radius

Everyday

1. (maths) A straight line from the centre of a circle to its edge, or the length of that line. tree →
2. A distance around a place in every direction: «within a 5-mile radius». tree →
3. (body) One of the two bones in the forearm, on the thumb side. tree →

In the lessons

In maths the radius is half the diameter: diameter = 2 × radius. The area of a circle is π × r × r and the circumference is 2 × π × r. In everyday life «within a radius of» means within that distance all around. Phrases: «within a radius of», «radius and diameter».

Examples

• The radius of this circle is 4 cm, so the diameter is 8 cm. tree →
• Draw a circle with a radius of 3 cm using a compass. tree →
• All pupils living within a two-mile radius can walk to school. tree →
• He broke his radius when he fell off the trampoline. tree →
• The radius goes from the centre to the edge. tree →
• A circle's area is π times the radius squared. tree →

Note

Plural: radii (RAY-dee-eye) in maths, radiuses also accepted. Maths: radius = diameter ÷ 2; diameter = radius × 2.