How to write a product requirements document that works

In short: A good product requirements document (PRD) describes which problem a product solves for whom, which flows it has to support and how you know it is done. It is short enough that everyone involved actually reads it, and precise enough that developers can estimate and build from it. The key parts are goal, target users, core flows, clear scope limits and acceptance criteria. Technical solutions usually do not belong in it.

What a requirements document is for

A PRD is the shared foundation for everyone on a software project: management, business teams, design and development. It answers "What are we building and why?" before anyone asks "How do we build it?".

In German projects you will also come across the Lastenheft and Pflichtenheft. The Lastenheft describes requirements from the client's side, the Pflichtenheft the contractor's planned implementation. A PRD is close to a Lastenheft in content, but usually leaner, and in agile projects it is updated continuously instead of being signed once and filed away.

The outline of a PRD that works

Problem and goal

Describe in a few sentences which problem you are solving and why it needs solving now. Add how you will measure success, for example less manual processing time, more completed orders or fewer support requests. Without a measurable goal you cannot tell later whether the project paid off.

Target users and roles

Who uses the product, and in what situation? A field sales rep on a smartphone has different needs from an office clerk at a desktop. List the roles and their permissions, such as admin, employee, customer.

Core flows as user stories

Describe the most important flows from the user's point of view, for example: "As a customer I want to repeat an order so I don't have to search for every item again." The format forces you to name the benefit of each feature. For the main flow, a sketch or clickable prototype is worth adding. We cover tools for that in our post on prototyping for non-technical founders.

Acceptance criteria

Every story needs criteria you can check to see whether it is done. "Search is fast" helps nobody. "Search shows results while typing and also finds items by article number" can be tested.

Scope limits: what is out

This section is the one most often forgotten and one of the most valuable. State explicitly what this version will not include. It prevents misunderstandings at quote and acceptance stage and protects the project from scope creep.

Constraints

These include existing systems you need to integrate with, data protection requirements, accessibility, languages, target devices and fixed deadlines. They often affect effort and architecture more than individual features.

Common mistakes when writing a PRD

A frequent mistake is a document that is too long. Nobody reads fifty pages in full, and contradictions go unnoticed. Describing solutions instead of problems is just as unhelpful: "We need a microservice architecture" does not belong in a PRD, while "The system must handle many simultaneous orders at peak times" does.

Many PRDs also rest on assumptions nobody has checked. If you are not sure users really need a feature, talk to them first. Our article on validating a product idea shows how.

The PRD during the project

A PRD is not a contract that never gets touched again. In agile projects it feeds the backlog and grows with what you learn in each sprint. What matters is that changes stay traceable and one person is responsible for priorities. Our post on agile for non-technical stakeholders describes how that collaboration works for business teams without a technical background.

For an MVP the document can be especially short. One or two pages with goal, main flow and scope limits are often enough. Our article on MVP development in eight weeks shows how that turns into a first product within weeks.

Frequently asked questions (FAQ)

How long should a product requirements document be?

As short as possible and as long as necessary. For an MVP one or two pages are often enough, for larger projects a few pages with links to sketches and prototypes. If nobody reads it to the end, it is too long.

What is the difference between a PRD and a Lastenheft?

Both describe requirements from the client's side. A Lastenheft tends to be formal and is often part of the contract, while a PRD is usually leaner and evolves throughout an agile project. In practice the terms overlap.

Do I have to write the PRD myself?

The content has to come from you, because you know the problem and the users. A development partner can help with wording, structure and the technical constraints. Many projects start with a joint workshop that produces the first PRD.

Do I need a PRD for agile development?

Yes, in a lean form. Agile development works without a rigid upfront specification, but it still needs a shared understanding of goal, users and scope. That is exactly what a good PRD provides.

Conclusion

A good requirements document mainly saves conversations that would otherwise happen too late: about scope, about priorities and about when something counts as done. The "what is out" section is often worth more than any feature list. If you want to start your next project with a clear PRD, we can support you through our web development services, from requirements to launch.