Python Program to Calculate Area of Circle

You can calculate the area of a circle in Python by using area = 3.14159 * radius ** 2, where radius is the circle's radius.

Examples

Inputradius = 0

OutputArea of circle: 0.0

Inputradius = 5

OutputArea of circle: 78.53975

Inputradius = 10.5

OutputArea of circle: 346.3600725

How to Think About It

To find the area of a circle, you need the radius. The formula multiplies pi (about 3.14159) by the square of the radius. So, you get the radius, square it by multiplying 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

Print the calculated area

Code

python

radius = float(input("Enter the radius of the circle: "))
area = 3.14159 * radius ** 2
print("Area of circle:", area)

Output

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

Step	Radius	Calculation	Area
1	5.0	N/A	N/A
2	5.0	3.14159 * 5.0 ** 2	78.53975
3	5.0	N/A	78.53975

Why This Works

Step 1: Getting the radius

We use input() to get the radius from the user and convert it to a number with float() so we can do math.

Step 2: Calculating the area

We use the formula area = π × radius², where radius ** 2 means radius times radius.

Step 3: Showing the result

We print the area so the user can see the answer.

Alternative Approaches

Using math.pi constant

python

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

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

Using a function

python

def circle_area(r):
    return 3.14159 * r ** 2
radius = float(input("Radius: "))
print("Area:", circle_area(radius))

Encapsulates calculation in a function for reuse.

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

Time Complexity

The program does a fixed number of operations regardless of input size, so it runs in constant time.

Space Complexity

It uses a few variables and no extra data structures, so space used is constant.

Which Approach is Fastest?

All approaches run in constant time; using math.pi is slightly more accurate but not slower.

Approach	Time	Space	Best For
Hardcoded pi (3.14159)	O(1)	O(1)	Simple quick calculation
Using math.pi	O(1)	O(1)	More accurate calculation
Function encapsulation	O(1)	O(1)	Reusable code