Simplified Metric Conversions with Common Core

Recently, one of my 7th grade students needed help with metric conversions. The teacher had given her a "staircase" diagram. From upper left to lower right, the steps were marked kilo, hecto, deca, (base unit m/L/g), deci, centi, and milli. The student was told that to convert by counting steps and then moving the decimal point. For example, meters to kilometers is three steps to the left, so move the decimal point three places to the left.

Even though using this type of diagram can lead to correct answers on homework, there are two reasons why this method troubles me.

• Students are dependent on the diagram. Most students will not remember how to draw and label it if they need to convert metric units a week, month, or year later.
• Many of the metric prefixes on the diagram are rarely used! In fact, the only prefixes needed for high school courses and tests such as ACT and SAT are kilo, centi, and milli.

I'm very glad that for CCSS, students are expected to convert measurements using two-column tables. In Grade 4, students convert from larger units to smaller units. In Grade 5, students convert in either direction. The units mentioned in the standards for Grade 4 are kilometer, meter, centimeter, kilogram, gram, liter, and milliliter.

To convert a measurement, first set up a table with headings for the two units. Under the correct headings, record the relationship and also the measurement to be converted. Here are two examples. The table at the left is set up for converting 12 liters to milliliters (grade 4). The table at the right is set up for converting 15 millimeters to centimeters (grade 5).

From tables like these, it is easy for students to see whether to multiply or divide to convert the measurements! Writing relationships in tables is also excellent preparation for variables and functions in higher grades.