Stop 05 · Requirements & DocumentationTopics · 6/9

Non-Functional Requirements

In short

Functional requirements say what the product does. Non-functional ones say how well: how fast, how safe, how reliable. Users notice them most when they're missing.

Requirements about how a system behaves rather than what it does — performance, security, accessibility, scalability — easy to forget because they're invisible until they fail.

The purple blob as a waiter rushes a hot dish to a spotless table, a stopwatch at its waist.

Think of it like this

The restaurant beyond the menu

The menu says you'll get paneer tikka. That's functional. But you also expect it within twenty minutes, on a clean plate, in a place that won't make you sick. Nobody prints that, yet you'd never return without it.

Watch it happen

Requirements for a fee payment page

Pay fees page

Functional

Non-functional

Pay with UPI

Loads in < 2 s

Up 99.9% of time

Card data secured

Screen-reader OK

1/4Functional: what the page does. A student can pay fees using UPI.

The big ideas

What you need to know

Performance

Load times and response times, measured on realistic phones and networks.

Security and privacy

Who can see what, how data is stored, and which rules or laws apply.

Reliability and scale

Uptime targets and how many users at once it must handle, like on result day.

Accessibility

Works with screen readers, keyboard navigation and enough colour contrast. Not optional.

Save yourself the pain

Common mistakes

Oops

Writing "should be fast and secure".

Do this instead

Make them measurable: "P95 load under 2 seconds on 4G", "only the student can see their receipts".

Oops

Adding non-functional requirements after the build.

Do this instead

Agree on them early. Speed and security are much harder to bolt on later.

Your mission

Measure real NFRs

~25 min

Quick check

Which is a non-functional requirement?

Go deeper

1Add a non-functional requirements section to a PRD you're practicing with.
2Learn to specify performance requirements concretely (e.g. 'loads in under 2 seconds').
3Understand why skipping these causes expensive rework late in development.