Parts of a circle · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Parts of a circle

Mathematics · Geometry · ages 10-11
Name ______________________   Date ____________

  1. A straight line from the center of a circle to its edge is called the...

    • Diameter
    • Radius
    • Circumference

  2. A circle has a radius of 4.5 cm. What is its diameter in cm?

    Answer: ______________

  3. What do we call the distance all the way around the outside of a circle?

    • Circumference
    • Radius
    • Diameter
    • Center

  4. Which part of a circle is the distance all the way around the outside edge?

    • Radius
    • Diameter
    • Circumference

  5. A circle with a radius of 3.2 m has a diameter of 1.6 m.

    Circle one:   True   False

  6. A circle has a circumference of 94.2 cm. Use 3.14 for π. What is its radius in cm?

    Answer: ______________

  7. A circle has a circumference of 62.8 cm. Use 3.14 for π. What is its diameter in cm?

    Answer: ______________

  8. Which sentence best explains how the radius and diameter are related?

    • The diameter is twice as long as the radius
    • The radius is twice as long as the diameter
    • The radius and diameter are always equal

  9. A circle's diameter is 20 cm. What is its radius, in cm?

    Answer: ______________

  10. Mia says: 'My circle has a radius of 6 cm, so the diameter must be 3 cm.' What mistake did Mia make?

    • She halved the radius instead of doubling it
    • She used the wrong shape
    • She measured the circumference instead
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Parts of a circle W1-mt_xq3YHZ2zeR-s1

  1. Radius · The radius is the line from the center of a circle out to its edge.
  2. 9 · The diameter is always 2 times the radius, so 4.5 cm x 2 = 9 cm.
  3. Circumference · The circumference is the distance around the outside edge of a circle, like its perimeter.
  4. Circumference · Circumference is the boundary you would trace if you walked around the outside of the circle.
  5. False · The diameter should be bigger than the radius, not smaller. 3.2 m x 2 = 6.4 m, not 1.6 m.
  6. 15 · The circumference of 94.2 cm means the diameter is 94.2 / 3.14 = 30 cm, and the radius is half of that, so 15 cm.
  7. 20 · Since circumference = π x diameter, dividing 62.8 by 3.14 gives a diameter of 20 cm.
  8. The diameter is twice as long as the radius · The diameter spans the whole circle through the center, which is two radius-lengths.
  9. 10 · The radius is half the diameter, so 20 cm / 2 = 10 cm.
  10. She halved the radius instead of doubling it · The diameter is always bigger than the radius: double it, don't halve it. 6 cm x 2 = 12 cm.
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