Differences Between Functional and Non-Functional Testing

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Relia Software

Software Development

Functional Testing focuses on what the software does, while Non-Functional Testing evaluates how the software performs. Together, they ensure high-quality software.

difference between functional and non functional testing

Table of Contents

Have you ever wondered how software gets thoroughly checked before it reaches your hands? There are actually two main types of testing that software goes through: functional testing and non-functional testing. Both play vital roles in ensuring a smooth and reliable software experience, but they focus on different aspects. 

This article will cut through the confusion and explain the key differences between functional and non-functional testing in a clear and easy-to-understand way. So, by the end, you'll be able to tell the difference between testing how a feature works and testing how well it performs!

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Key Differences Between Functional and Non-Functional Testing.

Key Differences Between Functional and Non-Functional Testing

Suppose that you're building a brand new car. Functional testing would be like checking if the engine starts, the steering wheel turns the wheels, and the brakes bring the car to a stop. It's all about ensuring the car performs its basic functions as intended.

Non-functional testing, on the other hand, would be like taking the car for a spin and evaluating how well it performs those functions. Does the engine roar to life smoothly, or does it sputter and cough? Does the steering respond quickly and precisely, or is there a lag between your turn and the car's movement? These aspects delve into the car's overall experience, not just its basic functionalities.

Focus vs. Functionality

Here's where the core difference lies:

  • Functional Testing: Focuses on what the software does. It verifies if features work according to their specifications and requirements. Think of it as checking off a list of functionalities, ensuring each one performs as designed.
  • Non-Functional Testing: Evaluates how the software performs. It goes beyond the basic "does it work?" question and delves into characteristics like speed, stability, security, usability, and scalability. Here, the tester is like a seasoned driver evaluating the car's handling, comfort, and overall performance.


Functional and non-functional testing also use different approaches:

  • Functional Testing: Often employs testing techniques like black-box testing, where the tester interacts with the software without knowing its internal workings, or white-box testing, where the tester has access to the software's code. Regression testing ensures that new features don't break existing functionalities.
  • Non-Functional Testing: This might involve performance testing to measure website loading times under various loads, usability testing to observe how real users interact with the interface, and security testing to identify and plug vulnerabilities.

Benefits and Goals

Ultimately, both functional and non-functional testing contribute to a successful software product. 

  • Functional testing ensures the software functions as intended, delivering the promised features and functionalities. 
  • Non-functional testing helps create a positive user experience by making sure the software performs well, is secure, and is easy to use. 

By combining these two testing types, developers and testers can create high-quality software that not only works but also works wonderfully.

Here is a table summarizing the key points of comparison for easier reference:


Functional Testing

Non-Functional Testing


What the software does.

How the software performs.


Verifies features work according to specifications.

Evaluates software characteristics beyond core functionalities.


  • User login and registration

  • Processing payments

  • Adding items to cart

  • Website loading speed

  • User interface intuitiveness

  • Security of user data


  • Black-box testing

  • White-box testing

  • Regression testing

  • etc.

  • Performance testing

  • Usability testing

  • Security testing

  • etc.


Ensures software functions as intended.

Creates a positive user experience.


Delivers promised features and functionalities.

Improves performance, usability, and security.

Practical Example: Comparing Functional vs. Non-Functional Testing in a Mobile App

Let's analyze the development of a mobile food ordering app to illustrate the distinct roles of functional and non-functional testing.

Functional Testing

  • Criteria: Verifying core functionalities as per specifications.
  • Example:
    • Users can successfully register and login.
    • Browse through restaurant menus and add desired items to the cart.
    • Apply discounts and coupons accurately.
    • Securely process payment information and confirm orders.

Non-Functional Testing

  • Criteria: Evaluating software characteristics beyond core functions.
  • Example:


  • Does the app load quickly on various devices (phones, tablets)?
  • Can it handle multiple users ordering simultaneously without lagging?


  • Is the app interface intuitive and easy to navigate for users of all ages and technical backgrounds?
  • Are menus and ordering steps clear and user-friendly?


  • Is user data (account information, payment details) encrypted and protected from breaches?
  •  Does the app have secure authentication methods to prevent unauthorized access?

Comparison Analysis

This example highlights the clear distinction between functional and non-functional testing. Functional testing ensures the core functionalities like user registration, order placement, and payment processing work as intended.

On the other hand, non-functional testing digs deeper. It evaluates the app's performance under real-world conditions, considering factors like user load and device variations. It assesses the user interface's usability, ensuring a smooth and intuitive ordering experience for everyone. Finally, security testing safeguards sensitive user data and ensures the overall safety of the app.

A Practical Example for Comparing Functional vs. Non-Functional Testing in a Mobile App
A Practical Example for Comparing Functional vs. Non-Functional Testing in a Mobile App.

When to Use Each Type: Functional vs. Non-Functional Testing

Here's a breakdown of when to use each type of testing:

  • Functional Testing First: Functional testing is usually done early in development to make sure the core features work as planned.  Think of it as catching any big mistakes before you invest more time and effort.
  • Non-Functional Testing Throughout: Non-functional testing can happen at different stages. You can check the website's loading speed as features are added, or test the app's usability with real people to see if it's easy to use.
  • Working Together: Functional and non-functional testing work best together. By combining them, you can create software that works properly and is also enjoyable to use!

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In the world of software development, both functional and non-functional testing play vital roles. Functional testing ensures the software fulfills its intended purpose by verifying core features work correctly. Non-functional testing delves deeper, evaluating how well the software performs in terms of speed, usability, security, and other user-centric aspects.

By working together, functional and non-functional testing create robust, high-quality software that not only functions as intended but also provides a positive user experience. This strong alliance ensures the software is built to last and delivers a truly satisfying experience for its users.

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