There is a straightforward formula that you can use to calculate a percentage in Python: (part / whole) * 100. That means the part is % of a whole number.

Use the division operator to divide one number by another, and then multiply the result by 100.

part = 3
whole = 5

percentage = (part / whole) * 100

print(percentage)

# Output: 60.0

That means 30 is 60% of 50.

Handling a ZeroDivisionError exception

What if the whole is 0? It will throw a ZeroDivisionError exception. To handle this exception, we can use a try-except block and manually return 0.

def calculate_percentage(part, whole):
    try:
        return (part / whole) * 100
    except ZeroDivisionError:
        return 0


percentage = calculate_percentage(3, 0)

print(percentage)

# Output: 0

Rounding percentages

If you want percentages as integers or with limited decimals, use the round() method.

For rounding up to two decimal points, pass 2 as a second argument to the round() method. For rounding up to four decimal points, pass 4.

part, whole = 300, 700

percentage = (part / whole) * 100

print(round(percentage))
# Output: 43

print(round(percentage, 2))
# Output: 42.86 (2 decimal places)

print(round(percentage, 4))
# Output: 42.8571 (4 decimal places)

Calculating percentage change

To calculate the percentage change between an old_value and a new_value, you can use the formula:

((new_value - old_value) / old_value) * 100

Increase percentage

Let’s take an old value of 75 and a new value of 100 and find the percentage increase from 75 to 100.

old_value = 75
new_value = 100

change = ((new_value - old_value) / old_value) * 100

print(f"Increase: {change}%")

# Output: Increase: 33.33333333333333%

That means it is a change of 33.33% increase.

Decrease percentage

Let’s take the old value of 100 and the new value of 75, and calculate the decreasing percentage.

old_value = 100
new_value = 75

change = ((new_value - old_value) / old_value) * 100

print(f"Decrease: {change}%")

# Output: Decrease: -25.0%

That means, it is a 25% reduction from 100 to 75.

You can see that when it comes to increase, it is up by 33.33% but when you decrease, it is 25%.

So, increase and decrease depend on which value you are calculating. It will make a base of that value and start calculating.

Percentage of a list

Let’s calculate the percentage of even numbers from the total number of elements in a list. For that, we need a list comprehension and the len() function.

We can calculate the number of even elements with list comprehension and len(), then divide by the total number of elements and multiply by 100.

num_list = [1, 2, 3, 4, 5]

percent = (len([a for a in num_list if a % 2 == 0]) / len(num_list)) * 100

print(str(percent))

# Output: 40.0

That means 40% of the elements in the input num_list are even numbers.

Alternate ways

Using a custom function

We can write a custom function that accepts two arguments: part and whole, and returns the percentage.

def calculate_percentage(part, whole):
    return str((part / whole) * 100) + "%"


percentage = calculate_percentage(3, 5)

print(percentage)

# Output: 60.0%

Using fractions.Fraction for exact percentages

The fractions.Fraction is a built-in class that represents numbers as exact rational values (numerator/denominator), avoiding floating-point rounding errors. This makes it great for precise percentage calculations.

from fractions import Fraction

part = 2
whole = 7

percentage = Fraction(part, whole) * 100

print(percentage)
# Output: 200/7

print(float(percentage))
# Output: 28.571428571428573

That means number 2 is 28.57142% of number 7.

Krunal Lathiya

With a career spanning over eight years in the field of Computer Science, Krunal’s expertise is rooted in a solid foundation of hands-on experience, complemented by a continuous pursuit of knowledge.