Body Mass Index (BMI) Calculator

What is the Body Mass Index?

Body mass index (BMI) is a measure of body fat based on height and weight that applies to adult men and women. Based off the the value of the index, a person can be categorized into one of the following:

• Underweight = <18.5

• Normal weight = 18.5–24.9

• Overweight = 25–29.9

• Obesity = BMI of 30 or greater

The general formula is as follows

where weight is in kilograms and height is in metres. Therefore, BMI's unit is in kilograms per metre squared.

How to create your own BMI calculator

In this blog I will show you how you can easily write simple code in python that calculates user's BMI.

Input:

• Weight in kilograms

• Height in metres

Output:

• Body Mass Index

• Category

So let's get started!

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First, let's prompt the user to enter their data

weight = float(input("Weight in kilograms: "))
height = float(input("Height in metres: "))

Notice how we cast the output to float as the input method returns a string.

Next, we will calculate the BMI using our formula. We will surround it with a try...except block in case the user hasn't entered valid values. We used abs() method in case the user accidentally inputted negative numbers.

bmi = 0
try:
    bmi = abs(weight / height ** 2)
except:
    print("Please enter a valid weight and height before proceeding!")

Finally, we can display to the user as a bonus their BMI category according to their input using a simple if...elif...else structure.

if bmi <= 18.5:
    print("You are underweight. Are you sure you're eating enough nutrient-rich and filling meals?")
elif bmi < 25:
    print("Congratulations! You're normal weight. Keep up your healthy diet :)")
elif bmi < 30:
    print("You are overweight. Try maintaining a balanced diet.")
else:
    print("You are obese. Obesity has many detrimental health implications.")

Do you think the commentary was unnecessary? Maybe. But in the end everyone knows their own health best ;)

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Full Code

weight = float(input("Weight in kilograms: "))
height = float(input("Height in metres: "))
bmi = 0
try:
    bmi = abs(weight / height ** 2)
except:
    print("Please enter a valid weight and height before proceeding!")
    if bmi <= 18.5:
    print("You are underweight. Are you sure you're eating enough nutrient-rich and filling meals?")
elif bmi < 25:
    print("Congratulations! You're normal weight. Keep up your healthy diet :)")
elif bmi < 30:
    print("You are overweight. Try maintaining a balanced diet.")
else:
    print("You are obese. Obesity has many detrimental health implications.")

Short and sweet.

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To learn more about the Body Mass Index, visit World Health Organization's website.