Bad requirements documents are the number one cause of failed software projects. After reviewing 500 plus PRDs and writing roughly the same number for our own builds, here is the practical guide to writing software requirements that produce predictable builds, accurate quotes, and shipped products. The bar is not perfection - it is enough clarity that a senior engineer can build it without guessing.
PRD vs SRS vs User Story - What You Actually Need
Product Requirement Document (PRD) is the modern term used by most product teams. It describes what to build, who it is for, and why it matters. It includes user flows, key features, success metrics, and constraints. It does not include detailed implementation specs.
Software Requirements Specification (SRS) is the older, more formal term used in enterprise and government contexts. It includes everything in a PRD plus formal functional and non-functional requirements, often numbered for traceability and compliance.
User stories are individual units of work written from the user perspective - As a [role], I want [action] so that [outcome]. They are the building blocks that fill out a PRD, not a replacement for one.
For most modern SaaS builds, write a PRD. For regulated industries (healthcare, fintech, government), write an SRS. For day-to-day backlog work, use user stories that ladder up to the PRD.
The Eight Sections Every PRD Needs
A PRD that an engineering team can actually build from has eight sections.
Problem statement. One paragraph describing the specific user pain you are solving. Concrete enough that you could imagine three users telling you about it in their own words.
Target users. Who specifically uses this product, what is their role, what is their context. Avoid hand-waving like SMEs in general - get specific to ops managers at logistics companies with 50 to 200 trucks.
Goals and success metrics. What does shipping this product change in the world. Define measurable outcomes - reduce processing time by 50 percent, capture 100 paying customers in first 6 months, achieve 90-day retention above 60 percent.
User flows. The 3 to 7 main flows through the product. Each described as a sequence of user actions and system responses, ideally with simple diagrams.
Feature list. The features that compose those flows. Each feature gets a name, a one-paragraph description, priority (P0 must-have for launch, P1 should-have, P2 nice-to-have), and acceptance criteria.
Non-functional requirements. Performance targets, security and compliance needs, accessibility level, scale assumptions, integration constraints.
Out of scope. What you are explicitly NOT building in this version. This is the section most teams skip and it causes more scope creep than any other.
Timeline and constraints. Target launch date, budget cap, must-use technologies, must-integrate-with platforms.
The Right Level of Detail
Too vague means engineers ask 50 questions per week and the project drags. Too detailed means you have written code in prose and limited engineer judgement.
The sweet spot: a senior engineer reading the PRD should be able to identify the major architectural decisions, the key data entities, the main user flows, and the success criteria - and still have judgement room for implementation details.
A useful test: have a senior engineer read the PRD without you in the room and write down their questions. Fewer than 5 questions means it is probably too detailed. More than 20 questions means it is too vague. 5 to 15 questions is the right zone.
User Flow Diagrams That Actually Help
Diagrams beat prose for user flows. Use simple boxes and arrows in Excalidraw, Figma, or even hand-drawn whiteboard photos. Polished diagrams are not the goal - clarity is.
For each major flow: starting state, the user actions in order, the system responses, the ending state, and the branches for the most important alternative paths (failure, edge case, alternate happy path).
Skip flows for trivial paths. Adding a login diagram to every PRD is busywork. Save flows for the 3 to 7 paths where your product actually delivers value.
Writing Acceptance Criteria
Every P0 feature gets acceptance criteria - the conditions that must be true for the feature to be considered shipped. Write them in Given-When-Then format or as numbered conditions.
Example: Feature - Customer can cancel subscription.
Acceptance criteria: 1) Customer can find the cancel option within the account settings page in fewer than 30 seconds. 2) Cancellation requires a confirmation step with clear messaging about end-of-billing-period access. 3) On confirmation, customer receives an email summarising what happens next. 4) Cancellation is reflected in the admin dashboard within 5 minutes. 5) Customer continues to have access until the end of their paid period.
Acceptance criteria force you to think through edge cases at requirements time rather than discovering them in QA.
The PRD Mistakes That Sink Projects
Treating PRD as a sales doc rather than a specification. Marketing language belongs in your pitch deck, not your PRD.
Writing requirements as solutions. The customer wants to filter by date range is a requirement. Implement a date range picker using react-day-picker is a solution. Stop after the requirement.
No prioritisation. A PRD with 100 features all marked must-have is a PRD with no must-haves. Force yourself to identify the P0 set - usually 10 to 20 features for an MVP.
Skipping out of scope. The most common cause of scope creep is failure to write down what you are not building.
Stale PRD. Your PRD has to evolve as you learn. A PRD that has not been updated since week one of the project is no longer a PRD - it is fiction.
Templates and Tools
Notion and Google Docs are the dominant tools for PRDs in 2026. Both let multiple stakeholders comment, suggest changes, and track decisions over time.
For larger PRDs, structured tools like Productboard, Linear, or Aha! let you link PRDs to product roadmaps and individual tickets.
For visual flows, Excalidraw, Figma, and FigJam are the modern defaults. Whiteboard photos still work fine for early-stage clarity.
Skip Confluence unless your organisation already lives in it. The friction of Atlassian tooling is higher than the value for most teams under 50 people.
How SpiderHunts Approaches Requirements
Every engagement starts with a one to two week discovery phase that produces a written PRD. We run it as a series of structured conversations with founders and key stakeholders, sketch user flows together, prioritise features against the launch budget, and document what is explicitly out of scope.
The PRD becomes the contract for the build phase. If something is not in the PRD, it is not in the build - until we agree a change, document it, and adjust scope and timeline accordingly.
This sounds slow. In practice it is the single biggest reason our projects ship on time. Slow at the start, fast at the finish.
Frequently Asked Questions
What is the difference between a PRD and a SRS?
A Product Requirement Document (PRD) describes what to build, who it is for, why it matters, and what success looks like. A Software Requirements Specification (SRS) includes everything in a PRD plus formal numbered functional and non-functional requirements for compliance and traceability. PRD is the modern default for SaaS. SRS is required in regulated industries like healthcare, fintech, and government.
How long should a PRD be?
A focused MVP PRD typically runs 8 to 20 pages. Anything under 5 pages is usually too vague to build from. Anything over 40 pages has crossed into implementation specification territory and is probably hiding scope problems. The right test is whether a senior engineer can read it and have 5 to 15 questions - more means too vague, fewer means too detailed.
Who should write the PRD?
In a startup, the founder or product manager owns the PRD with input from engineers, designers, and key customers. The PRD owner does not have to write every word, but they must own the decisions in it. In a larger team, the product manager owns it with formal contribution from engineering leads and design.
Should I write user stories instead of a PRD?
No - user stories are the building blocks that go into a PRD, not a replacement for one. A backlog of 200 user stories with no PRD makes it impossible for anyone to see the product as a coherent whole. Use both: a PRD for the strategic context, user stories for the day-to-day backlog work that ladders up to it.
How often should I update the PRD?
Every time you make a meaningful product decision that contradicts what is written. In practice, once per sprint or once per two weeks. A PRD that has not been updated in three months is no longer a PRD - it is fiction. The PRD should reflect current intent, not historical intent.
What is the most common mistake in writing software requirements?
Writing requirements as solutions instead of requirements. The customer wants to filter by date is a requirement. Implement a date range picker using react-day-picker is a solution. The first is what you specify. The second is what your engineer decides. Mixing them limits engineer judgement and creates artificial constraints.
How do I know if my PRD is ready for development?
Run this test: have a senior engineer read the PRD without you in the room and write down their questions. Fewer than 5 questions means the PRD is too detailed and you have not left engineer judgement room. More than 20 questions means it is too vague to build from. 5 to 15 questions is the right zone to start development.
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