A circle has always been anÂ important shapeÂ among all geometrical figures. There are various concepts and formulas related to a circle. The sectors and segments are perhaps the most useful of them. In this article, we shall focus on theÂ concept of a sector of aÂ circle along with area and perimeter of aÂ sector.
A sector is said to be a part of a circle made of theÂ arc of theÂ circle along with itsÂ two radii. It is a portionÂ of theÂ circle formed by aÂ portion of theÂ circumference (arc) and radii of theÂ circle at both endpoints of the arc.Â The shape of a sector of aÂ circleÂ can be compared with aÂ slice of pizza or aÂ pie. Before we start learning more about the sector, first let us learn some basics of the circle.
What is a Circle?
A circle is a locus of points equidistant from a given point located at the centre of the circle. The common distance from the centre of the circle to its point is called the radius. Thus, the circle is defined by its centre (o) and radius (r). A circle is also defined by two of its properties, such as area and perimeter. The formulas for both the measures of the circle are given by;

What is Sector of a circle?
The sector is basically a portion of a circle which could be defined based on these three points mentioned below:
 A circular sector is the portion of a disk enclosed by two radii and an arc.
 A sector divides the circle into two regions, namely Major and Minor Sector.
 The smaller area is known as the Minor Sector, whereas the region having a greater area is known as Major Sector.
Area of a sector
In a circle with radius r and centre at O, let âˆ POQ = Î¸ (in degrees) be the angle of the sector. Then, the area of a sector of circle formula is calculated using the unitary method.
For the given angle the area of a sector is represented by:
The angle of the sector is 360Â°, area of the sector, i.e. the Whole circle = Ï€r^{2}
When the Angle is 1Â°, area of sector = Ï€r^{2}/360Â°
So, when the angle is Î¸, area of sector,Â OPAQ,Â is defined as;
A = (Î¸/360Â°) Ã—Â Ï€r^{2} 
Let the angle be 45 Â°. Therefore the circle will be divided into 8 parts, as per the given in the below figure;
Now the area of the sector for the above figure can be calculated as (1/8)Â (3.14Ã—rÃ—r).
Thus the Area of a sector is calculated as:
A = (Î¸/360)Â Ã— 22/7 Ã— r^{2} 
Length of the Arc of Sector Formula
Similarly, the length of the arc (PQ) of the sector with angle Î¸, is given by;
l = (Î¸/360)Â Ã— 2Ï€rÂ Â (or)Â l = (Î¸Ï€r) /180Â 
Area of Sector with respect to Length of the Arc
If the length of the arc of the sector is given instead of the angle of the sector, there is a different way to calculate the area of the sector. Let the length of the arc be l. For the radius of a circle equal to r units, an arc of length r units will subtend 1 radian at the centre. Hence, it can be concluded that an arc of length l will subtend l/r, the angle at the centre. So, if l is the length of the arc, r is the radius of the circle and Î¸ is the angle subtended at the centre, then;
Î¸Â = l/r, where Î¸ is in radians.
When the angle of the sector is 2Ï€, then the area of the sector (whole sector) is Ï€r^{2}
When the angle is 1, the area of the sector = Ï€r^{2}/2Ï€Â =Â r^{2}/2
So, when the angle is Î¸, area of the sector = Î¸ Ã—Â Â r^{2}/2
A = (l/r) Ã— (r^{2}/2)
A = (lr)/2 
Some examples for better understanding are discussed here.
Examples
Example 1: If the angle of the sector with radius 4 units is 45Â°, then find the length of the sector.
Solution: Area = (Î¸/360Â°) Ã—Â Ï€r^{2}
= (45Â°/360Â°) Ã— (22/7) Ã— 4 Ã— 4
= 44/7Â square units
The length of the same sector = (Î¸/360Â°)Ã— 2Ï€r
l = (45Â°/360Â°) Ã— 2 Ã— (22/7) Ã— 4
l = 22/7
Example 2: Find the area of the sector when the radius of the circle is 16 units, and the length of the arc is 5 units.
Solution: If the length of the arc of a circle with radius 16 units is 5 units, the area of the sector corresponding to that arc is;
A = (lr)/2Â = (5 Ã— 16)/2Â = 40Â square units.
Perimeter of a Sector
The perimeter of the sector of aÂ circle is the length of two radii along with the arc that makes the sector. In the following diagram, a sector is shown in yellow colour.
The perimeter should Â be calculated by doublingÂ the radius and then adding it to the length of theÂ arc.
Perimeter of a Sector Formula
The formula for the perimeter of the sector of a circle is given below :
Perimeter of sector = radius + radius + arc length
Perimeter of sector = 2 radius + arc length
Arc length is calculated using the relation :
Arc length = l = (Î¸/360)Â Ã— 2Ï€rÂ
Therefore,
Perimeter of a Sector = 2 Radius + ((Î¸/360)Â Ã— 2Ï€rÂ ) 
Example
A circular arc whose radius is 12 cm, makes an angle of 30Â° at the centre. Find the perimeter of the sector formed. Use Ï€ = 3.14.
Solution : Given that r = 12 cm,
Î¸ = 30Â° = 30Â° Ã— (Ï€/180Â°) = Ï€/6
Perimeter of sector is given by the formula;
P = 2 r + r Î¸
P = 2 (12) + 12 ( Ï€/6)
P = 24 + 2 Ï€
P = 24 + 6.28 = 30.28
Hence, Perimeter of sector is 30.28 cm
Thanks for explaining so nicely it reallly helped me to learn the concept well