Short Answer

Six digits is a practical default for email OTPs when the rest of the system is strict. Pair it with a short TTL, small attempt budget, rate limits, purpose isolation, and one-time use. Use eight digits when the action risk or abuse environment justifies the extra friction.

A longer code increases the search space, but it also makes the user type more characters. The right answer depends on both the code space and the number of verification attempts your backend allows.

The Basic Math

Numeric OTPs have 10 choices per digit. A 6-digit code has 1,000,000 possible values. An 8-digit code has 100,000,000 possible values.

6 digits = 10^6 = 1,000,000 possibilities
8 digits = 10^8 = 100,000,000 possibilities

with 5 failed attempts:
6 digits: 5 / 1,000,000
8 digits: 5 / 100,000,000

That math assumes codes are generated uniformly and attackers only get a small number of tries. If verify attempts are unlimited, code length is not enough.

Attempt Limits Matter More Than Debates

A 6-digit code with five failed attempts is much safer than an 8-digit code with unlimited attempts. Verification should count failed attempts on the active challenge and lock it when the budget is exhausted.

  • Use a cryptographically secure random generator.
  • Store a hash of the code, not the raw code.
  • Expire the challenge after a short TTL.
  • Lock the challenge after a small number of failed attempts.
  • Rate-limit verify attempts by IP, tenant, and email.
  • Mark successful challenges as verified so they cannot be replayed.

Do Not Ignore Typing Friction

Email OTPs are often typed on mobile devices after a context switch. Numeric codes are easy to read aloud, copy, paste, and support. Alphanumeric codes increase possible values, but they also introduce mistakes like O versus 0 or l versus 1.

6 digits

Good default for login, signup verification, and access gates with tight verify limits.

8 digits

Useful for higher-risk actions or environments where you want more margin.

Alphanumeric

Larger code space, but more support friction and more transcription errors.

Long links

Better for click-first flows, but less useful when users need a manual fallback code.

A Practical Starting Policy

For most email OTP products, start with a 6-digit numeric code, a 10-minute TTL, one active challenge per email and purpose, and a small failed-attempt budget. Move to 8 digits for sensitive actions or when risk data says the extra friction is worth it.

Security shape

Code length is one control, not the whole system.

sendotp.email pairs stable email OTPs with purpose isolation, expiration, and verification semantics designed for real product flows.

Read the verify endpointRead about rate limits

FAQ

Is a 6-digit OTP secure enough?

A 6-digit OTP can be appropriate when it has a short TTL, a small failed-attempt budget, rate limits, and one-time use after successful verification.

When should I use an 8-digit OTP?

Use a longer code for higher-risk actions, weaker rate-limit environments, or workflows where the cost of a false accept is unusually high.

Should an email OTP contain letters?

Numeric codes are easier to read, type, and support. Alphanumeric codes increase the space, but also increase confusion between similar characters.

What matters more than code length?

Attempt limits, TTL, rate limits, purpose isolation, and replay prevention matter at least as much as whether the code is 6 or 8 digits.