BMI Calculator - Problem

Calculate the Body Mass Index (BMI) from a person's height and weight, then determine which category they fall into.

BMI Formula: BMI = weight (kg) / (height (m))²

Categories:

Underweight: BMI < 18.5
Normal weight: BMI 18.5 - 24.9
Overweight: BMI 25.0 - 29.9
Obese: BMI ≥ 30.0

Return the BMI value rounded to 1 decimal place and the corresponding category as a string.

Input & Output

Example 1 — Normal Weight

$ Input: weight = 70, height = 1.75

› Output: 22.9 Normal weight

💡 Note: BMI = 70 / (1.75)² = 70 / 3.0625 = 22.857... rounds to 22.9. Since 18.5 ≤ 22.9 < 25.0, category is Normal weight.

Example 2 — Overweight

$ Input: weight = 85, height = 1.70

› Output: 29.4 Overweight

💡 Note: BMI = 85 / (1.70)² = 85 / 2.89 = 29.411... rounds to 29.4. Since 25.0 ≤ 29.4 < 30.0, category is Overweight.

Example 3 — Underweight

$ Input: weight = 45, height = 1.65

› Output: 16.5 Underweight

💡 Note: BMI = 45 / (1.65)² = 45 / 2.7225 = 16.528... rounds to 16.5. Since 16.5 < 18.5, category is Underweight.

Constraints

1 ≤ weight ≤ 1000
0.1 ≤ height ≤ 3.0
BMI will be rounded to 1 decimal place

Visualization

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Asked in

H Healthcare Systems 25 F Fitness Apps 30

The key insight is to use the BMI formula (weight/height²) and apply efficient threshold checking. Best approach uses an if-else chain that checks categories in logical order. Time: O(1), Space: O(1)

Common Approaches

✓ Direct Calculation with Multiple Comparisons

⏱️ Time: O(1) Space: O(1)

Compute BMI using the standard formula, then use multiple separate if statements to check each category individually, making redundant comparisons.

Optimized If-Else Chain

⏱️ Time: O(1) Space: O(1)

Compute BMI with the standard formula and use a logical if-else chain that checks categories in order, avoiding redundant comparisons.

Direct Calculation with Multiple Comparisons — Algorithm Steps

Calculate BMI = weight / (height * height)
Round BMI to 1 decimal place
Check each category with separate if conditions
Return BMI and category

Visualization

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Step-by-Step Walkthrough

Calculate BMI

weight / (height²) = BMI value

Round BMI

Round to 1 decimal place

Check Categories

Test each category with separate conditions

Return Result

BMI value and category string

Code -

solution.c — C

#include <stdio.h>
#include <math.h>
#include <string.h>

int main() {
    double weight, height;
    scanf("%lf", &weight);
    scanf("%lf", &height);
    
    // Calculate BMI
    double bmi = weight / (height * height);
    double bmiRounded = round(bmi * 10.0) / 10.0;
    
    // Check each category separately
    char category[20];
    if (bmiRounded < 18.5) {
        strcpy(category, "Underweight");
    }
    if (bmiRounded >= 18.5 && bmiRounded <= 24.9) {
        strcpy(category, "Normal weight");
    }
    if (bmiRounded >= 25.0 && bmiRounded <= 29.9) {
        strcpy(category, "Overweight");
    }
    if (bmiRounded >= 30.0) {
        strcpy(category, "Obese");
    }
    
    printf("%.1f %s\n", bmiRounded, category);
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(1)

Fixed number of arithmetic operations and comparisons regardless of input size

✓ Linear Growth

Space Complexity

O(1)

Only stores a few variables for BMI calculation and result

✓ Linear Space

33.2K Views

Medium Frequency

~5 min Avg. Time

850 Likes

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BMI Calculator - Problem

Input & Output

Constraints

Visualization

Related Problems

Common Approaches

Direct Calculation with Multiple Comparisons — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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