Functional VS Non-Functional Testing

Functional VS Non-Functional Testing: Key Differences

Functional and non-functional testing are the two most commonly used terms in a software tester’s dictionary. Despite the fact that there are over 50 different types of tests, you can categorize all of them into these two categories.

Functional testing evaluates the software’s features to ensure they are bug-free. Non-functional testing, on the other hand, validates non-functional characteristics such as performance, reliability, usability, etc.

Nowadays, software firms strive not just for zero defects in the software but also for excellent performance. To achieve that, you must run both functional and non-functional tests on the software.

To run effective functional and non-functional testing on your software, you must first learn the difference between functional testing vs non-functional testing.

Don’t worry. We have discussed all that you need to know about these test types along with how to differentiate between them. Without further ado, let’s first understand each of them in detail.

What Is Functional Testing?

Functional testing is a method of software testing where the software is validated against the business or functional requirements. This test verifies that each feature of the software works according to the client’s specifications or requirements.

Functional testing is mainly a type of black-box testing. Here, major functionalities are tested using sets of predefined inputs to examine the software behavior. Then, this behavior is compared to the expected results to make sure everything works as planned.

Apart from major functionalities, functional testing also focuses on testing the basic usability and end-user perspective of the software. For example, it tests whether the user can navigate the software easily or not.

You can also use functional testing for error validation and ensuring proper exception handling.

Additionally, this test checks UIs, APIs, security, and database of the system under test. You can perform functional testing manually or take help from automation tools.

Why Functional Testing

Well, the main reason for performing functional tests is that without it you won’t be able to verify if your software functions as required. 

Your software can excel in performance, usability, and other non-functional aspects. However, if the features and functions do not match user expectations, it would be considered a failure. 

Since functional testing verifies every feature of the software against functional requirements, you can ensure the software performs as it should by this test method. 

Additionally, thorough testing allows you to detect all major and minor bugs to confirm the final product is bug-free.

Moreover, functional testing evaluates the product in a real-world setting from an end-user perspective. As a result, ensuring that the software meets user needs becomes easier with this test.

Functional testing also involves user acceptance testing where clients test the software before release to make sure it’s everything they expected. This reduces the risk of releasing a product that would dissatisfy the customers.

What’s more, you can find out if all the individual modules of the software work properly together or not with functional testing. This is important, otherwise, even if the modules work fine on their own, the software as a whole would fail to function properly.

Types Of Functional Testing

Here are the main types of functional testing used by testers:

Unit Testing

In this type of functional testing, individual units or components of the software are tested. Unit testing is performed to verify that each unit of the application functions as required.

It starts at the beginning of the development phase and continues as each software module is developed. Developers perform unit testing after building each software block to make sure it’s functioning properly.

Integration Testing

In this type of functional testing, individual software units are merged and tested as a single unit. 

The main purpose of performing integration testing is to ensure that separate software modules can communicate and function properly when combined. 

Additionally, most software bugs are found in integration points. Hence, integration testing also exposes interaction and integration faults in the system. 

Smoke Testing

Smoke testing is done on the initial program build to verify if the main functionalities work properly or not. It’s also known as build verification testing. 

If the basic features don’t work properly, there is no point in performing more extensive testing as they’ll obviously fail. It is a generalized way of testing the software.

Smoke testing saves time by checking the working condition of critical functionalities first before installing a new build and performing high-level tests.

Sanity Testing

Unlike smoke testing, sanity testing is done on a stable or mature software build. 

After the software undergoes changes or bug fixes, sanity testing checks to verify that the change didn’t have any negative effects on other parts of the software. 

It also makes sure that the previous bugs have been fixed and that no other bugs are present in the current build. If the sanity test fails, the current build gets rejected to reduce time and resource wastage on more extensive testing.

System Testing

In this software testing method, a complete and fully integrated software product is tested. The goal of this test is to determine whether the system meets the functional and business requirements. 

It is a series of tests designed solely to examine the system as a whole and make sure it is completely bug-free.

Regression Testing

Regression testing is an ongoing process as it is performed after every major change in the software. In this testing technique, the software is tested to verify that new feature addition or defect resolves do not affect the existing features. 

Regression testing is just the re-execution of previously performed test cases to confirm that existing functionality continues to function properly.

It assures that existing features haven’t been broken as a result of new changes.

User Acceptance Testing

User acceptability testing is the final stage of testing before the program is made available to the public. Here, the software is mainly tested for customer acceptability.

The main goal of this test is to determine software compliance with its business requirements and check whether it is ready for delivery. UAT is performed by end-users during beta testing.

Advantages And Disadvantages Of Functional Testing


  • Functional testing ensures the software is defect-free.
  • Verifies that the product meets all the functional requirements.
  • Confirms that all functions work as expected.
  • Prioritizes end-user satisfaction over anything else.
  • This testing confirms software safety and security.
  • It enhances software quality.
  • Reduces risks of releasing faulty products.
  • Improves company image by releasing satisfactory products.


  • Functional testing can miss critical logical errors.
  • Doesn’t confirm that the software is ready for real-world scenarios.
  • High possibility of performing redundant testing.
  • Requires an in-depth understanding of user requirements that may not be readily available.

Functional Testing Strategies

There are various useful testing strategies available to help you design your test suite. Here’s a brief description of the most popular ones:

Boundary Value Tests

Boundary value tests impose data limits on the software and check how it behaves for valid as well as invalid data. 

Generally, most of the bugs occur around the boundaries. Hence, testing inside, outside, and on the boundary can detect most of the bugs. 

Using this strategy also helps in reducing the number of test cases that need to be run. As a result, testing time and resources get saved while ensuring maximum test coverage.

Equivalence Partition Tests

In this testing strategy, test data are divided into equivalence data groups using various partitions. 

We assume that all the in each partitioned group will behave similarly. Hence, only one test case is tested from each partition.

Moreover, if one condition in a partition doesn’t work, then we assume all the conditions in that partition will also fail. This also helps in reducing test cases.

Decision-Based Tests

The decision-based testing strategy verifies the possible outcomes of the software when a specific condition is satisfied. Here, we test if certain user actions work as they should.

End-User Based Tests

This testing strategy evaluates how individual parts of the application work together. For example, it would test if the calculator stores history successfully in the cloud after each calculation.

Ad-hoc Tests

This strategy is done after all other tests to find bugs that other tests failed to detect. In this test, testers attack the software from an outsider’s point of view and try to break the application. This test reveals any security risks and confirms if the software handles exceptions properly.

Examples Of Functional Testing

Let’s look at some real-life examples of functional testing. So, for example, a banking software has many different components or units that need to be tested individually to see if they function properly.

For instance, the user has to log in using his username and password. Testing if the user can log in successfully using a valid username and password is a functional test. It’s known as unit testing. 

Then, after successful login, the user should be redirected to his home page where he can avail of other banking functionalities. But this home page and the login page are two different modules. Testing if these two modules can communicate properly is known as integration testing.

If a new feature such as transferring money to another account is added to this bank software, we can perform sanity testing or regression testing. This functional testing will see if the new feature has any negative effects on the existing software.

Lastly, the completed software is tested by an end user to see if it satisfies all the functional requirements in user acceptance testing.

What Is Non-Functional Testing?

Non-functional testing is basically testing the software against non-functional requirements. These tests are carried out in accordance with the client’s specifications.

The main goal of non-functional testing is to verify the non-functional parameters of the software. These parameters include reliability, usability, performance, load, and more.

This testing checks how the software performs in real-world scenarios. Unlike functional testing, it emphasizes more on product behavior and less on functionality. 

Non-functional testing verifies how well the software works. Additionally, it assesses software characteristics that improve the user experience. It is done after all the functional testing has been completed.

Why Non-Functional Testing

Even though it doesn’t focus on software functionality, non-functional testing is extremely important. Because while stability and performance under load are not functional components of a software system, they can still make or break the user experience. 

Non-Functional testing examines the software’s ability to perform in an external or business environment. If the software fails non-functional tests, it means there are problems in the system that might lead to a system crash in the future. 

Moreover, this type of testing plays a huge role in ensuring customer satisfaction. You can create bug-free software but if it isn’t user-friendly, the customer would definitely look for better alternatives.

If you overlook non-functional due to budget constraints or time shortage, persisting UI, UX, and performance defects can hamper customer experience and tarnish brand image.

That’s not all, testers can find out details about product behavior during non-functional testing. This knowledge will help to improve future software as well as reduce financial losses associated with software crashes.

So, to ensure your software is efficient, reliable, secure, and user-friendly, we must perform non-functional testing.

Types Of Non – Functional Testing

Non-functional testing is more of an umbrella term. Many different kinds of tests fall under this category. Some of the most popular tests have been mentioned below:

Performance Testing

When discussing non-functional testing, the first test that comes to mind is performance testing. This test determines whether or not the system responds promptly to a user request.

It is critical for determining the cause of slow software performance as poor latency ruins a user’s experience. Well-written performance tests can detect issues even before they become apparent to consumers.

Load Testing

Each system is designed to sustain a fixed amount of users or loads. Load testing checks if the software can handle the specified workload efficiently in a real working environment.

If a system fails load testing, it may not be able to function properly when deployed to a user environment with a heavy load. 

Reliability Testing

This test verifies the software’s reliability under specified conditions for a predefined amount of time and processes. Here reliability means the software performs as it should without fail.

If the system fails to perform under specified conditions, the reliability test would fail. This means the software is not ready to be released and needs further work to ensure proper functionality.

Usability Testing

This test examines how easy it is to use the software. Usability testing is performed by real-world users. 

In this test, mainly the user interface is tested for ensuring ease of use. If the software interface can not be smoothly navigated, no matter how good the software functionality is, the user won’t use it again.

Security Testing

Security testing is done to detect security flaws and system vulnerabilities that could result in critical information leaks.

The testing is carried out by examining system architecture from the perspective of an attacker. Test cases are developed to test the parts of the system where an attack is most likely to occur.

Scalability Testing

In scalability testing, the software’s capacity to scale up or scale down the number of user requests or other performance measure attributes is tested.

This is important for software projects that have plans to scale up in the future. 

Portability Testing

Portability testing is done to see if the system can work smoothly in diverse environments and operating systems. This test also examines the operation of the software in different hardware.

Apart from these, you can perform accountability testing, recovery testing, documentation testing, volume testing, endurance testing, and many more types of non-functional testing.

Advantages And Disadvantages Of Non – Functional Testing


  • Non-functional testing improves system performance.
  • Detects security issues early and protects the system from cyber-attacks.
  • Ensures that the system performs efficiently under a high workload.
  • Requires less time and effort compared to other test types.
  • Increases software ease of use, efficiency, and portability.
  • Reduces cost and production risk associated with non-functional aspects.
  • Provides detailed knowledge about software behavior in real environments.


  • After every software update, non-functional tests have to be performed again.
  • Repeating non-functional testing can become too expensive to maintain.

Non – Functional Testing Parameters

There are so many kinds of non-functional tests available that it becomes difficult to choose which to perform. However, there are some parameters that are mainly tested in non-functional testing. 

You can first select which of the parameters are necessary for your project. Then, choose non-functional test types based on these parameters.

The non-functional testing parameters are:

  • Security
  • Reliability
  • Survivability
  • Availability
  • Usability
  • Scalability
  • Interoperability
  • Efficiency
  • Flexibility
  • Portability
  • Reusability

Examples Of Non – Functional Testing

Let’s look at some real-life examples of non-functional testing. Let’s think of the banking software again. We have to specify a certain number of users that the software can handle at the same time as well as the maximum load time for non-functional testing.

Now, we test the software to see if it can load within 5 seconds while 1000 users are using the software at the same time. This is performance and load testing. 

For usability testing, we check if the user interfaces are working properly. Testing if the buttons are visible, the software workflow is clear to the user, and every element works smoothly is usability testing.

Then, we can test the software from a hacker’s perspective to see if it can be broken. This security testing will expose any security vulnerabilities of the software.

Parameters To Compare Functional And Non-functional Testing

Both functional and non-functional testing is extremely crucial for ensuring the performance and quality of software. Each of these test approaches targets different aspects of the software. 

To bring out the main differences between functional testing vs non-functional testing, we can compare them based on some parameters. The parameters we’ll be diving deep into are:

  • Objective
  • Focus
  • Execution
  • Usage
  • Functionality

Key Differences Between Functional Testing And Non-Functional Testing 

Let’s take a closer look at the parameters mentioned above to understand how functional and non-functional testing techniques differ based on them.


Functional testing verifies if the software is performing all the defined functions. It tests and validates software actions like transferring money, placing an order, creating an account, etc.

Non-functional testing, on the other hand, verifies if the software is performing all the functions in the right way. It tests and validates the software performance or behavior.


In terms of focus, functional testing follows and concentrates on validating the functional requirements given by clients & stakeholders. But non-functional testing focuses on user expectations about non-functional aspects such as speed, usability, and security. 


Functional testing verifies all the units and modules of the software. If the software doesn’t function properly, where would you run the non-functional tests? Therefore, functional testing is carried out before non-functional testing. Non-functional testing is done on the completed software as more of a refinement. Hence, it is executed after all functional testing is finished.


Functional requirements are clearly defined at the beginning of the project. Hence, planning and executing functional tests is comparatively easier than running non-functional tests.  

Moreover, the client may not be knowledgeable about the non-functional aspects that need to be tested. Hence, it can be difficult to prepare test specifications for non-functional requirements.

Also, while functional tests can be run manually with relative ease, non-functional tests require automation help to be performed properly.


Functional testing describes what the system should perform whereas non-functional testing describes how the system should perform. 

Functional testing ensures that all the defined features are working without error while non-functional testing makes sure the system is performing at high speed, the interface is user-friendly, user information is secured, etc.

Comparison Chart

Comparison Chart

ParametersFunctional TestingNon-functional Testing
ObjectiveFunctional testing verifies software functionalities and operations.Non-functional testing verifies software behavior.
FocusFocuses on client and business requirements.Focuses on end-user or customer expectations.
ExecutionIt’s performed before non-functional testing.It’s performed after all functional testing is completed.
RequirementCarried out following functional requirements.Carried out following performance requirements.
Use Validates the functionality of the application.Validates the behavior & performance of the application.
FunctionalityDescribes what the software does.Describes how the software works.
Manual TestingFunctional testing can be manually executed easily.Executing non-functional testing manually is highly difficult.
ExampleCheck the sign up functionality.The system should load under 5 seconds with 1000 users.

Frequently Asked Questions

Why Is Non-functional Testing Equally Essential As Functional Testing?

Non-functional testing is also a quality measure. It evaluates and validates the reliability, speed, scalability, and efficiency of software. 

As it helps to ensure that the software is reliable and capable of withstanding real-world situations, it is considered equally important as functional testing.

What Are The Main Types of Functional Testing?

Functional testing can be divided into two main types: Positive testing and Negative Testing. 

Which Tools Are Used For Non-functional Testing?

Some of the tools used for non-functional testing are JMeter, Loadster, Loadrunner, Loadstorm, Neoload, Forecast, and Load Complete.

Wrapping Up

While functional testing ensures that all the software functionality works properly, non-functional testing checks how well these functionalities perform in real-world scenarios. 

Both these testing techniques are imperative for making sure the end software product is bug-free as well as efficient. 

However, there are many types of functional and non-functional tests available to choose from. For choosing the right testing technique, you must know the differences between functional testing vs non-functional testing.

In this article, we have discussed everything you need to know about functional and non-functional testing along with a comparison chart between the two. 

With the help of this knowledge, you’ll be able to make an informed decision between these while choosing test types for your next project. 

Rahnuma Tasnim

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