Most requirement docs fail for one simple reason:
They are hard to execute.
Not because they are incomplete.
Not because they lack detail.
Because they are unclear.
This post is a practical checklist to fix that.
No theory. No fluff. Just a usable structure.
What counts as a functional requirement (quick reset)
Before writing anything, align on this:
A functional requirement = what the system does when a user does something
That’s it.
If a line does not describe:
- a user action
- a system response
…it’s not a functional requirement.
Step 1: Identify functional requirements (fast method)
Use this 3-question loop:
1. Who is using the system?
2. What are they doing?
3. What should the system do back?
Example:
User: logs in
Action: enters email + password
System: validates → allows access OR shows error
That becomes one requirement.
Repeat this loop until all key flows are covered.
Quick checklist
- [ ] Every user action is listed
- [ ] Every action has a system response
- [ ] No missing flows (login, reset, checkout, etc.)
Step 2: Write requirements in a standard format
Avoid long paragraphs.
Use a simple structure:
WHEN [user action]
THEN [system response]
Examples:
WHEN user submits login form
THEN system validates credentials and grants access if correct
WHEN user clicks reset password
THEN system sends reset link to registered email
Why this works
- easy to read
- easy to test
- no ambiguity
Step 3: Validate each requirement (engineer check)
Every requirement must pass these checks:
Clarity check
- [ ] Can two people read this and understand the same thing?
Testability check
- [ ] Can this be verified with a simple test case?
Scope check
- [ ] Does this describe only one action and one result?
If any answer is no → rewrite.
Functional vs non-functional requirements (quick table)
This is where many docs break.
They mix everything together.
Use this separation:
| Type | What it means | Example |
|---|---|---|
| Functional | What the system does | User logs in successfully |
| Non-functional | How the system performs | Page loads in 2 seconds |
Rule
If it describes behavior → functional
If it describes quality → non-functional
Keep them separate.
Common mistakes (and fixes)
1. Writing vague requirements
Bad:
System should handle login properly
Fix:
WHEN user enters valid credentials
THEN system grants access to dashboard
2. Combining multiple actions
Bad:
System validates login and sends welcome email and logs activity
Fix:
Split into 3 requirements.
3. Missing edge responses
Example:
- What happens on wrong password?
- What happens on empty input?
Fix:
Add explicit failure cases.
Mini template you can reuse
Copy this:
Feature: [Feature Name]
Requirement 1:
WHEN [user action]
THEN [system response]
Requirement 2:
WHEN [user action]
THEN [system response]
Failure case:
WHEN [invalid action]
THEN [system response]
How this improves testing
Every requirement becomes a test.
Example:
Requirement:
WHEN user enters correct password
THEN system grants access
Test:
- Input: valid credentials
- Expected: access granted
Clear input. Clear output.
No confusion.
Final checklist before handoff
Use this before sending to engineering:
- [ ] All user flows are covered
- [ ] Each requirement = one action + one result
- [ ] Requirements use consistent format
- [ ] No mixed functional and non-functional items
- [ ] Every requirement can be tested
If this passes, the doc is usable.
Bottom line
Functional requirements are not about writing more.
They are about writing clearly.
- Identify actions
- Define responses
- Keep everything testable
That’s the entire system.