How to Read a Ruler Explained (w/ Pictures)

Learn how to read a ruler in inches (imperial) and in centimeters (metric).

How to Read a Ruler in Inches

This first section will teach you how to read a ruler in inches. Note that this type of measurement correlates with the imperial measurement system that is most commonly used in the United States. If you are looking to learn how to read a ruler in centimeters (i.e. the metric measurement system), you can click here to skip ahead to the next section.

Step One: Get an Inch Ruler

The first step is to find an inch ruler. There are two common types of inch rulers: 6 inch and 12 inch. A 6 inch ruler has 6 equally-spaced lines that denote one inch each. This type of ruler is half of a foot long (i.e. 6 inches equals half of a foot in the imperial measurement system). A 12 inch ruler has 12 equally-spaced lines that denote one inch each. This type of ruler is one foot long (i.e. 12 inches equals one foot in the imperial measurement system). Each one inch segment on both types of rulers is divided into several shorter lines, all of which represent a fraction of an inch.

In this guide, we will be using a 6-inch ruler as shown in Figure 01 below.

Here a few pointers related to the lines on an inch ruler:

• The longest lines on an inch ruler represent inches (the longest measurement unit). As the lines get smaller, so does the measurement unit. The longest measurement unit on an inch ruler is one whole inch. The smallest measurement unit on an inch ruler is 1/16 of an inch.

• The markings on a ruler increase from left to right starting with zero. You always read a ruler from left to right. In other words, always line the left side of a ruler with the left edge of an object and see how far in length it extends to the right side of the object.

Step Two: Start with the Inch Marks

The longest lines on a ruler represent the inch marks. Each line is labeled with a whole number starting with 0 and going all the way to 6. The distance between each of these longer lines is exactly one inch.

For example, if a wanted to find the length of a pencil, you would lay it flat and line up the left edge with the 0 inch marker and then see which marker on the ruler lines up with the right edge. Whatever number is associated with this marker is the length of the object in inches. If the right edge of the pencil aligns with the 3 inch marker (as shown in Figure 02 below), then you can conclude that the pencil is 3 inches long.

Of course, not all objects can be measured in inches alone, which is why each one inch marker has smaller markers in between.

Step Three: Understand the 1/2 Inch Marks

Directly half-way between each inch marker is the second longest line on an inch ruler—the 1/2 inch marks. These marks represent half of an inch. For example, the mark that is directly midway between the 3 inch line and the 4 inch line represents the 3 1/2 inch line (or the 3.5 inch line). If the right edge of an object were to align with the 1/2 inch line between 3 and 4, we could say that the object has a length of 3 1/2 inches.

Figure 03 below illustrates an inch ruler being used to measure the length of a credit card. Notice that when the left edge of the credit card is aligned with the zero inch marker, the right edge lines up with the 1/2 inch line between the 3 inch marker and the 4 inch marker. So, we can say that the credit card has a length of 3 1/2 inches.

Step Four: Understand the 1/4 Inch Marks

The third longest lines on an inch ruler are the 1/4 inch markers. Direct between each 1/2 inch marker is a 1/4 inch marker. The 1/4 inch marker to the left of a 1/2 inch marker is 1/4 inches and the 1/4 inch marker to the right of a 1/2 inch marker is 3/4 inches.

The battery that is being measured in Figure 04 below has a right edge that aligns with the 1/4 marker between 2 and 2 1/2, so we can say that is has a length of 2 1/4 inches (i.e. two and a quarter inches).

On the other hand, the orange slice that is being measured in Figure 05 has a right edge that aligns with the 1/4 marker between 2 1/2 and 3, so we can say that it has a length of 2 3/4 inches (i.e. two and three-quarter inches).

Compare the objects being measured in both Figure 04 and Figure 05 below to better understand the difference between the 1/4 inch marker and the 3/4 inch marker.

Step Five: Understand the 1/8 Inch Marks

While some rulers include additional markers, the smallest lines that we have on our reference ruler are the 1/8 inch marks. If you want to learn how to read a ruler, then you will need to understand these shortest markers just as much as you understand the longest ones. In the case of the 1/8 inch markers, there are a few possibilities: 1/8, 3/8, 5/8, and 7/8. Between each 1/2 inch marker and 1/4 inch marker, there is an 1/8 inch marker. Additionally, between each whole inch marker and each 1/4 inch marker, there is an 1/8 inch marker.

The feather that is being measured in Figure 06 below has a right edge that aligns with the 7/8 marker between 4 and 4, so we can say that is has a length of 4 7/8 inches.

Similarly, the fish that is being measured in Figure 07 has a right edge that aligns with the 5 3/8 marker, so we can say that the fish is 5 3/8 inches long.

How to Read a Ruler in CM

This next section will teach you how to read a centimeter ruler. The skills for how to read a ruler in centimeters is exactly the same as reading an inch ruler. The other difference is that the marker lines on a centimeter ruler are different.

Step One: Get a Centimeter Ruler

Just like the previous section, you will need to start by getting a centimeter ruler (also known as a metric ruler). In this section, we will be using a 5-centimeter ruler as sown in Figure 08 below.

Here are a few pointers related to the marker lines on a metric ruler:

• The longest lines on a metric ruler represent centimeters (the longest measurement unit on the ruler). As the lines get smaller, so does the measurement unit.

• The markings on a centimeter ruler increase from left to right starting with zero. You will always read a metric ruler going from left to right. Whenever you use a metric ruler to measure the length of an object, you have to align the left side of a ruler with the left edge of the object and see how far in length the object extends to the right edge of the object.

• Unlike the inch ruler, the metric ruler uses decimals instead of fractions (e.g. 2 1/2 centimeters is expressed as 2.5 cm when using a metric ruler).

Step Two: Start with the Centimeter Marks

The longest lines on a metric ruler represent one whole centimeter marks. Each line is labeled with a whole number starting at zero and, in this case, going all the way to 5 (since we are using a 5cm long ruler). The distance between each centimeter marker is exactly one centimeter.

For example, if you wanted to find the length, in centimeters, of a bean, you would have to lay it flat and line up the left edge of the bean with the 0 centimeter marker and then see which marker on the metric ruler aligns with the right edge of the bean. Whatever value is aligned with the right edge marker is the length of the object in centimeters. If the right edge of the bean aligns with the 1 cm marker (as shown in Figure 09 below), then you can say that the bean is 1 cm long.

Step Three: Understand the 0.5 Centimeter Marks

The midway marker between each whole centimeter marker is the 0.5 (e.g. one-half) centimeter marks. These marks represent one half of a centimeter. For example, the mark that is directly between the 2 cm line and the 3 cm line represents the 2.5 cm line. If the right edge of an object lines up with the 0.5 cm line between 2 and 3, you could conclude that the object is 2.5 cm long.

Figure 10 shows a metric ruler used to measure the length of a paper clip. Notice that when the left edge of the paper clip is aligned with the zero centimeter mark, the right edge aligns with the 0.5cm marker between the 2cm marker and the 3cm marker. Therefore, the paperclip has a length of 2.5 cm.

Step Four: Understand the Millimeter Marks

The third longest lines on a metric ruler are the millimeter marls. Between each half centimeter there are four shorter lines that denote the millimeter marks on the ruler. Each centimeter is made up of 10 millimeters and each millimeter is equal to 0.1 cm.

The gummy bear being measured in Figure 11 below has a right edge that aligns with the 0.3 marker between 1 and 2, so we can say that it has a length of 1.3 centimeters.

Similarly, the lip balm stick that is being measured in Figure 12 below has a right edge that aligns with the 0.7 marker between 3 and 4, so we can say that the object has a length of 3.7cm.

How to Read a Ruler: Conclusion

Congratulations on making it through this free guide on how to use a ruler to measure the lengths of objects using both an imperial ruler (e.g. an inch ruler) and a metric ruler (e.g. a centimeter ruler).

Whether you are measuring lines and objects in math class, completing a personal project, or studying for an exam, knowing how to read a ruler to make accurate measurements is an important life skill that will help you inside and outside of the classroom. If working through this guide was your first experience with reading a ruler, it’s totally fine if you are still a little confused. If you fall into this category, we highly recommend that you work through this guide again to give yourself some practice with how to read a ruler. The more experience that you gain with working with rulers, the more confident and skilled you will become!

While the type of ruler you choose to learn how to read will likely depend on your location and/or courses (imperial for math students in the United States and metric for science classes or students outside of the United States), you may choose to learn how to use both the imperial ruler and the metric ruler to give yourself a deeper understanding of measurement as well as overall flexibility.

Either way, mastering how to read a ruler (whether it be imperial, metric, or both) is a skill that every student can achieve with practice and persistence. If you have a ruler handy, go ahead and use it to measure everyday items and household objects. Before you know it, reading a ruler and using it to make accurate measurements will be something you can do with little effort!

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