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Calculate BMI in Java

beginner· Basics & I/O · Calculation

Problem

Body mass index (BMI) is weight in kilograms divided by the square of height in metres; the result is grouped into ranges such as normal weight (18.5 to just under 25).

Given a weight in kilograms and a height in metres, calculate the BMI, round it to one decimal place and print its category.

Input

68 kg, 1.75 m

Output

22.2, Normal weight

Java Program

Java

public class CalculateBmi {
    public static void main(String[] args) {
        double weightKg = 68;
        double heightM = 1.75;

        double bmi = weightKg / (heightM * heightM); // weight divided by height squared

        String category;
        if (bmi < 18.5) {
            category = "Underweight";
        } else if (bmi < 25) {
            category = "Normal weight";
        } else if (bmi < 30) {
            category = "Overweight";
        } else {
            category = "Obese";
        }

        System.out.printf("BMI: %.1f%n", bmi);
        System.out.println("Category: " + category);
    }
}

▶ Run Code

Output

BMI: 22.2
Category: Normal weight

Core Logic

The formula gives a single number, and an else-if ladder checks it against the category boundaries from lowest to highest.

How It Works

1weightKg / (heightM * heightM) applies the BMI formula; the parentheses make sure height is squared first.
2The ladder tests bmi < 18.5, then < 25, then < 30, falling through to Obese.
3Because each test runs only if the earlier ones failed, every branch only needs its upper limit.
4printf("%.1f%n") prints the BMI rounded to one decimal place.

1.75 × 1.75 = 3.0625, and 68 / 3.0625 ≈ 22.2, which falls between 18.5 and 25.

💡

Key Point: Without the parentheses, weightKg / heightM * heightM divides and then multiplies back, giving the original weight instead of a BMI.

Key Concepts

double arithmeticelse-if ladderprintf

Approach 2: Imperial Units (Pounds and Inches)

Java

public class CalculateBmiImperial {
    public static void main(String[] args) {
        double weightLb = 160;
        double heightIn = 68;

        double bmi = 703 * weightLb / (heightIn * heightIn); // 703 converts lb/in² to kg/m²

        System.out.printf("BMI: %.1f%n", bmi);
        System.out.println("Category: " + (bmi < 18.5 ? "Underweight"
                : bmi < 25 ? "Normal weight"
                : bmi < 30 ? "Overweight" : "Obese"));
    }
}

▶ Run Code

Output

BMI: 24.3
Category: Normal weight

Core Logic

With pounds and inches the formula keeps its shape; a constant factor of 703 converts the result into the same scale as the metric formula.

How It Works

1703 * weightLb / (heightIn * heightIn) computes BMI from imperial units.
2The category is chosen with a chain of ternary operators instead of an if-else ladder.
3The same boundaries (18.5, 25, 30) apply, because the factor already matches the metric scale.

160 lb at 68 in: 703 × 160 / 4624 ≈ 24.3, still in the normal range.

💡

Key Point: Nested ternaries are compact but hard to read beyond three or four branches — the if-else ladder in the main program is easier to maintain.