Why Fibonacci? The Science Behind Agile Estimation

Why Fibonacci?

Agile teams use Fibonacci because humans are better at judging broad differences than tiny differences. It is useful to distinguish 5 from 8; arguing about 13 versus 14 is usually noise.

Estimation is not linear

Small stories can be estimated with confidence. Large stories contain unknowns, dependencies and edge cases. Fibonacci values grow with that uncertainty.

Why not use 1 to 10?

A 1 to 10 scale invites false precision. Teams spend time debating whether something is a 6 or 7 instead of discussing risk and scope.

When to split

A story estimated at 21 or higher should usually be split. Many teams also split anything above 8 if it cannot be finished comfortably in one sprint.

Team intuition

After several sprints, the team calibrates against real delivery. The shared mental model becomes more important than the number itself.