Python Program to Find Area of Circle

To find the area of a circle in Python, use the formula area = 3.14159 * radius ** 2 where radius is the circle's radius; for example, area = 3.14159 * r ** 2.

Examples

Inputradius = 0

OutputArea of the circle is 0.0

Inputradius = 5

OutputArea of the circle is 78.53975

Inputradius = 10.5

OutputArea of the circle is 346.3600725

How to Think About It

To find the area of a circle, first understand that the area depends on the radius squared multiplied by pi (about 3.14159). You take the radius number, multiply it by itself, then multiply by pi to get the area.

Algorithm

1

Get the radius value from the user

2

Calculate the area by multiplying pi with radius squared

3

Display the calculated area

Code

python

radius = float(input("Enter the radius of the circle: "))
pi = 3.14159
area = pi * radius ** 2
print(f"Area of the circle is {area}")

Output

Enter the radius of the circle: 5
Area of the circle is 78.53975

Dry Run

Let's trace the program with radius = 5 through the code

1

Input radius

User enters 5, so radius = 5.0

2

Calculate area

area = 3.14159 * 5.0 ** 2 = 3.14159 * 25 = 78.53975

3

Print result

Output: Area of the circle is 78.53975

Step	Variable	Value
1	radius	5.0
2	area	78.53975

Why This Works

Step 1: Input radius

We get the radius from the user as a floating-point number using float(input()) to allow decimal values.

Step 2: Calculate area

We use the formula area = pi * radius ** 2 where ** means 'to the power of'. This calculates the area.

Step 3: Display area

We print the result using an f-string to show the area clearly.

Alternative Approaches

Using math.pi constant

python

import math
radius = float(input("Enter the radius of the circle: "))
area = math.pi * radius ** 2
print(f"Area of the circle is {area}")

This uses Python's built-in math.pi for more precise pi value.

Using a function

python

def circle_area(r):
    pi = 3.14159
    return pi * r ** 2
radius = float(input("Enter the radius: "))
print(f"Area of the circle is {circle_area(radius)}")

Encapsulates calculation in a function for reuse.

Complexity: O(1) time, O(1) space

Time Complexity

The calculation involves only a few arithmetic operations, so it runs in constant time.

Space Complexity

Only a few variables are used, so the space needed is constant.

Which Approach is Fastest?

All approaches run in constant time; using math.pi is slightly more precise but similar in speed.

Approach	Time	Space	Best For
Fixed pi value	O(1)	O(1)	Simple quick calculation
math.pi constant	O(1)	O(1)	More precise calculation
Function encapsulation	O(1)	O(1)	Reusable code structure