OTP Generator

What Is an OTP Generator

A one-time password (OTP) is a short code that is valid for a single use, typically delivered by SMS, email, or a push notification. This generator creates random OTPs in three character sets: numeric codes such as 482913, alphanumeric codes such as K7QW2M, and hexadecimal codes such as A3F09C. Codes are produced with the Web Crypto API, so every value comes from a cryptographically secure random source rather than the predictable Math.random() used by quick scripts.

If you are looking for time-based codes that rotate every 30 seconds inside an authenticator app, that is a different job and is handled by our OTP Code Generator, which implements TOTP from a shared secret. This tool generates standalone random passcodes, the kind you send in an SMS or store against a test account.

How to Use It

1. Pick a character set: numeric, alphanumeric, or hexadecimal.
2. Set the code length between 4 and 12 characters. Six digits is the SMS standard.
3. Choose how many codes to generate, up to 20 per batch.
4. Optionally exclude ambiguous characters so 0, O, 1, and I never appear, which prevents transcription mistakes.
5. Copy an individual code with its copy button, copy the whole batch, or download everything as a text file. Generate New produces a fresh batch at any time.

How Much Entropy Does a One-Time Code Have

The strength of a random code is its entropy: $H = L \times \log_2 A$, where $L$ is the length and $A$ is the alphabet size. A 6-digit numeric code has $6 \times \log_2 10 \approx 19.9$ bits, roughly one million combinations. A 6-character alphanumeric code over 36 symbols has $6 \times \log_2 36 \approx 31$ bits, over two billion combinations. The result card updates the entropy for your current settings, so you can see exactly how much a longer code or a wider alphabet adds.

Those numbers are why one-time codes are always paired with an expiry and an attempt limit on the server side. Nineteen bits is trivially brute-forceable if an attacker can guess forever, but it is more than enough when the code dies in five minutes and the third wrong guess locks the flow.

The Difference Between a Static OTP and TOTP

A static OTP is generated once, sent to the user, and verified against that stored value until it expires. TOTP derives a code from a shared secret and the current time, so nothing has to be stored per user and the code rotates automatically every 30 seconds. Static OTPs are simpler and work over SMS or email with no clock synchronization; TOTP is stronger against interception because a leaked code expires almost immediately. When you need TOTP with QR code enrollment, use the OTP Code Generator instead.

Choosing a Character Set and Length

Numeric codes win on usability: they fit numeric keypads, are easy to read aloud, and are what users expect from SMS verification. Six digits is the near-universal convention, and eight digits is a sensible upgrade when the code survives more than a few minutes. Alphanumeric codes pack roughly 50 percent more entropy per character, which matters when the code must live longer or travel through a lower-trust channel. Hexadecimal is the natural choice when the code doubles as an identifier in technical systems, for example a pairing token that is also logged as a hex string.

Where Random OTPs Are Useful

Developers and testers use batches of codes to exercise sign-up and phone-verification flows without sending real messages. Teams use them for Wi-Fi guest passes, event door codes, and one-time discount vouchers. Support desks use them to hand out temporary access while a real account issue is resolved. In every case, generate the codes in a secure context, store only a hash of the value against the user or ticket, give the code a short expiry, and let it burn after one successful use.

Best Practices for One-Time Codes

• Keep the validity window short, five to ten minutes covers most flows.
• Limit verification attempts per code and per account.
• Compare submitted codes in constant time, or at least rate limit aggressively.
• Never log the OTP alongside the account it belongs to.
• Use a cryptographically secure source such as this tool or your server framework's built-in generator, never Math.random() or rand().

Related tools worth having nearby: the Strong Password Generator for permanent credentials, the PIN Generator for numeric PINs, the Random String Generator for arbitrary tokens, and the UUID Generator Tool for unique identifiers.