Conversion of Centimeters to Millimeters using Yarn

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SWBAT use the correct tool to measure and convert from a larger unit to a smaller unit.

Big Idea

Students have a blast cutting and measuring, in centimeters, 5 lengths of yarn. They then convert the units to millimeters using a centimeter ruler as the number line model.

Warm Up: Quick Review of our Metric Ladder

5 minutes


The goal of the core lesson today is to measure using centimeters and then learn to convert the measurements to a smaller unit by using the ruler ( number line).

Warm UP: Review of Relative Sizes

I reviewed relative sizes to set the stage for understanding what a cm and mm look like, beyond the ruler or measuring tape. This fulfills a portion of the standard as it helps students to understand relative sizes of units of measurement.

We first reviewed what a centimeter looked like.  All students took their thumb and index finger and used it against their other finger to the first knuckle to indicate the relative size of the centimeter. Some students had a little trouble. I told them that they should look at their peers and try to imitate them since most had it right. Those students adjusted their fingers and we were good to go!

The next item of business was for them to tell me what a millimeter looked like. They all grabbed a strand of hair. No one messed that up! It appears that using the piece of hair to model a mm. works quite well! I recommend sticking with that idea!

Then, I drew a stair step ladder model on the board with only centimeter and millimeter on it and had students draw it on their Educreations or Whiteboard ap.  We reviewed that moving from a centimeter to a millimeter would be multiplying by ten. I wrote 7 cm on the step and asked how many millimeters it was and all of them replied, 70 mm. (Resource: PDF example and explanation of Stair Step Model for further reference)

I  don't believe that they truly understand the why behind the conversion by just using a stair step ladder. The stair step model helps with visualizing  and organizing the units. I think it is the best tool because of the relevant direction it lends of "up and down" Today I want to make sure that they learn to count by 10's to fully understand the conversion.

Core Lesson: Learning to understand conversion from a larger unit to a smaller unit

10 minutes

I began today's lesson with my Educreations explanation of how to understand conversions by using the centimeter ruler. I had students seated on the floor in front of the SB as I presented my lesson whole group. I had emailed this lesson to all of my students for reference for their notes. This ap is a wonderful tool, especially if a student is absent. They can see their lessons immediately and it is personal because it records my familiar voice. I can individualize easily using this ap.

I presented this lesson whole group so that questions could be asked and shared together as we learn to measure and use a centimeter ruler. I was concerned about them understanding the concept of .5 centimeters and the conversion of that and other decimal (10ths) of the centimeter to millimeters.

I was surprised how easily they caught on. They could relate it to 1/2 being .5. So, they have a concept of the relationship between the decimal point and the fraction of 1/2. I did not delve into multiplying the decimal by ten, but focused on the idea that they calculated the whole number in millimeters first and then understood that the decimal point number is already representing millimeters.

Students were sent back to their desks as we prepared to practice reading the ruler as their number line.

Practicing in the classroom

30 minutes

Materials: 8.5 by 11 inch pieces of construction paper. Scissors and glue.  Balls of scrap yarn. ( I keep a bag in my cabinet. My friend who crochets gives me her scrap yarn all rolled up in a ball. It comes in really handy! ) A centimeter ruler for each student. The finished product will look like this: How they should look!

I told my students that they would be measuring and practicing their conversion skills using their rulers and counting. I wrote the instructions on my white board.


1. Take a piece of paper and choose a ball of yarn to use.

2. Cut five different size pieces of yarn making sure it isn't longer or wider than the paper.

3. Glue down each piece of yarn on the paper and measure it in centimeters.

4. Write the measurement in centimeters underneath or near the piece of yarn.

5. Using your ruler, count by tens to find the mm measurement.


As students started to cut, glue and measure, I roved around checking their understanding. I noticed one student having difficulty. I sat with him and worked one on one until he was Measuring Correctly. This young man did a great job! He's got it!