Nielsen's 5-User Rule

5 users uncover ~85% of usability issues. More users find the same problems repeatedly.

How to apply it

When you need 'why', talk to 5 people. When you need 'how many', use quantitative.

Nielsen's 5-User Rule in practice: a concrete example

Five people, most of the problems

Watch five people try to book a room on a new flow and you will see the same handful of walls: they miss the date picker, they can't tell which room is selected, they hesitate at the fee line. A sixth and seventh tester mostly re-confirm what the first five already showed. That is Nielsen's point: usability problems cluster, so a small round finds the big ones quickly and cheaply. Run several small rounds as you iterate, not one giant study at the end.

Common Nielsen's 5-User Rule mistakes

Treating five users as a statistic. It answers “what's broken and why”, never “what percentage of users convert”, which needs quantitative data.
Recruiting five of the wrong people. Five users unlike your real audience find the wrong problems.
Running one big study instead of several small ones. The value is in the test-fix-test loop, not the head count.

When Nielsen's 5-User Rule doesn't apply

When the question is “how many” rather than “why” (conversion rates, an A/B winner, sizing a market) five users tell you nothing reliable, and you need proper sample sizes. And if a product serves very distinct groups, say buyers and sellers, five per group beats five in total, because their problems barely overlap.

Practise it on a real challenge

Onboarding — users quit at step 3 and no one knows why

A team wants to know *why* users abandon onboarding step 3.

Nielsen's 5-User Rule

How to apply it