Tutorials · 27 Oct 2026
How to Build an OTP Verification Feature for Your Website
By Team SMS-COOL
Learning to build OTP verification feature is a core skill for any developer who handles accounts. An OTP adds a second proof of identity beyond a password and stops most account takeover attempts. This guide covers the design, the code flow, the security rules and the testing steps, without inventing numbers or promising unrealistic guarantees.
Direct answer: To build an OTP verification feature, generate a random code, store it with an expiry and attempt limit, send it by SMS or voice, verify it once against the stored value, then expire it immediately after success.
What Does an OTP Feature Need?
A basic what is OTP understanding helps: the code is temporary and single-use. Your feature needs generation, storage, delivery, verification and expiry.
- Random code generation
- Secure storage with expiry
- Delivery by SMS or voice
- Attempt limits
- Single-use verification
Step-by-Step: Build the Feature
- Generate a random numeric code.
- Hash or encrypt it before storage.
- Store it with a short expiry and an attempt counter.
- Send it through an SMS provider.
- Verify the code once, then delete it.
- Lock the account after too many attempts.
How Do You Send the Code?
Use a reliable SMS provider with an API. If you are handling virtual numbers for testing, the SMS verification API documentation shows how to order numbers and read codes in a test environment.
How Long Should Codes Last?
Keep the OTP code expiry time short, usually a few minutes. A short window limits the value of a stolen code. Pair the expiry with an attempt limit to stop brute force.
| Rule | Recommendation |
|---|---|
| Code length | 6 digits or more |
| Expiry | A few minutes |
| Attempt limit | Small, with lockout |
| Reuse | Never |
What Errors Should You Handle?
Handle expired codes, invalid codes and rate limits. Return clear messages so users know what to do. For a list of common errors, read our guide on SMS verification errors.
How to Test Your OTP Feature
- Test delivery with real numbers
- Test expiry with a short timer
- Test attempt limits and lockouts
- Test the resend path
- Test with virtual numbers for automation
Frequently Asked Questions
Should I store OTPs in plain text?
No. Hash or encrypt them before storage.
How many digits should an OTP have?
At least six. More digits add security but hurt usability.
Can I reuse a code?
Never. Delete it after successful verification.
What if the SMS fails?
Offer a voice call fallback and a resend option with rate limits.
Conclusion
Build OTP verification feature by generating a random code, storing it with a short expiry and attempt limit, delivering it by SMS, and verifying it once. Use the SMS-COOL API docs to test delivery and codes.
Related reading: https://sms-cool.com/blog.