How to measure Code Coverage effectively?

Basics of Code Coverage for your Unit Tests

A high coverage percentage does not guarantee freedom from bugs. It only shows which parts of your code execute during testing.

Below are three different types of coverage what you can use for your projects:

1. Statement Coverage
• Tracks each statement that runs.
• Formula:
Copy

Statement Coverage = (Number of executed statements / Total statements) * 100

2. Branch Coverage
• Checks each decision point (if/else) in the code.
• Formula:
Copy

Branch Coverage = (Number of executed branches / Total branches) * 100

3. Path Coverage
• Focuses on each unique path through the code.
• Useful in complex applications.

You can use these metrics with popular tools like JaCoCo (Java), Coverage.py (Python), and Istanbul (JavaScript).

Example in Python

def add_numbers(a, b):
    if a > 0 and b > 0:
        return a + b
    return 0

# Test cases
def test_add_positive():
    result = add_numbers(2, 3)
    assert result == 5

def test_add_non_positive():
    result = add_numbers(-1, 3)
    assert result == 0

• Two tests run different branches in this function.
• This covers statements and branches but does not check every path in larger code blocks.

Example in Java

public class Calculator {
    public int multiply(int x, int y) {
        if (x == 0 || y == 0) {
            return 0;
        }
        return x * y;
    }
}

// JUnit test
import static org.junit.Assert.*;
import org.junit.Test;

public class CalculatorTest {
    @Test
    public void testMultiplyNonZero() {
        Calculator calc = new Calculator();
        assertEquals(6, calc.multiply(2, 3));
    }

    @Test
    public void testMultiplyZero() {
        Calculator calc = new Calculator();
        assertEquals(0, calc.multiply(0, 5));
    }
}

• Each test covers a different branch.
• You can expand tests to include more input variations.

Below is a summary table of coverage types:

Focus on a reasonable coverage target. Some teams aim for 70–80%. Pick a standard that fits your application’s risk level and complexity. Remember that code coverage does not confirm absence of defects. It only shows test execution depth.

Use these metrics to improve your test practices and find gaps. Combine coverage data with reviews and other testing methods for a broader perspective.

Happy Testing,

Csaba

P.s.: oh btw. if you want to learn more about this, join our community of testers here