You Can attract It Yourself

Put a pin in a board, placed a loop that string around it, and insert a pencil right into the loop. Keep the cable stretched and also draw the circle!

Play through It

Try dragging the suggest to see just how the radius and also circumference change.

You are watching: A portion of the circumference of a circle

(See if you deserve to keep a consistent radius!)

Radius, Diameter and Circumference


The Radius is the street from the center outwards.

The Diameter goes straight across the circle, through the center.

The Circumference is the distance once around the circle.

And right here is the really cool thing:

So as soon as the diameter is 1, the one is 3.141592654...


We can say:

Circumference = π × Diameter

Example: girlfriend walk about a circle which has a diameter that 100m, how far have girlfriend walked?


Distance to walk = circumference = π × 100m

= 314m (to the nearest m)

Also note that the Diameter is double the Radius:

Diameter = 2 × Radius

And so this is likewise true:

Circumference = 2 × π × Radius

In Summary:

× 2× π


The size of the native may help you remember:

Radius is the shortest word and also shortest measureDiameter is longerCircumference is the longest



The one is a plane shape (two dimensional), so:



The area that a one is π time the radius squared, which is written:

A = π r2


A is the Arear is the radius

To help you mental think "Pie room Squared" (even despite pies room usually round):


Or, making use of the Diameter:


A = (π/4) × D2

Area compared to a Square


A circle has about 80% of the area the a similar-width square.The actual value is (π/4) = 0.785398... = 78.5398...%

And something amazing for you:

See circle Area through Lines


Because civilization have studied circles for countless years unique names have actually come about.

Nobody wants to to speak "that line that starts at one next of the circle, goes v the center and ends ~ above the various other side" once they can just to speak "Diameter".

So right here are the most common special names:



A line the "just touches" the circle as it overcome by is called a Tangent.

A line that cut the one at two points is dubbed a Secant.

A heat segment the goes from one allude to one more on the circle"s one is referred to as a Chord.

If that passes with the facility it is dubbed a Diameter.

And a component of the circumference is called an Arc.



There are two main "slices" the a circle.

The "pizza" slice is dubbed a Sector.

And the slice made by a chord is called a Segment.

Common Sectors

The Quadrant and Semicircle space two special species of Sector:


Quarter that a one is called a Quadrant.


Half a one is referred to as a Semicircle.

Inside and also Outside


A circle has an inside and also an exterior (of course!). Yet it likewise has an "on", because we could be appropriate on the circle.

Example: "A" is external the circle, "B" is within the circle and also "C" is top top the circle.

See more: Vòng Tay Lu Thống Cẩm Thạch, Ý Nghĩa Lu Thống Phong Thủy Là Gì



A circle is a "special case" of one ellipse.

Activity: Approximate value For Pi
Pi airplane Unit circle Circle Sector and Segment one Equations plane Geometry Geometry table of contents