Fibonacci or T-shirt sizes?

Planning poker works with any set of cards, but most teams use one of two: Fibonacci-style numbers for story points, or T-shirt sizes. They suit different jobs.

Fibonacci-style numbers

The cards go 0, ½, 1, 2, 3, 5, 8, 13, 20, 40 and 100. The gaps get bigger as the numbers grow, because the bigger a piece of work is, the less precisely anyone can size it. Nobody can honestly tell a 14 from a 15, but most teams can tell a 5 from an 8.

• Best for: stories the team will pick up soon, where the detail is known.
• Why teams like it: the numbers can be added up, so you can total a sprint or a release and track how much the team gets through (velocity).
• Watch out for: treating points as hours. A point is a relative size, and one team's 5 isn't another team's 5.

T-shirt sizes

The cards are XS, S, M, L, XL and XXL. There are no numbers, so people focus on how big something feels compared with other work, not on the arithmetic.

• Best for: features and epics early on, roadmap planning, and sizing a long list quickly.
• Why teams like it: it's fast, it's easy for people outside the team to understand, and it doesn't pretend to be more precise than it is.
• Watch out for: sizes can't be added up as they are. If you need a total, your team has to agree its own way of turning sizes into numbers, and should say so wherever the numbers are used.

The special cards

Both decks in EstimateIt also have three cards that aren't sizes: ? (I don't know enough to estimate this), ∞ (this is too big to estimate, so split it) and Pass (I'm not voting on this one). They don't count towards the average or median.

Which should you use?

Many teams use both: T-shirt sizes for features and epics while planning the roadmap, then numbers for the stories inside them when they're ready to be built. In EstimateIt the facilitator picks the cards for each item before voting starts, so one room can hold numbered stories and T-shirt-sized epics side by side. EstimateIt doesn't turn T-shirt sizes into points for you, so the two never get mixed up by accident.

Common questions

Why does planning poker use Fibonacci numbers?

Because the gaps grow as the numbers grow, matching how uncertainty grows with the size of the work. It stops teams arguing over differences too small to matter.

Can you add up T-shirt sizes?

Not directly. A team that needs a total has to agree its own conversion from sizes to numbers.

Can I use both decks in one EstimateIt room?

Yes. The facilitator chooses Numbers or T-shirt cards for each item before the votes are revealed.

Try it with your team

EstimateIt is free planning poker for agile teams, with nothing to install and no sign-up.

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