the length unit) end to end; understand that the length measurement of an object is the number of same-size length units that span it with no gaps or overlaps. Limit to contexts where the object being measured is spanned by a whole number of length units with no gaps or overlaps.
Notes for teachers
This lesson is aligned with CCSS.MATH.CONTENT.1.MD.A.2. Use it for whole-class instruction, independent practice, or homework. The content emphasizes the foundational concept of iterating length units (centimeters) to understand measurement, which is a key building block before introducing rulers as a tool in second grade.
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