Fibonacci vs T-shirt sizing

The quick answer

If your team plans numerically and talks about velocity, use Fibonacci. If you estimate roughly and mostly talk to stakeholders who don't care about points, T-shirt sizing is faster and less debate-prone. Both are legitimate — the mistake is pretending one is objectively "correct".

Fibonacci at a glance

Values: 1, 2, 3, 5, 8, 13, 21 — often with 0, ½, and ? for trivial, tiny, and unknowable.

• Arithmetic: velocity calculations, burndown charts, capacity maths.
• Honest about uncertainty: gaps widen as stories get bigger.
• Works well when the team has enough completed stories to anchor on.

T-shirt sizing at a glance

Values: XS, S, M, L, XL (sometimes XXL). No numeric sum.

• Removes false precision — perfect for brand-new teams or fresh domains.
• Faster to vote on — fewer decision points.
• Easier to communicate outside the team ("this feature is XL").
• Harder to use for velocity, though you can map sizes to points post-hoc.

When to switch

Teams often start with T-shirt sizing (low commitment, fast agreement) and move to Fibonacci once they've completed 2–3 sprints and want to predict capacity. Going back to T-shirts is also fine when you find the team debating 5 vs 8 for 15 minutes on every story — a sign the false precision is costing more than it gains.

Mapping between the two

A common cross-walk when teams transition:

• XS → 1
• S → 2 or 3
• M → 5
• L → 8
• XL → 13
• XXL → "split this story"

When neither fits

Some teams benefit from Powers of 2 (1, 2, 4, 8, 16, 32) — similar psychology to Fibonacci but arithmetically simpler. Others skip numeric estimation entirely and use #NoEstimates, slicing stories until each one is small enough that the question "how long?" isn't worth asking. Pokor supports Fibonacci, T-shirts, Powers of 2, and custom decks so you can pick what fits — or change your mind later.

Practical tips

• Stick with one deck per team for at least a full sprint cycle before switching. Re-calibrating mid-stream breaks velocity comparisons.
• Document the reference story. Whatever deck you pick, pin "what a Medium / 3-pointer looks like" somewhere. See story point estimation.
• Use the ? card. If a story's too fuzzy to estimate, saying so is more useful than guessing. Split it or spike it first.