Postel’s Law
Postel's Law, also known as the Robustness Principle, advises that systems should accept unexpected or incorrect inputs without failing. In simple terms, your system should handle user mistakes without breaking.
Surinder Thakur · AI Product Designer
Founder of ProUXUpdated
- Principle · 136
- CRO techniques
- UX concepts
- UX biases
- UI principles
- UX fundamentals
Why It Matters
Why Is Postel's Law Important for Your Business?
- Better User Experience: Users can interact freely without worrying about causing errors.
- Enhanced Stability: Your system stays functional even when users make mistakes.
- Cost Savings: Fewer errors mean less time and money spent on support.
- Stronger Reputation: A reliable system builds trust and encourages positive reviews.
Benefits of Following Postel's Law
- Increased Reliability: A stable system keeps users happy and reduces downtime.
- Lower Support Costs: Less troubleshooting frees up resources.
- Scalability: A robust system can handle more users and diverse interactions.
- Competitive Edge: Offering a user-friendly product sets you apart in the market.
How to Improve
How Can You Apply Postel's Law?
- Design Flexibly: Allow your system to accept a variety of inputs, even if they're not perfect.
- Anticipate Mistakes: Plan for common errors and unusual user actions.
- Provide Clear Guidance: Use helpful messages to steer users back on track without blaming them.
- Test for Errors: Regularly check how your system handles incorrect inputs to ensure it doesn't crash.
Key Takeaway
In Summary
By embracing Postel's Law, you create systems that are resilient and user-friendly. Handling unexpected inputs gracefully leads to happier users, reduced costs, and a stronger brand.
Questions about Postel’s Law
Accept leniently, store strictly. Take a phone number with spaces, dashes or a country code and normalise it yourself. Rejecting input you could have interpreted moves work onto the person for no benefit.
Reformat it silently and show what you understood. If it genuinely cannot be parsed, say what format you need with an example, and keep what they typed so they can edit rather than retype it.
These answers come from nineteen years of UX practice. More about Surinder →
AI Product Designer
19 years designing products, mobile first, grounded in UX psychology.
Founder of ProUX.
