What Is the Difference Between Functional and Non-Functional Testing?

Functional testing checks what a software application does, while non-functional testing checks how well it performs.

Overview

Software needs to do the right things and perform those functions reliably. QA teams therefore use both functional and non-functional testing to evaluate application quality.

What Is Functional Testing?

Functional testing verifies whether an application works according to its defined requirements.

For example, for a login page, functional testing may check:

• Can users enter their credentials?
• Does valid login information work?
• Is an error shown for incorrect credentials?
• Does password reset work?
• Are users redirected to the correct page?

Common types include:

• Unit testing
• Integration testing
• System testing
• Regression testing
• User acceptance testing

What Is Non-Functional Testing?

Non-functional testing evaluates qualities such as performance, security, usability, reliability, and scalability.

For example, an application may function correctly for 100 users but become extremely slow when 10,000 users access it simultaneously.

Non-functional testing can include:

• Performance testing
• Load testing
• Stress testing
• Security testing
• Usability testing
• Compatibility testing
• Scalability testing
• Reliability testing

Functional vs Non-Functional Testing

Functional Testing	Non-Functional Testing
Checks what the system does	Checks how well it works
Focuses on business functionality	Focuses on quality attributes
Tests features and workflows	Tests performance, security, usability, etc.
Example: Login works	Example: Login responds quickly under heavy load

Why Do Businesses Need Both?

A software application can be functionally correct but still provide a poor user experience.

For example, an online banking application may successfully process transactions but take 30 seconds to load each page. Functional testing may pass, but performance testing would identify the problem.

Final Thoughts

Functional testing ensures that the software works correctly, while non-functional testing ensures that the software works effectively under expected conditions. A strong QA strategy should include both.