C Program To Measure Body Mass Index (bmi) Online Calculator

Body Mass Index (BMI) is a widely used health indicator that helps assess whether an individual has a healthy body weight relative to their height. In this article, you will learn how to create a simple C program to calculate BMI, much like an online calculator, by taking user inputs for weight and height.

Problem Statement

Many individuals need a quick and easy way to determine their BMI to understand their weight status, which is crucial for health monitoring and fitness goals. Manually calculating BMI can be cumbersome, and an automated tool can simplify this process. The problem is to develop a console-based C program that accurately computes BMI from user-provided weight and height, making it accessible and convenient.

Example

Consider a person weighing 70 kg with a height of 1.75 meters. The BMI calculation would be: BMI = 70 / (1.75 * 1.75) = 70 / 3.0625 = 22.86

The program should output a similar numerical value.

Background & Knowledge Prerequisites

To understand and implement the C program for BMI calculation, readers should have a basic understanding of:

• C Language Fundamentals: Variables, data types (especially float or double for decimals).
• Input/Output Operations: Using printf for displaying output and scanf for reading user input.
• Arithmetic Operators: Basic operations like multiplication (*) and division (/).
• Standard Library: Familiarity with #include for standard input/output functions.

Use Cases or Case Studies

A BMI calculator, even a simple console-based one, has several practical applications:

• Personal Health Monitoring: Individuals can regularly check their BMI as part of a healthy lifestyle routine.
• Fitness Tracking: Used by fitness enthusiasts and trainers to assess progress and tailor workout plans.
• Educational Tools: Students learning programming can use this as a fundamental project to practice C concepts.
• Preliminary Health Assessments: Healthcare professionals might use a quick calculator for initial screenings or patient education.
• Dietary Planning: Individuals on a diet can use BMI to gauge their weight management efforts.

Solution Approaches

For calculating BMI, the most direct and common approach involves taking height and weight as input, applying the standard BMI formula, and displaying the result.

A Direct Calculation Approach

This approach focuses on implementing the standard BMI formula directly within a C program.

• One-line summary: Read weight and height from the user, apply the BMI formula (weight in kg / height in meters squared), and print the calculated BMI.

// Body Mass Index (BMI) Calculator
#include <stdio.h>

int main() {
    float weight_kg; // Variable to store weight in kilograms
    float height_m;  // Variable to store height in meters
    float bmi;       // Variable to store the calculated BMI

    // Step 1: Prompt user for weight input
    printf("Welcome to the BMI Calculator!\\n");
    printf("Please enter your weight in kilograms (e.g., 70.5): ");
    
    // Step 2: Read weight from user
    if (scanf("%f", &weight_kg) != 1) {
        printf("Invalid input for weight. Please enter a numerical value.\\n");
        return 1; // Indicate an error
    }

    // Step 3: Prompt user for height input
    printf("Please enter your height in meters (e.g., 1.75): ");
    
    // Step 4: Read height from user
    if (scanf("%f", &height_m) != 1) {
        printf("Invalid input for height. Please enter a numerical value.\\n");
        return 1; // Indicate an error
    }

    // Step 5: Check for invalid inputs (zero or negative height/weight)
    if (weight_kg <= 0 || height_m <= 0) {
        printf("Weight and height must be positive values. Please try again.\\n");
        return 1; // Indicate an error
    }

    // Step 6: Calculate BMI using the formula: weight / (height * height)
    bmi = weight_kg / (height_m * height_m);

    // Step 7: Display the calculated BMI
    printf("\\nYour calculated BMI is: %.2f\\n", bmi); // .2f formats to two decimal places

    // Step 8: Provide BMI category interpretation for context
    printf("BMI Categories:\\n");
    printf("  Underweight: < 18.5\\n");
    printf("  Normal weight: 18.5 - 24.9\\n");
    printf("  Overweight: 25 - 29.9\\n");
    printf("  Obesity: >= 30\\n");

    return 0; // Indicate successful execution
}

Run

• Sample output:

  Welcome to the BMI Calculator!
  Please enter your weight in kilograms (e.g., 70.5): 68
  Please enter your height in meters (e.g., 1.75): 1.65
  
  Your calculated BMI is: 24.98
  BMI Categories:
    Underweight: < 18.5
    Normal weight: 18.5 - 24.9
    Overweight: 25 - 29.9
    Obesity: >= 30

• Stepwise explanation:

1. Include Header: #include is used for standard input and output functions like printf and scanf.
2. Declare Variables: float data types are chosen for weight_kg, height_m, and bmi to handle decimal values.
3. Get Input: The program prompts the user to enter their weight in kilograms and height in meters using printf.
4. Read Input: scanf("%f", &variable_name) reads the floating-point values entered by the user and stores them in the respective variables. Basic error handling is included to ensure valid numerical input and positive values.
5. Calculate BMI: The core calculation bmi = weight_kg / (height_m * height_m); applies the standard BMI formula.
6. Display Output: printf is used to display the calculated bmi to two decimal places using the %.2f format specifier.
7. Interpret BMI: For user convenience, standard BMI categories are printed to help interpret the calculated value.
8. Return 0: Indicates that the program executed successfully.

Conclusion

Creating a BMI calculator in C is a straightforward yet practical exercise that demonstrates fundamental programming concepts such as user input, arithmetic operations, and formatted output. This console application effectively mimics the core functionality of an online BMI calculator, providing immediate results based on user data.

Summary

• BMI helps assess body weight relative to height for health monitoring.
• The C program takes weight (kg) and height (m) as input.
• It uses the formula: BMI = weight / (height * height).
• Error handling is included for invalid or non-positive inputs.
• The output provides the calculated BMI and common category interpretations.
• This serves as a good example for learning basic C programming for practical applications.