Table of Contents

Functional Vs Non-Functional Requirements

Table of Contents
Functional Vs Non-Functional Requirements

Can you agree that Functional and Non-Functional requirements set the foundation for a successful software development project? If done wrong, product requirements distort the final result. Most importantly, this stage is difficult to redo later. Well, how do you avoid miscommunication between the product owner and the dev team to set clear benchmarks for the products?

Functional and Non-Functional requirements are often really hard to grasp, but we’ll use our own experience to explain the differences. So, let’s get right into it.

What are Functional Requirements?

A functional requirement is a function of a software system with inputs, required for a system to function and outputs that it produces. Functional requirements usually describe the following aspects of the system’s functionality:

  • Calculations, performed by the system
  • Data processing and consumption
  • Use cases for the system: All possible scenarios where a system might be involved.
  • Used technology: Which languages APIs, database solutions and other tools are used to implement and maintain a system.

What are the main Types of Functional Requirements Documents?

  • To describe all the requirements, our team always creates a functional requirements specification document that collects all the main features of the application.
  • A use case document describes user software interactions in a generalized impartial way.
  • A user stories document goes through the same situations but from the users point of view after making user stories documents. Sometimes, we discovered that some use cases weren’t necessary at all.

Functional Decomposition

Here, complex functionality is broken down into structural parts.
For us, it helps to simplify the development process early on and estimate project duration and costs.

What are Non-Functional Requirements?

These are attributes that will help you define crucial user expectations and control software quality. The list of non-functional requirements is very specific to the type of the product and industry. But we can boil it down to four main:

  • Usability: evaluating the ability of a user to easily interact with a design product to achieve their own needs.
  • Legal or regulatory requirements: you should know which industry’s laws your product has to comply with and be familiar with fines.
  • Reliability: describe your products’ vulnerable points and solutions, analyze similar existing applications, examine safety risks and evaluate typical problems.
  • Performance: assess how a product reacts to users actions, the speed of reaction to clicks, mouse suffering and keypresses.

How to Determine and Measure Non-Functional requirements?

Non-functional requirements describe not what should be implemented but rather how. From our experience, you need to adopt the following practices.

  • Discuss non-functional criteria with all involved parties early on
  • Assess each requirement with a numeric score
    Re-evaluate non-functional requirements during development, testing, maintenance and scaling.
  • Make sure to integrate users feedback.

Actually, it will be easier to draw a line between functional and non-functional requirements by analyzing a real-life example.Let’s take Instagram as an example and hypothesize how a development team would write their documentation. You can use the same logic for your development project.

What Functional Requirements the development team can come up with before Developing this Service?

Functional Requirements

User Stories

Instagram users want to upload image and video content, write captions and publish the updates. Similarly, they would like to react to content posted by other accounts.

Use cases

When a user opens an Instagram mobile, web or desktop app, the first page is the feed. Here a user can interact with the content leaving likes and comments.

Additional functionality

Push notifications, automated location tagging, automated expiration of Instagram stories, photo filters, search bars.

Non-functional Requirements


Content loading speed is the priority. Instagram is a read based program because most users spend time viewing content rather than uploading their own media files.


Content loading speed is the priority. Instagram is a read based program because most users spend time viewing content rather than uploading their own media files.

Reliability and Scalability

The system should perform well in low latency conditions, show media content with the smallest delay possible.


Instagram has detailed protocols for authentication, photo upload, API integration, photo embedding and encryption of direct messages.

We can make the only conclusion, clear functional and non-functional requirements set the foundation for a successful software project. This is why we at EZtek always start by defining product requirements, not just functional and non-functional but the whole package.
EZtek is a software development company providing UI/UX design, engineering, testing and tech consulting services to top brands worldwide.
Need experts’ help, contact us!


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