by Adam Sandman on Tuesday, January 28, 2025

White Box vs. Black Box Testing: What You Need to Know

Software testing can be categorized in a variety of different ways, from functional vs. non-functional to manual vs. automated. Today, we’re breaking down the differences between white box and black box testing to understand when each should be used (and why).

What is White Box Testing?

Put simply, white box testing involves evaluating an application’s systems where the tester can understand and assess the internal workings of the software. This includes knowledge of the source code, which informs test case design and analysis of results. White box tests hone in on output validation and specific paths that the program takes to verify its performance and efficiency. It also encompasses techniques like security testing, because this often necessitates knowledge of the software’s structure and code to identify vulnerabilities.

Example

Consider the internal code for a website’s shopping cart feature. The tester examines the code for:

Logic that calculates the total price, including taxes and discounts

Proper handling of edge cases, such as adding zero-quantity items or invalid product IDs

Secure storage of sensitive data like product IDs and pricing to prevent vulnerabilities (e.g. SQL injection)

Based on this, they might write unit tests to confirm that:

A 10% discount on a $100 item reduces the price to $90

Adding an item with a negative quantity displays the correct error message

No sensitive data is saved or transferred inappropriately

White box testing verifies that the internal workings of the software are robust, reliable, and efficient. It’s especially useful during unit testing and integration testing phases to identify and fix bugs early in the development cycle.

What is Black Box Testing?

On the other hand, black box testing takes the opposite approach — analyzing an application’s functionality without knowledge of the internal workings and code. The person executing black box testing isn’t concerned with how efficient the code is, as long as the system acts as expected. This kind of testing focuses exclusively on how the software behaves from the outside. The goal is to validate functionality simply based on predetermined specifications and requirements, often using higher-level testing methods like functional testing, non-functional testing, and regression testing.

Example

The same shopping cart feature from above is tested without examining the code. The tester simulates real-world scenarios:

Adds items to the cart and verifies the displayed total is accurate

Applies discount codes to confirm that the discount is calculated correctly

Attempts invalid actions (like entering a negative quantity) to check for proper error handling

Here, we see that the tester observes only the outputs and verifies they match the expected behavior based on the specifications — without internal knowledge of the code providing those outputs.

Black box testing is excellent for validating the software from the end user's perspective. It’s used during functional testing, system testing, and acceptance testing to check that the application meets user requirements and performs well in real-world scenarios.