Math Background

Lesson: Measurement
Introducing the Concept

In this lesson, the students find the perimeter of a rectangle in inches and centimeters. They will then convert the centimeter measurement to millimeters. Then they will measure in millimeters to check their work.

Materials: small index card, an inch ruler, and a centimeter ruler for each pair of students

Prerequisite Skills and Concepts: Students should be able to measure to the nearest one-half inch and to the nearest centimeter and millimeter.

  • Say: Today, we are going to find the perimeter of several rectangles in both customary and metric units of measurement. Remember that perimeter is the distance around a figure. It is measured in units of length.
  • Ask: Which units are used to measure length in the customary system? (inch, foot, yard, and mile) Which unit of length should we use to measure this rectangular card?
    Discuss why the inch is the best unit to measure the card. Distribute inch rulers.
  • Say: Measure each side of the card to the nearest one-half inch. Label each side with the correct measurement.
  • Ask: What is the length of the card? (5 in.) What is the width of the card? (3 in.)
  • Say: To find the perimeter, add the lengths of each of the sides. So to find the perimeter of this card we should add 5 in. + 3 in. + 5 in. + 3 in. Point to the appropriate sides of the card as you say this.
  • Ask: What is the perimeter of this card in inches? (16 in.)
  • Say: Now, let's measure the index card again, this time using metric units. I will come around to collect your inch rulers and to give you centimeter rulers.
  • Ask: What are the units of length in the metric system? (millimeter, centimeter, decimeter, meter, and kilometer) Which metric unit of length should we use to measure this rectangle?
    Discuss with the students why the centimeter is the best unit to use to measure the perimeter of the card.
  • Say: Now measure each side using the centimeter ruler. Measure to the nearest 0.5 centimeter. Label each side with the correct measurement.
  • Ask: What is the length of the card? (12.5 cm) What is the width of the card? (7.5 cm) What is the perimeter of the card in centimeters? (40 cm)
    You may wish to add the numbers with the students, since they may have trouble adding with decimals.
  • Say: Now let's convert the perimeter into millimeters. To convert within either the customary or metric systems, you will either multiply or divide, depending upon whether you are going from larger units to smaller units or from smaller units to larger units. In this case we are converting from centimeters to millimeters.
  • Ask: Which are larger—centimeters or millimeters?
    Students should say centimeters. Place a clear centimeter ruler on the overhead. Compare the size of a centimeter to the size of a millimeter.
  • Say: When you convert from a larger unit to a smaller unit, you will have more of the new units than of the old ones. So to convert from a larger unit to a smaller unit you must multiply.
  • Ask: One centimeter equals 10 millimeters. So to convert the perimeter measurement of 40 centimeters to millimeters, we need to multiply 40 by 10. What is the perimeter in millimeters? (400 mm)
  • Say: When you convert units within the metric system, you will always multiply or divide by a power of ten. Now let's measure the card again in millimeters to check that we have converted correctly.

    Give students time to measure the rectangle in millimeters and to find the perimeter. Discuss the results. If students' measurements are slightly different from their calculations, discuss the concept that measurements are not exact.

    For further practice, have student measure the perimeter of various items in the classroom in inches, centimeters and millimeters. Items might include index cards, sticky notes, textbooks, pencil cases, and folders.

Houghton Mifflin Math Grade 4