Is each line on a ruler a millimeter?

How long is that line? How wide is that suitcase? Will that box fit on this shelf? Questions like that are one of the few things you can't necessarily Google an answer to nowadays. But they can be solved with the aid of a simple measuring tool, the ruler, or its grown-up cousin the yardstick. While some rulers will show only the US customary measures of inches and feet, most also have a metric side that measures length in millimeters and centimeters.

Choosing the Right Ruler Measurements

Before you start lining your ruler up and measuring things, take a look at the numbers running down the sides. If there are numbers on just one side of the ruler, they almost definitely show US customary measures: inches and feet. If the ruler has measurements down both sides, one side will show US customary measures; look for the side where the larger marks (inches) are numbered up to 12.

The other side of the ruler will have markings for cm and mm measurement. The distance between numbered marks on that side will be shorter than on the inches side, and the numbered marks will go as high as 30, because there are approximately 30 centimeters in 12 inches (the length of the standard ruler). The small lines between the larger, numbered lines represent millimeters.

Line It Up

Now that you've identified which side of the ruler has metric measurements, line that side of the ruler up with the object you're measuring. The "zero" line on the ruler usually doesn't line up exactly with the edge of the ruler, so make sure you put that zero line even with one edge of what you're measuring.

Tips

• Millimeters are usually used to measure very small things. If you're measuring a very small object, it may be easier to bring the object to the ruler instead of the other way around.

Read Your Millimeter Ruler

Now that the "zero" mark on your ruler is lined up with one edge of the object you're measuring, read along the ruler until you reach the far edge of the object being measured. Because the marks on a mm ruler are quite small and aren't numbered, it may help to put your finger, or the point of a pen or pencil, down to help you keep your eye on the correct mark.

Next, count the number of millimeter marks, starting from the zero line of the ruler and continuing until you reach the mark that lined up with the far edge of your object. The number of marks equals the object's measurement in millimeters.

Converting From Centimeters to Millimeters

You don't actually need to count every single millimeter mark along the ruler – you can use the numbered centimeter marks as a shortcut. Each centimeter is equal to 10 millimeters, so if your object measures 4 centimeters long, that's equivalent to:

4\times 10=40\text{ millimeters}

Often, your measurement in millimeters will fall between centimeter marks on the ruler. In that case, count centimeters up until the mark just before your measured object, then add in however many more millimeter marks it takes to reach the line you measured to.

Once you understand that 1 centimeter is 10 millimeters, you don't need to do the multiplication to convert between these two units of measure. Just count by tens for each centimeter mark.

For example, if you'd measured an object that reaches up to the 5 centimeter mark and then another 5 millimeters beyond that, instead of counting "one... two... three... four... five..." for the centimeters and then multiplying to convert them into millimeters, you can just count millimeters by tens: "ten... twenty... thirty... forty... fifty..." and then add in the remaining 5 millimeters for a total measurement of 55 mm.

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Need to measure something but getting hung up on all those lines on a ruler? You're in the right place. We're here to explain what the ruler markings mean so taking measurements will be a breeze. Whether you need to know how to read an inch ruler or how to read a metric ruler (cm ruler), this easy guide to ruler measurements has got you covered.

1. 

   1

   Get an inch ruler. You'll know it's an inch ruler because it will have 12 lines that denote inches on the ruler. 12 inches equals 1 foot (0.305 m). Each foot is broken down into inches. Each inch is broken down into 15 smaller marks, equaling 16 marks in total for each inch on the ruler.[1]

   • The longer the line on the surface of the ruler, the bigger the measurement is. Ranging from 1 inch to 1/16 of an inch, the lines decrease in size as the unit of measurement does.
   • Make sure you read the ruler from left to right. If you are measuring something, align it with the left side of the zero mark on the ruler. The left side of the line where the object ends will be its measurement in inches.

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   2

   Learn the inch marks. A ruler is made up of 12 inch marks. These are typically the numbered marks on the ruler and are denoted by the longest lines on the ruler. For example, if you need to measure a nail, place one end directly on the left side of the ruler. If it ends directly above the long line next to the large number 5, then the nail is 5 inches long.

   • Some rulers will also denote 1/2 inches with numbers, so make sure you are using the largest numbers with the longest lines as your inch markers.

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   Learn the 1/2 inch marks. The 1/2 inch marks will be the second longest lines on the ruler, half as long as the inch marks. Each 1/2 inch mark will come midway between each inch number because it is half of an inch. This means that marks directly between the 0 and 1 inch, 1 and 2 inches, 2 and 3 inches, and so on across the ruler, are the 1/2 inch marks. In total, there are 24 of these marks on a 12 inch ruler.[2]

   • For example, place the ruler against a pencil with the eraser at the far left of the ruler. Mark where the tip of the pencil lead ends on the ruler. If the pencil point ends at the shorter line halfway between the 4 and 5 inches marks, then your pencil is 4 and 1/2 inches long.

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   Learn the 1/4 of an inch marks. Halfway in between each 1/2 inch line, there will be a smaller line that denotes a 1/4 of an inch. In the first inch, these marks will mark 1/4, 1/2, 3/4, and 1 inch. Although the 1/2 inch and 1 inch marks have their own lines, they are still part of the 1/4 of an inch measurements because 2/4 of an inch equals half an inch and 4/4 of an inch equals 1 inch. There are a total of 48 of these marks on a 12 inch ruler.[3]

   • For example, if you measure a carrot and the tip falls on the line halfway between the 6 1/2 and 7 inch lines, the carrot is 6 and 3/4 inches long.

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   5

   Learn the 1/8 of an inch marks. The 1/8 of an inch marks are the smaller marks found directly in between the 1/4 of an inch marks on the ruler. Between 0 and 1 inch, there are marks that denote 1/8, 1/4 (or 2/8), 3/8, 1/2 (or 4/8), 5/8, 6/8 (or 3/4), 7/8, and 1 (or 8/8) of an inch. In total, there are 96 of these marks on a 12 inch ruler.[4]

   • For example, you measure a piece of fabric and the edge falls on the 6th line after the 4 inch mark, which is directly in between the 1/4 of an inch mark and the 1/2 inch mark. This means that your fabric is 4 and 3/8 inches long.

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   Learn the 1/16 of an inch marks. The small lines halfway between each 1/8 of an inch denote 1/16 of an inch. These are also the smallest lines on the ruler. The very first line on the left hand side of the ruler is the 1/16 of an inch mark. Between 0 and 1 inch, there are marks that denote 1/16, 2/16 (or 1/8), 3/16, 4/16 (or 1/4), 5/16, 6/16 (or 3/8), 7/16, 8/16 (or 1/2), 9/16, 10/16 (or 5/8), 11/16, 12/16 (3/4), 13/16, 14/16 (or 7/8), 15/16, 16/16 (or 1) of an inch. There are a total of 192 of these lines on the ruler.[5]

   • For example, you measure a flower stem and the end of the stem falls on the 11th line after the 5 inch mark. The flower stem is 5 and 11/16 inches long.
   • Not every ruler will have the 1/16 inch mark. If you plan on measuring things that are small or you need to be extremely accurate, make sure the ruler you use has these marks.

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1. 

   1

   Get a metric ruler. A metric ruler is based on the International System of Units (SI), sometimes called the metric system, and is divided into either millimeters or centimeters instead of inches. Rulers are often 30 centimeters long, which are designated by large numbers on the ruler. Between each centimeter (cm) mark, there should be 10 smaller marks called millimeters (mm).

   • Make sure you read the ruler from left to right. If you are measuring an object, align it with the left side of the zero mark on the ruler. The left side of the line where the object ends will be its measurement in centimeters. This way the line thickness will not affect the measurement.
   • Unlike with the English ruler, the measurements for the metric ruler are written in decimals instead of fractions. For example, 1/2 a centimeter is written as 0.5 cm. [6]

2. 

   2

   Learn the centimeter marks. The large numbers next to the longest lines on the ruler denote the centimeter marks. A metric ruler has 30 of these marks. For example, place the bottom of a crayon on the far left side of the ruler to measure it. Note where the tip falls. If the crayon ends directly on the long line next to the large number 14, your crayon is exactly 14 cm long.[7]

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   3

   Learn the 1/2 of a centimeter marks. Halfway between each centimeter, there is a slightly shorter line that denotes 1/2 of a centimeter, or 0.5 cm. There are a total of 60 of these marks on a 30 cm ruler.[8]

   • For example, you measure a button and the edge ends on the fifth line right between the 1 and 2 centimeter marks. Your button is 1.5 cm long.
   • For example, to measure 0.6 cm, count one thick line (5 mm) and one thin line (1 mm).

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   4

   Learn the millimeter marks. Between each 0.5 cm line, there are four additional lines that denote the millimeter marks. There are a total of 10 lines per centimeter, with the 0.5 cm line acting as the 5 millimeter mark, making each centimeter 10 mm long. There are 300 millimeter marks on a 30 cm ruler.[9]

   • For example, if you measure a piece of paper and it ends on the 7th mark between the 24 and 25 centimeter mark, it means your object is 247 mm, or 24.7 cm long.

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Add New Question

• Question

  What is 55.5? Is that larger than 55 1/4?

  

  The 55.5 is larger than 55 1/4. the .5 on the 55.5 would equal 1/2. Therefore, 55.5 is equal to 55 1/2 which is 1/4" larger than 55 1/4.

• Question

  Can I learn to read a ruler in one day?

  

  Yes, but it really depends on what type of ruler you want to learn as well as how fast you pick up new material

• Question

  What does it mean when mm is shown just beside the 0 in a ruler?

  

  Each small line represents 1mm. Therefore, the first line past the big number (for instance 25) will represent 25.1cm or 251mm.

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• Make sure you always use the correct side of the ruler for the task at hand. You don't want to get the centimeters and the inches mixed up or your measurements won't be correct. Remember that there are 12 large numbers on an English ruler and 30 numbers on the metric ruler.

• Learning to read a ruler takes practice, especially converting the numbers in the measurements. Just remember to practice using your ruler and you'll get better at it.

Thanks for submitting a tip for review!

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Article SummaryX

To read a metric ruler to measure centimeters, look at the long lines on the ruler that are numbered 1-30, which are the centimeter marks. The distance between them is equal to 1 centimeter. There are smaller lines between the larger centimeter lines, which represent millimeters. Alternatively, to read an English ruler, start by looking for the inch lines, which are the longest lines on the ruler that are numbered 1-12. The distance between those lines is 1 inch, and then the smaller lines between them are 1/2, 1/4, 1/6 and 1/8 inches! For tips on how to count the smaller lines and figure out how many millimeters you're working with, read on!

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