How to Write Functional Specifications Documents

If you’re looking for ways to write your first Functional Specifications Documents, then the following tutorial will help. One way to develop Functional Specifications is to use free Microsoft Word templates from the web and modify different chapters or buy professional template sets with pre-formatted samples.

What are Functional Specifications Documents?

Functional Specifications Documents (also called functional specs, functional specifications document (FSD), or Program specification) are used software development to describe how the system works from a functional perspective.

The Functional Specifications focus on two areas:

  • Functions – what the system actually does
  • Specifications – what you want each function to do, i.e. as defined to a granular level.

These technical documents describe what the user needs and what inputs and outputs are expected from the system.

Systems Engineering Models

During system development, business analysts create specifications to describe the system at different levels of detail.

These unique specifications form the core of the configuration baseline. As well as referring to different levels within the system hierarchy, these baselines are defined at different phases of the design process.

Understanding Functional Specifications

A Functional Specification (FS) describes the technical requirements for each item, material, or services, including the procedures for ensuring that the requirements have actually been met.

The function of Functional Specifications is to:

  • Help avoid duplication and inconsistencies,
  • Allow for accurate estimates of necessary work and resources
  • Act as a reference document for change control
  • Provide technical documentation for configuration management
  • Allow for consistent communication across the software development lifecycle
  • Enable developers design the system and estimate the cost of design alternatives
  • Provide guidance to testers for verification of each requirement

What Functional Specifications Do Not Do

Functional Specifications do not define how the proposed system work or how the system functions will be implemented.

Instead, it focuses on how users might observe when interacting with the system.

Sample Functional Specifications

An example of a typical functional specification:

When the user clicks OK, the focus is returned to the main window in the state it was in before this dialog was displayed.

This requirement describes the interaction between the user and the system.

When the user provides input to the system, e.g clicks OK, the program closes the dialog window containing the OK button.

Functional Specification Purpose

Functional Specifications have different purposes, such as:

  • Achieve consensus in the development team on how the software should work before writing source code and test cases.
  • Consensus is reached after peer reviews by the stakeholders.

Process of Creating Functional Specifications

In the software engineering life-cycle (e.g. waterfall model), the Functional Specifications describes what needs to be implemented. The system specification document describes how the functions will be realized using a specific software environment.

In other methodologies, e.g. prototypical systems, Functional Specifications are written after or as part of Requirements Analysis phase.

When the team finishes the functional specification review consensus, the Understanding Functional Specifications is typically "complete" or "signed off".

After this, the software development team writes source code using the Functional Specification as their reference point. While testing the system, the functionality is compared against the expected behavior as defined in the Functional Specification.

Download Sample Functional Specifications

Click here to download sample Functional Specifications in Microsoft Word as part of the Software Development LifeCycle Template Kit.

PS – you can also download a single Functional Specifications template from Klariti