Parts of a circle
A circle is the set of all points that are the same distance from a fixed point called the centre. That fixed distance is the radius (r). A straight line through the centre joining two points on the circle is the diameter (d), and it is always twice the radius.
Key idea / Key idea
Diameter = 2 × radius, so d = 2r and r = d ÷ 2. The diameter is the longest chord in a circle.
Chords, arcs and sectors
A chord is a straight line joining any two points on the circle. An arc is part of the circle's edge (circumference). A sector is the region enclosed by two radii and an arc — like a slice of pizza. A segment is the region between a chord and an arc.
| Part | Description |
| Radius | Centre to the circle |
| Diameter | Across, through the centre |
| Chord | Joins two points on the circle |
| Arc | Part of the circumference |
| Sector | Two radii + an arc |
Using the relationships
Worked example / Worked example
A circle has a diameter of 18 cm. Find its radius.
r = d ÷ 2 = 18 ÷ 2 = 9 cm.
If instead the radius were 7 cm, the diameter would be d = 2 × 7 = 14 cm.
Remember / Remember
- All radii of one circle are equal.
- Diameter = 2 × radius; it is the longest chord.
- A sector needs two radii and an arc.
- The circumference is the full distance around the circle.