Guide to Writing Functional Requirements

Functional requirements differ from non functional requirements. Both are necessary to product and software development. But if you’re unfamiliar with their differences, it may be easier to fully understand each separately. Use this guide to understand functional requirements and how to write them.

Functional Requirements Definition

Put simply, functional requirements define what a system is supposed to do. If functional requirements are not met, the system won’t meet the expectations of its users and stakeholders. It won’t work correctly or as intended.

You can also think of a functional requirement as a product feature that a user can detect. This might be an obvious feature, for example, a large Add to Cart button. But it can also be a less obvious feature like correctly calculating the sales tax for the user’s online purchase.

Also learn about non-functional requirements. Read the blog >>

How to Write Functional Requirements

To write functional requirements, include a unique identifier, a concise summary of the functionality required, and the reason why it is required.

How you write your functional requirements will depend on your product development methodology.

Agile software teams generally call their functional requirements user stories and might write them on Post-Its or cards in an online system.

Teams developing products for a regulated industry might still be using Agile best practices, but because of the size and complexity of their products, will use a more structured approach to documenting requirements. Requirements are usually outlined as written descriptions in a document — like an SRS or PRD.

No matter the methodology you use, when writing a functional requirement, you want to include:

• Identifier — unique name and number
• Summary
• Reasoning

The identifier is used to help track the requirement through the system, and the other information helps clarify why the requirement is needed and what functionality must be provided.

Other Guidelines for Writing Functional Requirements

There are differences between well-written and poorly-written requirements. You want to encompass all the relevant information as thoroughly, clearly, and concisely as possible. Here are some general best practices to writing useful requirements:

• Use an active voice.
• Avoid vagueness — make them as complete and accurate as possible.
• At the same time, avoid extraneous information that can be confusing.
• Be consistent in terminology and format.
• Use “must” instead of “should.” Separate meta-data fields are a better way to indicate priority and whether the requirement is in or out of scope.
• Quantify requirements — if a stakeholder wants a website to load “quickly,” ask what that means (3 seconds or less? 2 seconds or less?)
• If you intend to reuse the requirement, write it as such — use “accept payment” rather than “accept payment through iTunes,” for example.
• Include requirements that detail what the system should not do to cover every scenario.
• Focus on features the users truly need.
• Each requirement needs to be testable.
• Each requirement needs to track back to one of the objectives.
• Document assumptions.
• Use visuals to reinforce information when possible.
• As best you can, make them understandable to non-technical stakeholders.

Even More on Writing Requirements

Want a deeper dive into writing good requirements? Download the white paper “9 Tips for Writing Useful Requirements.” It includes visual examples and proven techniques to give you the strongest requirements possible.

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