Here's a lesson idea for elementary grades that practices estimation and measuring with a metric ruler. Kids will like the game aspect.
For this lesson plan on teaching students to read a metric ruler, each student will need a ruler with millimeters and centimeters (or you can have one ruler for every two students), several objects for students to measure, pencils, and notebook paper.
Start the lesson by asking students to look at their rulers and find the side that measures centimeters and millimeters. This side should be facing up, so they can use it. (This will be true if you have rulers that have inches on one side and centimeters on the other.) Explain to students that measuring with centimeters and millimeters is very similar to measuring with inches. Review how to measure an object, especially starting at the zero mark and measuring straight across. Also, you will want to remind them that things don't always measure perfectly--an object might be 2 and 3/4 inches long, not just 3 inches.
Finally, tell students that it takes 10 millimeters to equal one centimeter. Therefore, if something isn't an exact amount of centimeters, you can use decimal points to show how many centimeters and how many millimeters an object is such as 3.4 cm or 5.6 cm.
Lesson and Practice
During this lesson you will ask students to measure the same objects and compare what measurements students got. Before students start measuring, you will need to show them a poster of a metric ruler and show them how to read the millimeters and centimeters. Post the drawing of the ruler on the chalkboard and draw a line to 3 cm and 4 mm. (This line will be much larger than 3 cm 4 mm, and the ruler will be enlarged also, so all students can see.)
Show students how to start at the centimeter mark that's closest to the end of the line and then count the small millimeter marks to get the measurement. Also demonstrate how to write this measurement as 3.4 cm or 34 mm or 3 cm and 4 mm. You can draw several lines and practice with the large class ruler until you think students understand the process.
Next ask students to measure the same objects with their own rulers such as the width and length of their desks or different textbooks. Anything that all students have access to that is the same will work for this practice. You will be able to get an accurate measurement and check if students are measuring correctly. When students get a measurement, ask them to write it down on a piece of notebook paper in one of the three ways you have showed them and large enough, so you can quickly walk around the classroom and see the measurement to gauge if students are measuring correctly or not.
Practice with a Game!
To provide more practice for students in this lesson plan on teaching students to read a metric ruler, you can play a game with students. Divide students into two teams. Each team sends a member with his ruler to the front of the class. You show them something to measure such as a pencil. Before students measure, they estimate how many centimeters and millimeters the pencil is. They write this estimation down on a scrap of paper, but they don't tell what it is. Then each student measures the object and writes on the chalk board their answer. They get one point for the correct measurement and one point if their estimation is within 5 mm of the actual measurement. Play the game until class time is up or determine a score a team must reach to win.
At the end of the game, ask students to tell you some things they learned while playing the game and emphasize teaching points such as 10 mm equals 1 cm or starting on the zero mark when measuring.