Why to avoid t-shirt sizes for user story estimation

The more I work on agile software development teams who use t-shirt sizes (S,M,L,XL etc.) to estimate user stories the more I dislike this approach. Here’s why:

  • In my opinion, the most import thing about user story sizing is relativity, and t-shirt sizes are a subjective measure of relativity: someone in the team might think a large is two times as big as a small, whereas another person might think it’s three times as big. This isn’t helped by the t-shirt analogy where it’s actually hard to determine how much bigger is a large t-shirt than a small one?
  • You can’t create a single measure of team velocity unless you define a scale that converts t-shirt sizes into a numeric size so you can measure t-shirt size relativity and velocity.
  • As soon as create a scale to convert t-shirt sizes into a numeric size you’ve essentially started using story points (in a convoluted way).

TL;DR: Using t-shirt sizes for user story estimation is confusing and ultimately leads the team to using story points so just skip t-shirt sizes and use relative story points instead.

Author: Alister Scott

Alister is an Excellence Wrangler for Automattic.