Why Is an OTP Usually Six Digits? The Maths Behind Code Length
Six-digit codes feel standard, but the number is a trade-off. It is long enough that guessing is unlikely within a few tries, and short enough that a person can read it from one screen and type it in without mistakes.
The numbers
| Length | Possible codes | Chance of one random guess |
|---|---|---|
| 4 digits | 10,000 | 1 in 10,000 |
| 6 digits | 1,000,000 | 1 in 1,000,000 |
| 8 digits | 100,000,000 | 1 in 100,000,000 |
Why length alone is not the answer
A six-digit code with unlimited tries can be guessed by a script. A four-digit code with strict limits can be safe enough for a low-risk step. What protects users is the combination of a short life, a small number of attempts and a fresh code on every request.
What attackers actually do
- Try many codes quickly if the server allows it.
- Trick the user into reading the code aloud.
- Intercept messages through a SIM swap or malware.
What developers should do
- Generate the code with a secure random source.
- Expire it in minutes.
- Allow only a few wrong tries.
- Block reuse of an old code.
- Log failures and alert on bursts.
When to use longer or different codes
For high-risk actions, use a stronger factor such as an authenticator app or passkey. See how one-time passwords are generated and the OTP best practices.
Real-world risk
In practice, most account takeovers do not involve guessing codes at all. They involve talking people into reading the code out, or taking over the phone number. Better code length helps little against those, which is why user education and stronger factors matter.
A quick design checklist
- Six digits by default.
- Five attempts at most.
- Five minutes of life.
- New code for every request.
- Alerts for repeated failures.
Questions people ask
Is an eight-digit code safer?
Slightly, if the attempt limits are the same. It is harder to type.
Why do some codes have letters?
They allow more combinations per character, but they are harder to read aloud and type.
Related guides and pages
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