The diameter of a circle is the length of a straight line that passes through the center of the circle and touches the circle at two opposite points on its edge.

Quick Scoop: Core idea

  • The diameter is the longest straight line you can draw across a circle, center to center-edge-to-edge.
  • It always passes through the center and its endpoints lie on the circumference (the outline of the circle).
  • All diameters in the same circle have the same length.

Mini formulas (super short)

  • If you know the radius rrr:
    • Diameter d=2rd=2rd=2r (twice the radius).
  • If you know the circumference CCC:
    • Diameter d=C÷πd=C\div \pi d=C÷π.

Example: if the radius is 5 cm, then the diameter is 2×5=102\times 5=102×5=10 cm.

Quick HTML table

html

<table>
  <tr>
    <th>Concept</th>
    <th>Meaning</th>
  </tr>
  <tr>
    <td>Diameter (geometric idea)</td>
    <td>Any line segment passing through the center with both ends on the circle.[web:1][web:5]</td>
  </tr>
  <tr>
    <td>Diameter (length)</td>
    <td>The distance across the circle through the center, equal to twice the radius.[web:1][web:7]</td>
  </tr>
  <tr>
    <td>Relation to radius</td>
    <td>d = 2r (diameter is two times the radius).[web:1][web:3]</td>
  </tr>
  <tr>
    <td>Relation to circumference</td>
    <td>d = C / π (circumference equals π times diameter).[web:1]</td>
  </tr>
</table>

Little story to remember it

Imagine you’re cutting a perfectly round pizza right through the exact middle so that both slices are equal.

  • That straight cut from one edge, through the center, to the opposite edge is the diameter.

TL;DR: The diameter of a circle is the straight line going through the center from one side of the circle to the other, and its length is always 2×2\times 2× the radius.

Information gathered from public forums or data available on the internet and portrayed here.