T-Shirt Sizing: A Simple Model for Product Estimation and Sizing

Updated 18. Jul 2023

T-Shirt sizing is a simple prioritization model used for estimating larger bodies of work. It is a quick and easy way to estimate the relative size of tasks, features, or requirements within a project. This model is commonly used in Agile methodologies. It is called T-Shirt sizing because it uses a simple sizing scale that mimics T-Shirt sizes: XS, S, M, L, and XL.

When is it useful?

Imagine a scenario where you are trying to make a rough plan for the next quarter or more. Your analysis gave you a bunch of ideas that your team could be working on, but if you want to make the best decision and make the most out of your time you have to understand how long each task may take.

Benefits

Compared to other estimation models, the main benefit of T-Shirt sizing is that you do not need to understand all the details of the task, but rather it provides a more high level estimation for bigger tasks. This means that you can run through a list of tasks and figure out the estimation much faster than with other methods.

How do you use it?

To use T-Shirt sizing, the team first needs to agree on the sizing scale and its definitions. Then, each task, feature, or requirement is assigned a T-Shirt size based on its complexity and effort required. This can be done through a collaborative discussion or a voting process.

ℹ️

☝️ Remember, do not discuss the nitty gritty details of a task, but focus on a high level estimate.

As mention, T-Shirt sizing is usually meant for bigger tasks and features, so think weeks, months. Here is an example we are using:

Size	Estimation
XS	1-2 Weeks
S	2-4 Weeks
M	1-2 Months
L	2-4 Months
XL	4 Months +

In general, T-Shirt sizing can be done without any special tools. It is usually done through a collaborative discussion or a voting process where team members verbally express their opinions on the size of each task.

One way to use T-Shirt sizing is to have team members individually assign a T-Shirt size to each task, feature, or requirement. Then, the estimates are revealed and discussed as a group. If there are discrepancies between estimates, the group can discuss and re-vote until a consensus is reached. We have also built a tool that can help facilitate this process, which you can try out at this link.

Regardless of the specific method used, the goal of T-Shirt sizing is to provide a quick and easy way to estimate the relative size of tasks without getting bogged down in details.

Often, to better understand the requirements a team may decide to do a “spike” task to explore what needs to be done and have a better estimate.

What to be careful of

Lack of Consistency: It's crucial to establish clear guidelines and definitions for each size (e.g., Small, Medium, Large) to ensure everyone is on the same page.
Focusing on Precision: T-Shirt sizing is meant to be a quick and relative estimation technique, not an exact science. Overanalyzing and striving for precise measurements can defeat the purpose and slow down the prioritization process.
Skipping Collaboration: Prioritization using T-Shirt sizing should involve collaboration and input from multiple stakeholders. Making decisions in isolation without seeking diverse perspectives can result in biased prioritization and limited buy-in from the team.
Using T-Shirt Sizing as the Sole Criterion: T-Shirt sizing is just one technique among many for prioritization. Relying solely on T-Shirt sizing without considering other factors like strategic value, customer impact, or business objectives can result in suboptimal prioritization decisions.

Real-world examples of T-Shirt sizing:

As an example let's say a software development team is using T-Shirt sizing to estimate the time it will take to complete a set of new features for their application. They estimate that the most complex feature will take XL (4 months +), while the majority of the others are M (1-2 months) or L (2-4 months) based on the amount of development work and testing required.

Here are 5 example tasks for a software development team, with their estimated T-Shirt sizes and reasoning:

Task	Estimation	Reason
Implement a new authentication system	L (2-4 months)	This feature requires significant development work and testing, but is a known problem with clear requirements and a well-defined solution.
Integrate with a third-party API	M (1-2 months)	This feature requires some development work and testing, but is a well-documented and straightforward integration.
Add support for multiple languages	XL (4 months +)	This feature requires significant development work, testing, and coordination with other teams to ensure translations are accurate and complete.
Improve performance of a key feature	S (2-4 weeks)	This is a small change that requires some optimization work but does not have significant impact on the rest of the system.
Refactor legacy code	XL (4 months +)	This feature requires significant development work and testing to modernize and improve an existing codebase that is difficult to work with.

Alternatives to T-Shirt Sizing

While T-Shirt sizing is a quick and easy way to estimate the relative size of tasks, it is not the only method for estimation or prioritization. If you have more time and want to be more accurate, you can try two other methods we have developed:

Estimation Based on Complexity: This method is similar to T-Shirt sizing, but instead of assigning sizes based on a pre-defined scale, you assign sizes based on the complexity of the task. This method requires a bit more effort and discussion, but it can be more accurate than T-Shirt sizing. You can try our tool for complexity-based estimation here.
Estimation Based on Time Buckets: This method involves assigning each task to a specific time bucket, such as "Less than 1 week," "1-2 weeks," "2-4 weeks," and so on. This method provides a more accurate estimate but can take longer to complete. You can try our tool for time bucket estimation here.

In conclusion, T-Shirt sizing is a simple and effective way to estimate the relative size of tasks, features, or requirements within a project. It is a great tool to use for high-level estimation and can help teams make better decisions about how to prioritize their work.