When Agile teams estimate work, they often use story points instead of hours or days. If you’ve played Planning Poker, you have probably seen numbers from the Fibonacci sequence: 1, 2, 3, 5, 8, 13, 21. But have you ever wondered why agile coaches prefer Fibonacci numbers instead of simple sequences like 1, 2, 3, 4, 5? The answer lies in human psychology, and it’s connected to something called Weber’s Law.
Here you will explore why Fibonacci is so effective, why some teams use a modified version, and how this practice helps agile teams make better decisions.
What is the Fibonacci Sequence?
The traditional Fibonacci sequence is a series of numbers where each one is the sum of the two before it: 1, 2, 3, 5, 8, 13, 21, 34 and so on. In Agile, teams often use a modified Fibonacci sequence for estimation: 1, 2, 3, 5, 8, 13, 20, 40, 100. The goal is not to be mathematically perfect but to make estimation practical and realistic for teams.
Why Fibonacci Works for Agile Estimation?
Simple numbers don’t help much, if teams used a simple series like 1, 2, 3, 4, 5, the numbers are too close together. It would be hard to tell whether a task is a 3 or a 4 because the difference feels too small.
Weber’s Law explains it, Weber’s Law says humans notice differences better when they are expressed as percentages rather than exact numbers. Example: If you hold a 1kg weight in one hand and a 2kg weight in the other, the difference is 100%, easy to notice. If you hold a 20kg weight and a 21kg weight, the difference is only 5%. That’s much harder to notice. The Fibonacci sequence works well because each number is roughly 60% larger than the previous one. This matches how our brains judge effort differences.
Why Modify the Fibonacci Sequence?
Early Agile teams used the exact Fibonacci numbers. But over time, teams noticed some problems: A number like 21 made stakeholders think the team was being too precise, even though estimation is never exact. To simplify, many teams replaced 21 with 20.
Once teams started adjusting, they also added 40 and 100 to represent very large, uncertain tasks. These numbers don’t strictly follow Fibonacci, but they make sense in Agile because very large estimates are often imprecise anyway.
Why Fibonacci Stuck Around?
In 2007, when Planning Poker cards became popular, the modified Fibonacci sequence was printed on thousands of decks. This gave it widespread adoption. Today, most scrumteams use the modified Fibonacci sequence, but doubling systems are still fine if a team prefers them. The important part is consistency within the team, not the exact numbers.
The Fibonacci sequence works well for Agile estimation because it matches human psychology and makes it easier to distinguish meaningful differences in effort. Whether you use the classic sequence, a modified version, or even a doubling sequence, the key is to promote good team discussions about work complexity, not exact precision.
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Frequently Asked Questions
Why do Agile teams use the Fibonacci sequence for estimation?
Because it matches how humans perceive effort differences. The jump between numbers is big enough to avoid false precision but small enough to guide meaningful discussions.
What’s the difference between Fibonacci and modified Fibonacci?
Modified Fibonacci replaces some numbers (like 21 → 20) and adds larger ones (40, 100) to keep things simple and realistic for teams.
Can teams use doubling numbers instead of Fibonacci?
Yes. Sequences like 1, 2, 4, 8, 16 also work, but they often lead to less useful discussions. Fibonacci tends to encourage better conversations about complexity.
What is Weber’s Law and how does it apply here?
Weber’s Law says people notice differences as percentages, not exact numbers. Fibonacci fits this law because each number is roughly 60% bigger than the last.
Does the choice of sequence affect project outcomes?
Not directly. Both Fibonacci and doubling sequences can work. What matters most is that the team agrees on one system and uses it consistently.
Where can I learn Agile estimation techniques in depth?
At HelloSM Scrum training, the best Scrum training institute in Hyderabad and a top training institute in India, you’ll gain hands-on practice with estimation, backlog refinement, and other Agile practices that make teams successful.