Java Program to Calculate the Area of a Triangle

Last updated: October 17, 2026Oct 17, 2026

A triangle is a shape with three sides and three corners. Its area can be calculated in two common ways:

from the base and height: area = (base × height) / 2,
from the three sides a, b and c, using Heron's formula: area = √(s(s - a)(s - b)(s - c)), where s = (a + b + c) / 2 is half the perimeter.

1. Area from Base and Height

The height is the perpendicular distance from the base to the opposite corner.

TriangleArea.java

public class TriangleArea {
    public static void main(String[] args) {
        double base = 21.75;
        double height = 10.5;

        double area = (base * height) / 2;

        System.out.println("Base: " + base);
        System.out.println("Height: " + height);
        System.out.println("Area of the triangle: " + area);
    }
}

Output

Base: 21.75
Height: 10.5
Area of the triangle: 114.1875

2. Area from Base and Height with User Input

TriangleAreaInput.java

import java.util.Scanner;

public class TriangleAreaInput {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter the base: ");
        double base = scanner.nextDouble();
        System.out.print("Enter the height: ");
        double height = scanner.nextDouble();
        scanner.close();

        if (base <= 0 || height <= 0) {
            System.out.println("Base and height must be positive.");
            return;
        }

        System.out.printf("Area of the triangle: %.2f%n", 0.5 * base * height);
    }
}

Output (with sample input)

Enter the base: 30.5
Enter the height: 20.75
Area of the triangle: 316.44

3. Area from Three Sides Using Heron's Formula

Three lengths only form a triangle if each side is shorter than the sum of the other two (the triangle inequality). The program checks this first, then applies Heron's formula with Math.sqrt().

TriangleAreaHeron.java

public class TriangleAreaHeron {

    static double area(double a, double b, double c) {
        if (a + b <= c || a + c <= b || b + c <= a) {
            return -1;   // not a valid triangle
        }
        double s = (a + b + c) / 2;
        return Math.sqrt(s * (s - a) * (s - b) * (s - c));
    }

    public static void main(String[] args) {
        double[][] triangles = {{3, 4, 5}, {7, 8, 9}, {1, 2, 10}};

        for (double[] t : triangles) {
            double area = area(t[0], t[1], t[2]);
            if (area < 0) {
                System.out.println("Sides " + t[0] + ", " + t[1] + ", " + t[2] + " do not form a triangle.");
            } else {
                System.out.printf("Sides %.1f, %.1f, %.1f -> area %.2f%n", t[0], t[1], t[2], area);
            }
        }
    }
}

Output

Sides 3.0, 4.0, 5.0 -> area 6.00
Sides 7.0, 8.0, 9.0 -> area 26.83
Sides 1.0, 2.0, 10.0 do not form a triangle.

4. Summary

With base and height, the area is base * height / 2.
With three sides, use Heron's formula: Math.sqrt(s * (s - a) * (s - b) * (s - c)) with s as half the perimeter.
Check the triangle inequality before using Heron's formula.
Use double for lengths and printf("%.2f") to format the result.

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