I’m in a lot of agile planning meetings, and a major part of any meeting is spent estimating agile story points and determining the amount of work for a story. Deciding how much work there is for a team to do is actually an important task of a successful planning meeting! We often use Fibonacci numbers and it seems to work well, but I never knew why. So I decided to do a little research.

In math, the Fibonacci sequence follows the following integer sequence:

1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144… Can you guess the next one?

Named after the Italian mathematician Fibonacci, it can be expressed mathematically as a recurrence relation: $F_{n} = F_{n-1} + F_{n-2},\,$

where the nth Fibonacci number is the sum of the previous two Fibonacci numbers.

The reason it is used in agile planning for story estimation is that we are human. We have difficulty discerning close numbers. For instance, using a relatively easy example, if a story is estimated as 3 or 4 points, can the level of accuracy be certain?

Not really. We just can’t estimate that well. On the other hand, can we discern between 5 and 8 or 8 and 13? Yes, because of the relativity of the numbers.

As humans, it is much easier for us to estimate in comparison rather than using absolutes. Given that, it helps to eliminate arguments when estimating story sizes as well, thus reducing the length of planning meetings. It’s fairly easy to agree that a story is a 13 versus 21, because they are far enough apart to discern a difference. However, we might argue if we were deciding on 13 versus 16.

The Fibonacci sequence also occurs in nature, and has been called the “golden ratio.” In nature, you often see flowers with numbers of petals in the sequence.

Thanks to Fibonacci, while estimating agile story points is still not easy it’s fairly simple. And we don’t argue much about the effort required for a story because it’s easy to agree on the relative effort using the Fibonacci sequence.