What does it mean to measure?
Measuring means finding out how long something is. We measure to compare objects and see which is longer or shorter. When we use an inch scale, we are finding the length of an object in inches.
- Your teacher wants to know if your desk is longer than a book.
- You want to see if a new pencil will fit in your pencil box.
- You need to cut a piece of string that is the same length as your friend's string.
In all of these cases, you need to measure!
Measuring helps us describe the world exactly. Instead of saying "a really long book," we can say "the book is 8 inches long."
What is an inch scale?
An inch scale is a tool that helps us measure length. It is a ruler that is marked with numbers and lines. Each space between two big lines is called an inch. The abbreviation for inches is in or a double quotation mark " (for example, 5 inches can be written as 5 in or 5").
- Find the numbers: 1, 2, 3, 4, and so on.
- Each number stands for one inch from the start of the ruler.
- The line under the number 1 is exactly 1 inch from the left edge.
- If an object stretches from the left edge to the line under the 3, it is 3 inches long.
Normally in first grade, we always measure in whole inches. We do not worry about half inches or quarter inches yet. We just count the whole numbers!
How to line up the ruler
To measure correctly, you must line up the ruler carefully. The end of the object must match the end of the ruler. In first grade, we always line up the left side of the object with the zero edge of the ruler. (Sometimes the very left edge of the ruler is the zero, even if it is not written.)
- Lay the ruler flat next to the object.
- Make sure the very left end of the object is exactly at the very left end of the ruler. We call this lining up the zero.
- Look at the right end of the object. Find the closest number on the ruler that is under the right end.
- That number is the length in inches.
Example: A crayon is placed with its left end at the ruler's left edge. The right end of the crayon is under the number 4. The crayon is 4 inches long.
If you start measuring from the number 1 instead of zero, your measurement will be wrong by 1 inch! Always check that the left edge lines up with the start of the ruler.
Measuring to the nearest whole inch
When we measure in first grade, we measure to the nearest whole inch. That means if an object is a little bit longer than 3 inches but not quite 4 inches, we say it is 3 inches long. But in first grade, we mostly practice with objects that are exactly on the inch marks.
- A paperclip is placed on a ruler. Its right end is exactly on the 2. The paperclip is 2 inches long.
- A small toy car is lined up at zero. Its front bumper stops exactly at the 5. The car is 5 inches long.
- A paintbrush is measured. Its tip is at the 7. The brush is 7 inches long.
Practice with objects that have straight edges, like index cards or new pencils. They are easier to line up.
What if the object is longer than the ruler?
Sometimes you want to measure something that is longer than your ruler, like a big book or a table. In first grade, we learn that we can move the ruler and measure in parts, but we must be very careful. The best way is to use a longer ruler or a measuring tape. But for practice, we can measure one section, remember that number, and then measure the next section.
Your ruler is only 6 inches long. The book looks longer than 6 inches.
- Line up the left edge of the book with zero. Mark where the 6-inch mark ends with your finger. That is 6 inches.
- Now move the ruler so that your finger is at zero again. The rest of the book goes to the 4-inch mark.
- Add the two measurements: 6 inches + 4 inches = 10 inches. The book is 10 inches long.
In first grade, we usually only measure objects that are shorter than a ruler to make it easier. But it is good to know this trick!
Comparing lengths using inches
Once we know how many inches long something is, we can compare lengths. We can say which object is longer, which is shorter, or if they are the same length. We use words like longer than, shorter than, and equal to.
- A pencil is 5 inches long. A crayon is 3 inches long. The pencil is longer than the crayon. The crayon is shorter than the pencil.
- A glue stick is 4 inches long. A small toy is also 4 inches long. The glue stick and the toy are the same length. They are equal.
- A paintbrush is 7 inches long. A marker is 5 inches long. Which is longer? The paintbrush is longer.
Longer, shorter, taller, wider — these are all words we use to compare. When we measure with inches, we can prove who is right!
Using an inch scale that starts at 1
Some rulers have the number 1 written at the very first line, not 0. That can be tricky! But remember, the space before the 1 is still the first inch. So if you line up the left edge of an object with the left edge of the ruler (before the 1), then the object's right end will line up with a number. That number is the length in inches.
- On a ruler that starts with 1, the distance from the left edge to the 1 is exactly one inch.
- So if an object goes from the left edge to the 4, it is 4 inches long.
- If you accidentally line up the object with the 1 instead of the left edge, you will be off by one inch!
Always check where your ruler starts. Some rulers have a little space before the 1. That space is part of the measurement.
Estimating length before measuring
Estimating means making a smart guess. Before you measure with an inch scale, you can look at an object and guess how many inches long it is. Then you measure to check if your guess was close. Estimating helps you become better at understanding how big an inch is.
- Look at a new crayon. Do you think it is about 3 inches or about 6 inches? (A new crayon is about 3 inches.)
- Look at a dollar bill. Do you think it is about 6 inches? (It is actually about 6 inches!)
- Look at your pinky finger. Is it about 2 inches? (For many first graders, yes!)
Did you know that the distance from the tip of your thumb to the first knuckle is about 1 inch for many people? You can use your own body to estimate!
Inches are part of the customary system
In the United States, we often use the customary measurement system. Inches, feet, and yards are part of this system. There are 12 inches in 1 foot. First graders do not need to memorize that yet, but it helps to know that inches are small units, and feet are bigger. We use inches to measure small things.
- A smartphone is about 5 or 6 inches long.
- A pair of scissors is about 5 inches long.
- A juice box is about 4 inches tall.
- A new pencil is about 7 inches long.
If you ever travel to another country, you might see centimeters instead of inches. That is the metric system. But here in the USA, we use inches every day!
Common mistakes to avoid
Even grown-ups make mistakes when measuring! Here are some common errors and how to fix them. Learning these now will make you a measuring expert.
If you start at the 1-inch mark, your measurement will be 1 inch too short. Always check the left edge!
The ruler must be flat and straight alongside the object. If it is crooked, you will not get the right length.
Sometimes we look quickly and guess the number. But we must look exactly at where the object ends and read the closest number.
After you measure once, do it again! Measuring twice helps you catch mistakes.
Practice with real objects
The best way to learn is to practice. Find small objects around your home or classroom. Measure them with an inch ruler. Write down the length. Then compare with a friend.
- A building block
- A spoon
- A key
- A sticky note
- A small book
Write each length in inches. For example: "The key is 2 inches long."
Always write the unit "inches" or use the symbol " after the number. For example: 3 inches or 3".
Putting it all together
Now you know the most important facts about measuring with an inch scale. Remember these big ideas:
- An inch is a unit of length.
- We use a ruler to measure in inches.
- Always line up the left end of the object with the left end of the ruler.
- Look at the number under the right end of the object.
- That number tells you the length in inches.
- We can compare lengths: longer, shorter, or equal.
Sofia has a ribbon that is 6 inches long. Her friend gives her another ribbon that is 4 inches long. Which ribbon is longer? How much longer?
Answer: The first ribbon is longer. It is 2 inches longer because 6 − 4 = 2. (We don't need to subtract in first grade, but we can see that 6 is more than 4.)
Keep practicing! The more you measure, the better you will understand how big an inch is. Soon you will be able to estimate lengths very well.
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.
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Instructional note: Ensure students have access to inch rulers where the zero is clearly indicated or the left edge is the starting point. Model measuring several times before asking students to work independently. Emphasize the importance of aligning the zero edge with the object's end.