Measurement: Radius and Diameter of Circle

Understanding a Circle

Take a circular paper disc, fold it in half and then unfold it.

The line of the fold that divides the circle into two halves is a diameter of the circle.
In the figure below, XY is a diameter.
Trace the outline of a CD, a round plate or a ring on paper and cut it along its edge to get a circular paper disc.
diameter of circle
Fold the paper disc in another way and you will get another diameter of the circle.
Name it ST as below.
diameter of circle
Measure the lengths of the two diameters.
You will note that XY = ST.

Also, XY and ST meet at point O, which is known as the centre or center of the circle.

Any line drawn from the centre O, to the boundary or edge of the circle is a radius of the circle.
In the figure above, OP is a radius.
OQ is another radius.
Measure the lengths of OP and OQ and you will notice that OP = OQ.
You will also note that the radius is equal to half the diameter.
OP  = 
1
2
XY
OP  = 
1
2
ST
Radius = Diameter ÷ 2
Or,
Diameter = 2 × Radius

1. Find the radius of the circle below.

find radius
Diameter  =  6 cm
Radius  =  6 cm  ÷  2  =  3 cm

2. The diameter of a circle if 20 m. What is its radius?

find radius
Radius  =  20 m  ÷  2  =  10 m

3. Find the diameter of the circle below.

find radius
Radius  =  7 cm
Diameter  =  7 cm  ×  2  =  14 cm

4. The radius of a circle is 15 m. Find its diameter.

find radius
Diameter  =  15 m  ×  2  =  30 m

GLOSSARY of terms related to circle

lines on circle     slices on circle
Circumference: The perimeter of a circle.

Arc: A part of the circumference.

Chord: A line inside a circle that crosses the circumference at two points. A diameter is a chord that passes through the centre of the circle.

Tangent: A straight line that touches the circle at only one point.

Quadrant: A quarter circle.

Semicircle: A half circle.

Sector: A slice of the circle made by two radii. [Think of a pizza slice.]

Segment: A slice of the circle made by a chord.