Table of Contents
1.  Introduction 
2.  What is a Radian? 
4.  Unit Circle 
5.  Degree to Radian Conversion 
6.  What is the need for Radian? 
7.  FAQs 
8.  Summary 
September 1, 2020
Reading Time: 5 minutes
Introduction
Radian, as a concept, was first described by Roger Cotes back in 1714. But the term 'Radian' first appeared in print in 1873. The name radian was not universally adopted for a while, in fact, Longmans' school of trigonometry still called radian the 'circular measure' when published in 1890.
As shown in the picture,
 There are 2 pi radians in a circle (2 * 3.14159..). That equates to 6 complete radians and a little more than a quarter of a radian (0.2831853..).
 The length of each arc (s) subtended by an angle of one radian is equal to the radius itself. s = r ( for 1 radian arc).
 The magnitude of such a subtended angle (in radians) is equal to the ratio of the arc length subtended to the radius of the circle; which means θ=s/r, where θ is the subtended angle in radians.
 The length of the enclosed arc is equal to the radius multiplied by the magnitude of the angle in radian; that is s=rθ.
Also read:
What is a Radian?
Radian is the plane angle subtended by a circular arc and its value is given by the length of the arc divided by the radius of the arc. One radian is the angle subtended at the center of a circle by an arc that is equal in length to the radius of the circle.
The unit is also called a circular measure of an angle.
Unit symbol: The international bureau of weights and measures specify rad as a symbol of the radian. Alternative symbols used 100 years ago are c (the letter), or a superscript R.
Radian 

Unit System 
SI derived unit 
Unit of 
Angle 
Radian Symbol 
rad or c 
In unit 
dimension with the arc length equal to the radius i.e 1mm 
Conversions 

Units 
1 rad is equal to 
Milliradians 
1000 mrad 
Turns 
12π turns 
180 degrees 
180 =57.296˚ 
gradians 
200=63.662g 
Unit Circle
The unit circle is a circle with the value of its radius as 1 unit. It makes learning and talking about lengths and angles much simpler.
The x and yaxes divide the entire coordinate plane into four quarters (90˚ each) called quadrants. The trigonometry quadrants are as follows:
Quadrant I 
090˚ or (0  π/2) rad 
Quadrant II 
90180˚ or (π/2  π) rad 
Quadrant III 
180270˚ or (π  3π/2) rad 
Quadrant IV 
270360˚ or (3π/2  2π) rad 
Any angle with a value greater than 360˚ or 2π can be calculated by subtracting a multiple of 360, which is also the closest value lesser than the angle itself from it so that we can reduce all angle values to a range of (0,360) or (0,2π).
Eg: 17.5π = 17.5π  8*2π = 1.5π or 270˚
For any angle, we can label the side and the unit circle by its coordinates(x,y). The coordinates x and y will be the output of the trigonometric functions
where,
X=cos t; Y=sin t
The diagram of the unit circle is as follows:
Coordinates of a point on a unit circle where the central angle is t radians: The value of x and y is given by the length of the two triangles legs. This is a right triangle, and it displays how the lengths of these two sides are given by trigonometric functions of t.
Degree to Radian Conversion
1 rad = 180/π
1 rad= 1 * (180˚)/ π ≈ 57.2958˚
Angle in degrees = Angle in radians * (180˚)/π
For example,
1. 2.5 rad = 2.5 * (180˚) /π
≈ 143.2394˚
2. π/3 rad = π/3* (180˚)/π
≈ 60˚
To convert from degrees to radians, multiply the angle in degrees by π/180 such that,
Angle in radians= angles in degrees * π/(180˚)
For example:
1. 1˚ = 1 * π/(180˚)
≈ 0.0175 rad
2. 23˚= 23 * π/(180˚)
≈ 0.4014 rad
Radian is converted to turns by dividing the number of radians by 2π.
Radian to degree conversion derivation
The circumference of a circle is 2πr (r is the radius of the circle)
Using this,
360˚↔ 2πr (since a complete circle sweeps 360˚)
By the definition of radians, a full circle has:
2πr/r rad= 2π rad
Combining both the above relations
2π rad=360˚
≡ 1 rad = (360°)/2π
≡ 1 rad = (180°)/π
Example 1: Convert 50˚to radians
Answer: 50˚ x π/180 = 50π/180 = 5π/18
Example 2: Convert π/6 to degrees
Answer: π/6 x (180˚)/π = 180/6 = 30˚
If an angle is given as a value without the degree symbol, then the angle is generally assumed to be in radian measure.
Conversion of common angles:
Turns 
Radians 
Degrees 
0 
0 
0˚ 
1/24 
π/12 
15˚ 
1/12 
π/6 
30˚ 
1/10 
π/5 
36˚ 
1/8 
π/4 
45˚ 
1/2π 
1 
57.3˚ 
1/6 
π/3 
60˚ 
1/5 
2π/5 
72˚ 
1/4 
π/2 
90˚ 
1/3 
2π/3 
120˚ 
2/5 
4π/5 
144˚ 
1/2 
π 
180˚ 
3/4 
3π/2 
270˚ 
1 
2π 
360˚ 
What is the need for Radian?
Since radian is the angle subtended by an arc equal in length to the radius of the circle, the definition is far less arbitrary than a degree so you could claim it is a better unit purely.
Both radian and degree are units of angle measurement, but the angle 360 in degree is arbitrary since there is no mathematical base to choose that value from.
Also, 1 radian = 2*pi…where pi comes in a picture. Pi is the dearest number to mathematics and we are talking about circles, who else stands a chance?
 A oneradian arc has an arc length equal to the radius
 It also makes the value of trigonometric functions of very small angles much easier to assume and compute.
 It makes differentiation of trigonometric functions more straight forward.
 It makes radial motion formulas, and some harmonic motion formulas, more straightforward as well.
Summary
A radian equal to 57.3˚ and an angle subtended by an arc whose length is equal to the radius of the circle.
Radian = length of arc/ radius of circle
Unit circle is a circle with a radius of 1. It is a great way to learn and talk about lengths and angles
The length of the circumference of a circle is given by 2πr, where r is the radius of the circle The radian is the angle of a circle subtended by an arc equal in length to the radius
one radian arc has an arc length equal to the radius.
Written by Sakshi Goel, Cuemath Teacher
Frequently Asked Questions (FAQs)
Value of 360?
6.283 radians
Rad full form?
Radian
Radians to Degrees Formula?
Angle in degrees = Angle in radians * (180˚)/π
Degree to Radian Formula?
Angle in radians = Angles in degrees * π/(180˚)
1 radian?
1 radian ≈ 57.2958˚ or 180/π
Radian Symbol?
rad or c
Radian Definition?
Radian = length of arc/ radius of circle