The Ratio of Circumference to Diameter
Students measure the circumference and diameter of circular objects. They calculate the ratio of
circumference to diameter for each object in an attempt to identify the value of pi and the
Measure the circumference and diameter of various circular objects
Calculate the ratio of circumference to diameter
Discover the formula for the circumference of a circle
Pieces of string, approximately 48" long
Circular objects to be measured
Apple pies (or other circular food item, to be measured at the end of the lesson)
Apple Pi activity sheet
Circle Tool applet
Spread Sheet program
Prior to this lesson, ask students to bring in several flat, circular objects that they can measure.
As a warm-up, ask students to measure the length and width of their desktops. Ask them to
decide which type of unit should be used. Then, have students measure or calculate the distance
around the outside of their desktops.
With the class, discuss the following questions:
1. What unit did you use to measure your desks? Why?
[Because of the size of desks, the most appropriate units are probably inches or
2. Why did some of your classmates get different measurements for the dimensions of their
[Measurements will obviously differ because of the units. In addition, the level of
precision may give different results. For instance, a student may round to the nearest
inch, while another may approximate to the nearest ¼-inch.]
3. What do we call the distance around the outside of an object?
[The distance around the outside of a polygon is known as the perimeter. The distance
around the outside of a circle is known as the circumference.]
Inform the class that they will be measuring the circumference of several circular objects during
today’s lesson. Also, alert them that, just as there is a formula for finding the perimeter of a
rectangle (P = 2L + 2W), there is also a formula for finding the circumference of a circle. They
should keep their eyes open for a formula as they proceed through the measurement activities.
Divide the class into groups of four students. Within the groups, each student will be given a
different job. (If class size is not conducive to four students per group, form groups of three —
one student can be assigned two jobs.)
Task Leader: Ensures all students are participating; lets the teacher know if the group
needs help or has a question.
Recorder: Keeps group copy of measurements and calculations from activity.
Measurer: Measures items (although all students should check measurements to ensure
Presenter: Presents the group’s findings and ideas to the class.
Students should measure the "distance around" and the "distance across" of the objects that they
brought to school. Students will likely have little trouble measuring the distance across, although
they may have some difficulty identifying the exact middle of an object. To measure the distance
around, students will likely need some assistance. An effective method for measuring the
circumference is to wrap a string around the object and then measure the string. To ensure
accuracy, care should be taken to keep the string taut when measuring the outside of a circular
Students should be allowed to select which unit of measurement to use. However, instruct
students to use the same unit for the distance around and the distance across.
Students should record the following information in the Apple Pi activity sheet:
Description of each object
Distance around the outside of each object
Distance across the middle of each object
Distance around divided by distance across
Apple Pi Activity Sheet
After the measurements have been recorded, a calculator can be used to divide the distance
around by the distance across. Students should answer both questions on the worksheet. As
students are working, take note of their results. Push students to note any numbers in the last
column that seem to be irregular, and have them check their measurements for these rows.
When all groups have completed the measurements and calculations, conduct a whole-class
discussion. Rather than present each individual object, students should discuss the average and
note any interesting findings. Students should also compare their averages with those of other
You may wish to use the Circle Tool applet as a demonstration tool. This applet allows students
to see many other circles of various sizes, as well as the corresponding ratio of circumference to
Explain that each group has found an approximation for the ratio of the distance around to the
distance across, and this ratio has a special name: π. (It may also be necessary to explain that the
"distance across" is known as the diameter and that the "distance around" is known as the
circumference. Because of this relationship, algebraic notation can be used to write
circumference ÷ diameter = π
or, said another way,
π = C/d
which leads to the following formula for circumference:
Point out that groups within the class may have obtained slightly different approximations for π.
Explain that determining the exact value of π is very hard to calculate, so approximations are
often used. Discuss various approximations of π that are acceptable in your school’s curriculum.
1. In this lesson, students use a numeric approach to see the relationship between
circumference and diameter. That is, students compute the ratio of circumference to
diameter and then take the average for several objects. For a visual approach, have
students plot the diameter of those objects along the horizontal axis of a graph and plot
the circumference along the vertical axis. As shown below, a line of best fit with slope
of roughly 3.14, or π, will approximate the points in the resulting scatterplot.
2. Students can read and react to the book Sir Cumference and the First Round Table: A
Math Adventure by Cindy Neuschwander. Within their groups, students can pose
questions about the book and its mathematical accuracy, realism, and other components.
3. In their groups, students can research the history of π and its calculation, approximation,
and uses. In particular, they can research Archimedes method for estimating the area of
a circle using inscribed polygons. The students could report their findings to the class.
Student Assessment: Students will turn in their activity sheets and write
conclusions in the math composition books that will also be checked weekly.
What prior knowledge did students have of π (if any)? How did student’s prior
knowledge affect the delivery of the lesson? What modifications did you need to make
as a result, and how effective were these adjustments?
How precise were student measurements? How did you assist students with their
How did students react to the use of 3.14 as an approximation of π? Were there any
adverse reactions due to conceptual misunderstandings?
How did students show that they were actively learning?
Did students understand that the ratio of circumference to diameter (i.e., π) is an
approximation? Did they understand why they had obtained different values for this
approximation during the activity?
NCTM Standards and Expectations
1. Understand both metric and customary systems of measurement.
2. Select and apply techniques and tools to accurately find length, area, volume, and angle
measures to appropriate levels of precision.
3. Develop and use formulas to determine the circumference of circles and the area of
triangles, parallelograms, trapezoids, and circles and develop strategies to find the area
of more-complex shapes.
Neuschwander, Cindy, and Wayne Geehan. 1997. Sir Cumference and the First Round
Table: A Math Adventure. Watertown, MA: Charlesbridge Publishing.
This lesson prepared by Christopher Johnston.