Postel's Law
Also known as Robustness Principle
Be liberal in what you accept and conservative in what you send: accept messy user input gracefully, and respond in clear, consistent ways.
See it in action
The same screen without and with Postel's Law. Watch both sides play out, then compare the outcome.
A perfectly understandable number is rejected; the person must guess the “right” way to type it.
Any reasonable format is accepted and normalised behind the scenes; the output is consistent.
What to watch: the same input — rejected on the left, quietly cleaned up on the right.
Accept varied input (spaces, dashes, case, formats) and normalise it yourself.
Push your system's format rules onto people, or be sloppy in what you send back.
Key takeaways
- Accept many input formats (dates, phone numbers, spaces) and normalize them yourself.
- Don't reject input for formatting the system could fix.
- Keep output — messages, data, layouts — predictable and consistent.
Quick facts
- Also known as
- Robustness Principle
- Described by
- Jon Postel
- Year
- 1980
- Category
- Interaction & speed
- Related laws
- Doherty Threshold, Tesler's Law, Cognitive Load
Postel's Law explained
Postel's Law, also known as the robustness principle, says: be conservative in what you do, be liberal in what you accept from others. It began as advice for network protocols and applies equally to interface design.
In UX terms, products should accept a wide range of user input (different formats, typos, partial information) and translate it into what the system needs, while providing clear, consistent output.
This shifts the burden of precision from the user to the system. People should not have to know the exact format a database expects.
Where it comes from
Formulated by internet pioneer Jon Postel as the robustness principle in the early TCP specification (RFC 761, 1980), and since adopted as a general design principle.
How it shows up in apps
Forms that reject a phone number because of spaces or a card number because of dashes create pointless errors at high-intent moments like sign-up and checkout.
Postel's Law examples
A sign-up form accepts '(415) 555 0132', '415-555-0132' and '4155550132', formats them consistently, and never shows a format error.
Flexible phone number fields
Accepting phone numbers with spaces, dashes, brackets or country codes, and normalising them behind the scenes, prevents needless validation errors.
Natural-language dates
Calendar and task apps that understand 'next Friday at 3pm' let users enter dates in their own words rather than a fixed format.
Search tolerance
Search that corrects typos and understands synonyms returns useful results even when users do not type the exact product name.
How to apply Postel's Law
- Accept common variations in input formats and normalise them automatically.
- Correct or suggest fixes for typos instead of rejecting input.
- Validate only what really needs validating, and explain errors clearly.
- Keep your outputs consistent and predictable.
- Design for different devices, input methods, screen sizes and assistive technologies.
Want Postel's Law checked in your own product? Our mobile app ux audit evaluates it alongside the other principles that shape how users decide, onboard and stay.
Common mistakes
- Rejecting valid input because of formatting such as spaces in card numbers.
- Being so lenient that ambiguous input is silently misinterpreted.
- Inconsistent output formats that confuse users.
How to measure it
Track validation error rates per field and the number of attempts users need to submit forms. Search logs showing zero-result queries reveal where the system is not tolerant enough.
Frequently asked questions
What is Postel's Law?
Be liberal in what you accept and conservative in what you send: accept messy user input gracefully, and respond in clear, consistent ways.
Where does Postel's Law come from?
Formulated by internet pioneer Jon Postel as the robustness principle in the early TCP specification (RFC 761, 1980), and since adopted as a general design principle.
How does Postel's Law apply to app design?
Forms that reject a phone number because of spaces or a card number because of dashes create pointless errors at high-intent moments like sign-up and checkout.
Who is Postel's Law named after?
It is named after Jon Postel, an internet pioneer who wrote the principle into early TCP specifications around 1980.
What is an example of Postel's Law in UX?
A form that accepts phone numbers in any common format and formats them consistently, instead of rejecting input that does not match a strict pattern.
How do you measure Postel's Law in a product?
Track validation error rates per field and the number of attempts users need to submit forms. Search logs showing zero-result queries reveal where the system is not tolerant enough.
Sources
- Postel, J. (1980). RFC 761: DoD Standard Transmission Control Protocol. Internet Engineering Task Force. Find on Google Scholar ↗
Related laws
Interaction & speedDoherty Threshold
Productivity and engagement rise sharply when a system responds in under about 400 milliseconds, so neither the user nor the computer is left waiting on the other.
Product strategyTesler's Law
Every system has a certain amount of complexity that can't be removed — only moved — so the product should absorb it instead of passing it to users.
Memory & attentionCognitive Load
Cognitive load is the amount of mental effort needed to understand and use an interface; when it exceeds what people can handle, they slow down, make mistakes or give up.
Postel's Law in practice
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Certifications that test it
- Free certification · UX/UI, Design, Research & CreativeUX Design CertificationCertifies that you can take a product problem from user need to a tested, usable flow — research, information architecture, interaction design and usability evaluation.
- Free certification · UX/UI, Design, Research & CreativeUX Writing & Content Design CertificationCertifies that you can write interface copy that helps people act — microcopy, errors, onboarding, voice and tone, content structure and testing words.
- Free certification · UX/UI, Design, Research & CreativeAccessibility Certification (WCAG)Certifies that you can design and review digital products for people with disabilities — WCAG 2.2 principles, inclusive design patterns, assistive technology and accessibility testing.
- Free certification · AI & Generative AIAI UX Design CertificationCertifies that you can design AI-powered products people trust — setting expectations, conversational and agent UX, handling errors and uncertainty, and responsible AI.
- Free certification · UX/UI, Design, Research & CreativeInteraction Design CertificationCertifies that you can design how interfaces behave — usability principles and mental models, feedback and states, forms and input, and motion and timing.
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