Python Program to Calculate Perimeter of Circle

You can calculate the perimeter of a circle in Python using perimeter = 2 * 3.1416 * radius, where radius is the circle's radius.

Examples

Inputradius = 1

OutputPerimeter of circle: 6.2832

Inputradius = 5

OutputPerimeter of circle: 31.416

Inputradius = 0

OutputPerimeter of circle: 0.0

How to Think About It

To find the perimeter of a circle, think about the distance around it. The formula is 2 * π * radius. So, you get the radius from the user, multiply it by 2 and π (about 3.1416), and that gives you the perimeter.

Algorithm

1

Get the radius value from the user

2

Set π (pi) as 3.1416

3

Calculate perimeter by multiplying 2, π, and radius

4

Print the perimeter result

Code

python

radius = float(input("Enter the radius of the circle: "))
pi = 3.1416
perimeter = 2 * pi * radius
print("Perimeter of circle:", perimeter)

Output

Enter the radius of the circle: 5
Perimeter of circle: 31.416

Dry Run

Let's trace the example where radius = 5 through the code

1

Input radius

User enters 5, so radius = 5.0

2

Set pi

pi = 3.1416

3

Calculate perimeter

perimeter = 2 * 3.1416 * 5.0 = 31.416

4

Print result

Output: Perimeter of circle: 31.416

Step	Variable	Value
1	radius	5.0
2	pi	3.1416
3	perimeter	31.416
4	output	Perimeter of circle: 31.416

Why This Works

Step 1: Get radius input

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: Use pi value

We set pi to 3.1416, which is a close value for π, the circle constant.

Step 3: Calculate perimeter

We multiply 2 * pi * radius to find the perimeter, which is the distance around the circle.

Step 4: Show result

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

Alternative Approaches

Use math.pi constant

python

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

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

Define a function

python

def circle_perimeter(r):
    return 2 * 3.1416 * r
radius = float(input("Radius: "))
print("Perimeter:", circle_perimeter(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

Only a few variables are used, so memory use is constant.

Which Approach is Fastest?

All approaches run instantly for any input size; using math.pi is preferred for accuracy without speed loss.

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