The Fibonacci sequence in agile estimation

Described in the 13th century by the Italian mathematician Leonardo Fibonacci, the sequence starts with 0 and 1, and each number is the sum of the two before it: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55…

Agile estimation keeps the first terms, without the duplicate: 0, 1, 2, 3, 5, 8, 13, 21. Each value is roughly 60% bigger than the previous one.

The bigger a task, the harder it is to estimate its effort precisely. Telling a 1 from a 2 is easy, but nobody can really say whether a story is a 12 or a 13. The Fibonacci sequence builds this in: values are close together for small tasks and spread out for big ones.

• Fewer pointless debates: on a 1-to-10 scale, a team can spend ten minutes deciding between 7 and 8. With Fibonacci, the choice is between 5, 8 and 13, which forces a clear position.
• Relative estimation: you are not looking for a duration, but a size compared with a reference story. “This one is about twice as big” naturally maps to going from 3 to 5 or 8.
• A warning sign: when a story reaches 21, the uncertainty is too high to commit to it.

Story points measure a mix of complexity, amount of work and uncertainty. Each team calibrates its own scale, but these benchmarks are common:

• 0: nothing to do, or almost (the story is already covered by another one).
• 1 and 2: a small, well-understood change with no unknowns.
• 3 and 5: a typical story, with some complexity or several parts to change.
• 8: a sizeable story with some grey areas.
• 13: a big story, at the limit of what fits in a sprint.
• 21 and above: too big or too vague. The story should be split or clarified before being estimated.

Many card decks use a variant popularized by Mike Cohn: 0, ½, 1, 2, 3, 5, 8, 13, 20, 40, 100. The large values are rounded to make it clear that beyond 13, you are no longer estimating but giving an order of magnitude. The ½ is for tiny tasks.

Both versions work. What matters is that the whole team uses the same scale and keeps it over time, so velocity can be tracked.

Most decks add two cards that are not numbers:

• “?”: I don’t have enough information to estimate. It is a signal for the Product Owner to clarify the story.
• “☕”: I need a break. An estimate made by a tired team is rarely worth much.

• Pick a reference story worth 2 or 3 points and estimate the others by comparison.
• Don’t convert points into hours: you would lose the benefit of relative estimation.
• Don’t average the votes: a 3 and an 8 don’t make 5.5, they make a discussion.
• Split any story above 13 points.

In The Planning Poker, the Fibonacci deck has the cards 0, 1, 2, 3, 5, 8, 13 and 21, plus “?” and “☕”. You can also build your own deck, for instance to add ½, 20, 40 and 100.