Introduction to Rulers and Measuring Tapes - First Grade Mathematics

Have you ever seen a carpenter building a bookshelf or a doctor measuring how tall you have grown? They do not use paperclips or connecting cubes; they use a standard ruler or a measuring tape! A ruler is a remarkable tool that combines a straightedge with a number line of fixed, unchanging units called inches or centimeters. Instead of having to line up twelve loose cubes, a ruler has twelve units already connected into one sturdy wooden or plastic stick. In this chapter, you will be introduced to standard rulers, learn how to read the markings, and discover the secret of lining up the zero mark.

What Is a Ruler?

A ruler is essentially a straight number path that has been permanently drawn onto a stick!

The Anatomy of a Standard 12-Inch Ruler:

|---|---|---|---|---|---|---|---|---|---|---|---|
0   1   2   3   4   5   6   7   8   9  10  11  12
|<- 1 Inch ->|

Inches: The Standard Units

On a standard ruler, the distance between one big numbered line and the next is called one inch. From 0 to 1 is 1 inch. From 1 to 2 is 1 inch. From 0 to 6 is 6 inches. A standard classroom ruler is 12 inches long, which is also called one foot!

The Most Important Rule: Start at Zero!

When measuring with a ruler, the most common beginner mistake is lining up the object with the number 1 instead of the number 0!

Where to Start on a Ruler:

WRONG WAY (Starting at 1):
Object:          +------------------------+
Ruler:   |---|---|---|---|---|---|---|---|
         0   1   2   3   4   5   6   7   8
             \___________________________/
                 Object ends at 7, but its length is only 6 inches! (Started at 1!)

RIGHT WAY (Starting at 0!):
Object:  +------------------------+
Ruler:   |---|---|---|---|---|---|---|---|
         0   1   2   3   4   5   6   7   8
         \_______________________/
             Object starts at 0 and ends at 6: Length = 6 INCHES!

Finding the Zero Mark

Look closely at your ruler. Sometimes the number 0 is printed clearly. Other times, the zero mark is a special line drawn slightly in from the left plastic edge. Always align the start of your object with the ZERO LINE, not the edge of the stick and not the number 1!

Reading to the Nearest Inch

In first grade, we measure objects to the nearest whole inch.

Reading to the Nearest Inch:
Object:  +-----------------------------+
Ruler:   |---|---|---|---|---|---|---|---|
         0   1   2   3   4   5   6   7   8
                                 ^
Notice: The object passes 5 and is very close to 5.
It has not reached the halfway mark to 6.
We say: "The object is about 5 inches long!"

The Word "About"

Objects in real life do not always end perfectly on a whole number mark. If an object is slightly past 4, we look to see which number line it is closest to and say it is "about 4 inches long."

Practice Exercises

Exercise 1: What is the name of the standard unit of length shown on a 12-inch classroom ruler? How many inches make up one foot?

Exercise 2: Where must you align the left edge of an object when measuring with a ruler: at the number 1, at the number 0, or at the number 12?

Exercise 3: You place a crayon against a ruler. The bottom of the crayon is aligned with 0, and the tip reaches the number 4 line. How long is the crayon?

Exercise 4: A student aligns a pencil with the number 2 on a ruler. The pencil ends at the number 7 mark. How long is the pencil? Explain why the pencil is not 7 inches long.

Solutions and Explanations

Solution to Exercise 1: The standard unit of length is the inch. Exactly 12 inches make up one foot.

Solution to Exercise 2: You must always align the left edge of the object with the number 0 line (the zero mark).

Solution to Exercise 3: The crayon is 4 inches long.

Solution to Exercise 4: The pencil is 5 inches long (7 - 2 = 5). It is not 7 inches long because the student started measuring at 2 instead of starting at 0, giving the measurement a 2-inch false start.