How to Read a Metric Ruler in Millimeters and Centimeters

A metric ruler measures length using millimeters (mm) and centimeters (cm). Reading one correctly is useful for measuring school projects, craft materials, small household objects, and distances on paper. You will identify the zero point, follow the scale, and read measurements that fall between numbered marks without confusing the ruler’s physical edge with its measurement line.

What You’ll Need

A metric ruler with millimeter markings. A dual-scale ruler is fine, but use the side labeled in centimeters or millimeters.
The object or distance you want to measure.
A pencil, if you need to mark a measured point.

Step-by-Step

Find the metric scale. Look for numbers labeled in centimeters, often marked “cm,” and shorter divisions between them. On a standard metric ruler, each centimeter is divided into 10 millimeters. If the ruler also has inches, make sure you are following the metric side rather than the inch scale.
Locate the zero mark. Find the line labeled 0. It may sit slightly in from the ruler’s physical end, especially if the end is worn or rounded. The zero line—not necessarily the edge—is the point where measuring begins.
Place the ruler beside the object. Lay it flat and straight along the length you want to measure. Put the zero mark level with one end of the object. Keep the ruler parallel to the object; an angle can make the reading too long or too short.
Keep the scale steady and level. Look directly down at the marks rather than from the side. This reduces parallax, the apparent shift that happens when you view a line at an angle. Make sure the object has not moved and that the ruler does not curve away from it.
Read the centimeter number. Find the nearest numbered centimeter mark at or before the object’s far end. The large numbers usually show centimeters. For example, if the end reaches the number 6, it is at least 6 cm long.
Count the small divisions. Each small interval between centimeter marks is 1 mm. Count the intervals after the last whole-centimeter mark to find the extra millimeters. If the end is 4 small marks after 6 cm, the length is 6 cm 4 mm.
Write the measurement clearly. You can record that example as 6 cm 4 mm or 6.4 cm. These are equivalent because 4 mm is 0.4 cm. Use the unit with the number so another person knows whether the value is centimeters or millimeters.
Measure from a different starting point if zero is damaged. Line a clear mark, such as 2 cm, up with one end of the object. Read the mark at the other end, then subtract the starting reading. If the far end reads 9 cm, the object is 9 − 2 = 7 cm long.

Common Mistakes

Starting at the physical edge: The ruler’s end can have a small blank margin or may be chipped. Align the object with the zero line. Starting at the edge when it is not at zero adds or removes that margin from the result.

Counting lines instead of spaces: A millimeter is the interval between two adjacent marks. When counting from a numbered centimeter line, count each space you move across, not both endpoint lines as extra units.

Mixing up millimeters and centimeters: The number of millimeters is ten times the number of centimeters. A length of 35 mm is 3.5 cm, not 35 cm. Check the unit before copying a measurement into instructions or a form.

Reading from the wrong scale: Some rulers show inches on one edge and centimeters on the other. Inch markings are not divided into ten millimeters. Confirm the unit labels before measuring.

Holding the ruler at an angle: A ruler that runs diagonally from the object’s end does not show its true length. Align it with the object, and view the endpoint from directly above to avoid a misleading reading.

Tips

Use millimeters for small objects or when a close fit matters. Use centimeters for longer lengths or when a simpler number is easier to read.
Remember the conversion: 1 cm = 10 mm, and 100 cm = 1 m. For example, 125 cm equals 1 m 25 cm.
If the object is longer than the ruler, measure one section at a time. Mark the endpoint of each section carefully, then add the section lengths. Keep the ruler in a straight line.
For a round or irregular object, measure the specific dimension you need—such as its width or diameter—and say which one you measured. A straight ruler may not capture a curved surface’s path accurately.
Use a ruler with clear, undamaged marks for precise work. If you need greater precision than the smallest divisions allow, use a more suitable measuring tool rather than estimating many extra digits.

Troubleshooting

The ruler has no zero line at the end: Find the printed zero and align that line with the object. If it is obscured, start from another intact numbered line and subtract as described in the steps.

The endpoint falls between millimeter marks: Record the nearest mark for an ordinary measurement, or estimate between marks only if the task allows it. Do not report a highly precise value from a ruler whose divisions cannot support that precision.

The object will not stay straight: Hold it gently in place or set it on a flat surface. For a flexible object, avoid stretching it unless the instructions specifically require measuring it under tension.

Your measurement seems far too large or small: Check whether you used the centimeter or inch scale, began at zero, and counted millimeter spaces correctly. Convert the result as a quick check: centimeters multiplied by 10 gives millimeters.

The ruler and object are too short to measure together: Mark a clear starting point, move the ruler without changing the object’s alignment, and continue from that mark. For better accuracy, use a longer ruler or tape measure when available.

Frequently Asked Questions

How many millimeters are in a centimeter?

There are 10 millimeters in 1 centimeter. To convert centimeters to millimeters, multiply by 10; to convert millimeters to centimeters, divide by 10.

What do the small lines on a metric ruler mean?

On a standard ruler, each small interval between numbered centimeter marks represents 1 millimeter. Ten of those intervals span one centimeter.

How do I read 6.7 cm on a ruler?

Find 6 cm, then move 7 small millimeter intervals farther along. The same length is 67 mm.

Should I measure from the edge or the zero?

Measure from the zero line. Use the physical edge only if it exactly lines up with zero.

Can I measure an object that is longer than my ruler?

Yes. Measure aligned sections, mark each stopping point, and add the section lengths. Keep each placement straight to avoid gaps or overlaps.

Further reading: National Institute of Standards and Technology. Details can change, so check the official page for anything that matters to you. Spotted a mistake? Tell us. How we write an answer.