--- title: "Parts of a circle" description: "Illustrate and name parts of circles, including radius, diameter, and circumference; know that the diameter is twice the radius" canonical: https://lightmysky.com/learn/mathematics/parts-of-a-circle-mt_xq3YHZ2zeR source: https://lightmysky.com/learn/mathematics/parts-of-a-circle-mt_xq3YHZ2zeR.md retrieved: 2026-10-19 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Parts of a circle Illustrate and name parts of circles, including radius, diameter, and circumference; know that the diameter is twice the radius Subject: Mathematics · Area: Geometry · Ages 10 to 11 Page: https://lightmysky.com/learn/mathematics/parts-of-a-circle-mt_xq3YHZ2zeR ## Ready when they can - Label the radius, diameter, and circumference on a circle diagram - Calculate the diameter given a radius of 4.5 cm (diameter = 9 cm) - Explain the relationship between radius and diameter in their own words ## Lesson: Parts of a circle Every circle has a middle point called the center. The radius is the straight distance from the center to any point on the edge. No matter which direction you measure, the radius stays the same length. *(drawing: Imagine a straight line from the center of a circle out to its edge. That line is the radius.)* The diameter is a straight line that crosses the whole circle, passing through the center and touching the edge on both sides. Since it covers two radius-lengths, the diameter is always twice the radius. **Example.** Maya measures a round plate and finds the radius is 4.5 cm. To find the diameter, she doubles it: 4.5 cm x 2 = 9 cm. The plate's diameter is 9 cm. **Tip.** Circumference is the distance around the outside edge of a circle, like the edge of a clock face. Keep this rule handy: diameter = radius x 2. **Recap.** Radius runs from the center to the edge, diameter crosses through the center at twice the radius, and circumference is the distance around the outside. ## Practice 16 questions on this page, each with its working shown. ## Needs first - [Classifying shapes by line properties](https://lightmysky.com/learn/mathematics/classifying-shapes-by-line-properties-mt_r8c43QB6wx) ## Opens up - [Advanced Maths Vocabulary](https://lightmysky.com/learn/mathematics/advanced-maths-vocabulary-mt_3rTIyJDw7-) - [Properties of triangles, quadrilaterals and circles](https://lightmysky.com/learn/mathematics/properties-of-triangles-quadrilaterals-and-circles-mt_DNYQLahbfa) - [Circles: Circumference & Area](https://lightmysky.com/learn/mathematics/circles-circumference-and-area-mt_svFa6_mjO_)