Math: Circles: Radius, Diameter, Circumference
Find and use circle measurements with pi
Math: Circles: Radius, Diameter, Circumference
Find and use circle measurements with pi
Math - Grade 6-8
- 1
A circle has a radius of 6 cm. What is its diameter?
Use d = 2r.
The diameter is 12 cm because the diameter is twice the radius, so 2 × 6 = 12. - 2
A circle has a diameter of 18 inches. What is its radius?
Use r = d ÷ 2.
The radius is 9 inches because the radius is half the diameter, so 18 ÷ 2 = 9. - 3
A circle has a diameter of 10 meters. Find its circumference using 3.14 for pi.
When you know the diameter, use C = πd.
The circumference is 31.4 meters because C = πd, so C = 3.14 × 10 = 31.4. - 4
A circle has a radius of 4 feet. Find its circumference using 3.14 for pi.
When you know the radius, use C = 2πr.
The circumference is 25.12 feet because C = 2πr, so C = 2 × 3.14 × 4 = 25.12. - 5
A bicycle wheel has a diameter of 26 inches. How far does the bike move in one full turn of the wheel? Use 3.14 for pi.
The bike moves 81.64 inches in one full turn because the distance is the circumference, and 3.14 × 26 = 81.64. - 6
The circumference of a circular table is 94.2 inches. What is the diameter of the table? Use 3.14 for pi.
Rearrange C = πd to solve for d.
The diameter is 30 inches because d = C ÷ π, so 94.2 ÷ 3.14 = 30. - 7
The diameter of a round clock face is 12 inches. What is the circumference of the clock face? Use 3.14 for pi.
The circumference is 37.68 inches because C = πd, so 3.14 × 12 = 37.68. - 8
A circle has a circumference of 62.8 centimeters. What is its radius? Use 3.14 for pi.
Find the diameter first, then divide by 2.
The radius is 10 centimeters because the diameter is 62.8 ÷ 3.14 = 20, and the radius is 20 ÷ 2 = 10. - 9
Circle A has a radius of 5 cm. Circle B has a diameter of 8 cm. Which circle has the greater circumference? Use 3.14 for pi.
Find the circumference of each circle and compare the results.
Circle A has the greater circumference. Circle A has circumference 2 × 3.14 × 5 = 31.4 cm, while Circle B has circumference 3.14 × 8 = 25.12 cm. - 10
A circular garden has a radius of 7 meters. A gardener wants to place edging around the outside. How many meters of edging are needed? Use 3.14 for pi.
The gardener needs 43.96 meters of edging because the edging length is the circumference, and 2 × 3.14 × 7 = 43.96. - 11
A student says, "The radius of a circle is always longer than the diameter." Explain whether the student is correct.
Think about how a radius and a diameter are drawn inside a circle.
The student is not correct. The diameter is always twice the radius, so the diameter is longer than the radius. - 12
A circle has a diameter of 15 units. Write the circumference in terms of pi, then estimate it using 3.14 for pi.
The exact circumference is 15π units. The estimated circumference is 47.1 units because 15 × 3.14 = 47.1. - 13
A jar lid has a circumference of 25.12 cm. What is the diameter of the lid? Use 3.14 for pi.
Use d = C ÷ π.
The diameter is 8 cm because d = C ÷ π, so 25.12 ÷ 3.14 = 8. - 14
A circular running track has a diameter of 100 meters. A runner completes 3 laps. How far does the runner travel? Use 3.14 for pi.
The runner travels 942 meters. One lap is 3.14 × 100 = 314 meters, and 3 laps is 314 × 3 = 942 meters. - 15
Two circles have the same center. The smaller circle has a radius of 3 cm, and the larger circle has a radius of 9 cm. How many times greater is the larger circle's circumference than the smaller circle's circumference?
You can compare the radii instead of calculating both circumferences.
The larger circle's circumference is 3 times greater than the smaller circle's circumference. Circumference is proportional to radius, and 9 ÷ 3 = 3.