UX Design Principle #005: Postel's Law

William Brodlo
Marketing & Design Associate
Read Time
4 min read
Published On
June 30, 2023

In the field of User Experience Design, certain principles from psychology can greatly impact the overall user experience. These laws help create more intuitive and human-friendly experiences.

UX Design involves not only creating visually appealing interfaces but also interfaces that are easy to use and understand. Designers use psychological principles to ensure that interfaces meet users’ needs and tendencies, we’ve already tackled Hick’s Law, Fitts’s Law, Jakob’s Law, and Miller’s Law. The next principle in our series is Postel's Law (also known as the Robustness Principle).

What is Postel's Law?

Postel's Law, named after Jon Postel, an American computer scientist and contributor to the development of the Internet, is a principle that guides the design of network protocols. It states: "Be conservative in what you send, and liberal in what you accept." This quote was pulled from an early specification for the Transmission Control Protocol (1980) which is a communications standard of the Internet.

To put the principle in UX Design terms, when designing user interfaces, designers should aim to be lenient in the information they accept, allowing for flexibility and accommodating different user inputs, while being strict in the output and feedback they provide to maintain a consistent and intuitive user experience.

Examples of Postel's Law in Practice

Email Address Variations

Image Source: Google

Email services like Gmail follow Postel's Law by accepting a wide range of email address variations. For example, they treat "example.email@gmail.com," "exampleemail@gmail.com," and "ex.ample.email@gmail.com" as the same address. This is true for any variations of your email with periods throughout. This leniency in accepting different variations reduces user frustration and allows for greater flexibility in how users can format their email addresses while ensuring successful message delivery.

URL Handling

Image Source: Google Search of perpetualny.com and www.perpetualny.com

Web browsers adhere to Postel's Law by being lenient in the interpretation of URLs. For instance, if a user mistypes a URL by omitting the "www" prefix or accidentally adds a trailing slash, modern browsers often correct these errors and redirect the user to the intended page. This leniency helps users access websites even with minor URL mistakes, enhancing the overall browsing experience.

Search Query Auto-Correction:

Image Source: Google Search and Microsoft Bing Search

Search engines like Google and Bing apply Postel's Law by intelligently interpreting user queries. They automatically correct common spelling errors or suggest alternative search terms. This leniency in interpreting user queries allows for a more forgiving search experience, helping users find relevant results even if they make mistakes or use different variations of keywords.

Applying Postel's Law in UX Design

While Postel's Law is primarily associated with network protocols, its principles can be applied in UX Design to create robust and user-friendly experiences.

User Input Validation

When designing forms or input fields, it is essential to be liberal in accepting user input. Users may provide information in different formats or may make minor mistakes. By implementing lenient input validation, such as automatically formatting phone numbers or auto-completing fields in forms, designers can minimize user frustration and errors. However, when it comes to displaying output or providing feedback, designers should adhere to strict standards and present information consistently and clearly.

Error Handling and Feedback

Following Postel's Law, error handling and feedback mechanisms should be forgiving and informative. Instead of displaying cryptic error messages, designers should aim to provide helpful guidance that assists users in understanding and correcting errors. For example, date entry fields can give you error messages about having the format incorrectly, but don’t tell you which format they need. To combat this, designers can make multiple date formats acceptable or be specific and informative in their error messages. By being liberal in accepting user input and guiding them toward the correct format or action, designers can ensure a smoother user experience and prevent frustration.

Password Strength Requirements

For password strength requirements, striking the right balance between security and usability is crucial. To make your designs accessible and dependable, avoid overly strict rules that can frustrate them. Instead, provide guidance and suggestions to help users willingly create strong passwords. Consider promoting the use of “passphrases”, which are longer and easier to remember than complex character combinations. Implementing a password strength meter can offer real-time feedback on the complexity of the chosen password. Allowing longer passwords and increasing the character limit while providing educational resources empower users to create strong passwords while maintaining a positive user experience and ensuring account security.


Postel's Law, or the Robustness Principle, is a concept that promotes taking in a wide variety of acceptable inputs and only giving out filtered and essential outputs. By being liberal in what we accept from users and accommodating variations and diverse inputs, we can create more flexible and user-friendly interfaces. However, when it comes to what we send or present to users, we should adhere to strict standards and maintain consistency. By applying Postel's Law in UX Design, we can enhance the overall user experience and ensure robustness in our designs.

If you would like to learn more about Postel's Law, we recommend these further readings:

If you haven't read the other four installments in our UX Design Principle series, take a look at Hick’s Law, Fitts’s Law, Jakob’s Law, or Miller’s Law.