Fibonacci, T-shirts or powers of two? Picking a deck

2026-10-08 · The Scrums team

Estimation decks skip numbers on purpose. People judge differences in proportion, as Ernst Weber found in the 1830s. Here's how to pick the deck that suits what your team is estimating.

Why does a planning poker deck jump from 8 to 13, and never offer a 10? Because the gap between 8 and 10 is one nobody can estimate honestly.

In the 1830s, the German physiologist Ernst Weber found that we notice differences in proportion, not in absolute amounts: you can tell a 1 kg weight from a 2 kg one, but not 20 kg from 21. Estimating work is the same. A one-day task and a two-day task feel different; a twenty-day job and a twenty-one-day job don't. A deck whose gaps grow as the numbers grow asks only the questions people can answer.

Choosing a deck

Scrums has four built in, and you can make your own.

Fibonacci (0, 1, 2, 3, 5, 8, 13, 21) is the classic. It suits story points, where the numbers are relative sizes, not hours.
Modified Fibonacci adds ½ and rounds the top to 20, 40 and 100. Use it when your backlog mixes small fixes with big, vague items.
T-shirt sizes (XS to XXL) are for early, rough sizing, such as a roadmap or a new project, when numbers would suggest more precision than anyone has.
Powers of 2 (1 to 64) suit teams that think in doubling: "this is twice as big as that".
A custom deck of up to 16 cards fits whatever scale your team already uses.

Every deck in Scrums includes ? for "I can't estimate this yet" and a coffee cup for "we need a break". Both are useful signals: a row of question marks means the story needs more detail before anyone should guess.

More than one number

Size isn't the only thing worth estimating. A small change in a fragile part of the system can carry more risk than a big change somewhere safe. Scrums lets each story be estimated on up to four dimensions, each with its own deck: story points and risk, say, or effort and complexity. Most teams use one; the second is there when the conversation keeps coming back to "yes, but it's risky".

Try sprint poker in your browser, or sign up and pick your team's deck.