You Can draw It Yourself

Put a pin in a board, placed a loop the string about it, and insert a pencil into the loop. Keep the string stretched and draw the circle!

Play through It

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

You are watching: Half of a circle is called

(See if you can keep a consistent radius!)

Radius, Diameter and Circumference


The Radius is the street from the facility outwards.

The Diameter goes straight throughout the circle, with the center.

The Circumference is the distance once approximately the circle.

And right here is the really cool thing:

So when the diameter is 1, the circumference is 3.141592654...


We can say:

Circumference = π × Diameter

Example: friend walk about a one which has actually a diameter that 100m, how much have girlfriend walked?


Distance walked = one = π × 100m

= 314m (to the nearest m)

Also keep in mind 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 length of the indigenous may assist you remember:

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



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



The area the a one is π time the radius squared, i m sorry is written:

A = π r2


A is the Arear is the radius

To help you remember think "Pie space Squared" (even despite pies space usually round):


Or, utilizing the Diameter:


A = (π/4) × D2

Area compared to a Square


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

And something exciting for you:

See one Area through Lines


Because people have studied one for countless years distinct names have actually come about.

Nobody wants to speak "that line that starts at one side of the circle, goes with the center and ends on the other side" when they have the right to just to speak "Diameter".

So below are the most usual special names:



A line the "just touches" the circle together it passes by is called a Tangent.

A heat that cut the one at 2 points is dubbed a Secant.

A heat segment that goes indigenous one point to another on the circle"s one is dubbed a Chord.

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

And a component of the circumference is called an Arc.



There room two key "slices" the a circle.

The "pizza" slice is referred to as a Sector.

And the slice made by a chord is referred to as a Segment.

Common Sectors

The Quadrant and also Semicircle are two special types of Sector:


Quarter the a one is called a Quadrant.


Half a one is called a Semicircle.

Inside and Outside


A circle has an inside and an exterior (of course!). However it likewise has an "on", because we might be best on the circle.

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

See more: How Many Atoms Are In Vinegar ? How Many Atoms Make Up One Molecule Of Vinegar



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

Activity: Approximate worth For Pi
Pi aircraft Unit circle Circle Sector and Segment circle Equations aircraft Geometry Geometry table of contents