# OTP security calculator. - Canonical URL: https://www.broadnet.me/tools/otp-security-calculator - Language: en - Last updated: 2026-10-21 - Publisher: BroadNet (https://www.broadnet.me) - Markdown version of the page above, generated from the rendered page. Free developer tool · runs in your browser Configure a one-time-passcode scheme — length, character set, expiry and attempt limit — and see the exact combination space, entropy in bits, the odds of a successful guess, brute-force resistance, and a strength rating with concrete hardening advice. The math is exact and runs entirely in your browser. Configure your OTP scheme 6 Number of digits or characters in the code (4–12). Alphabet size: 10 How long the code stays valid before it rotates or expires. Verification attempts allowed before the code is locked or invalidated. attempts / second An assumed automated guessing rate. Used only to estimate brute-force time — leave blank if unknown. Try an example Strength analysis Strong Strength rating Total combinations 1,000,000 ≈ 1.000e+6 · alphabet ^ length Entropy 19.9 bits length × log₂(alphabet) Guess chance per attempt 1 in 1,000,000 Chance within the attempt limit(5×) 1 in 200,000 5.00e-4% Time to try every code 13.9 hours Average time to a hit 6.9 hours Attempts possible per validity window: 6.000e+3 The attempt limit caps a real attacker at 5 — the lockout, not the rate, is the binding constraint. At this rate the code expires long before the space can be exhausted — online brute-force is effectively impossible. Security rests on the attempt limit and short expiry, not on the code length alone. Rating thresholds (guess chance within the attempt limit) - Very strong — 1 in 1,000,000 or better (meets the NIST-referenced target) - Strong — better than 1 in 100,000 - Fair — better than 1 in 10,000 - Weak — worse than 1 in 10,000 This weak / fair / strong / very-strong rating is the tool's own heuristic — it is not an external standard. It scores the chance of a successful guess within your attempt limit against the thresholds below. The combination space, entropy and probabilities are exact math. Recommendations - Numeric OTPs are the standard for SMS and are easy for users, but their small space means security depends on rate-limiting and short expiry — not on the code itself. Keep both tight. - Enforce a hard server-side lockout after a few failed attempts. Without it, the 'chance within the attempt limit' above is meaningless and the full space becomes brute-forceable. - Shorten the validity window. A code valid for several minutes gives an attacker far more guessing time than one valid for 30–60 seconds. - For high-value actions such as payments or account recovery, add device binding, step-up auth, or a second factor. An OTP alone is a single shared secret over one channel. The short answer ## How strong is an OTP, really? An OTP's raw strength is just its combination space: the alphabet size raised to the code length. A 6-digit numeric code has one million combinations and about 19.9 bits of entropy — trivially small by cryptographic standards. What actually keeps it safe is not the code but the controls around it: a strict attempt limit before lockout and a short expiry window. With a 5-attempt lockout, the chance of guessing a 6-digit code is 5 in 1,000,000 — roughly 1 in 200,000. Remove the lockout and the same code falls in seconds. This tool computes both the raw math and the practical, lockout-bounded odds so you can see exactly where your scheme's security comes from. ### The math behind OTP strength Total combinations = alphabet ^ length, computed exactly with big-integer arithmetic so even large spaces are shown to the digit. Entropy in bits = length × log₂(alphabet); every extra character adds log₂(alphabet) bits. The chance of a single random guess is 1 ÷ combinations. Across an attempt limit of N, the chance of at least one hit is approximately N ÷ combinations, which we express as a clear '1 in X' and a percentage. If you supply an assumed attacker rate, the time to try every code is combinations ÷ rate, and the average time to a hit is half that — but a real attacker is stopped first by whichever comes sooner: the attempt lockout or the code's expiry. ### What NIST 800-63B says NIST Special Publication 800-63B treats a short OTP as a low-entropy secret whose safety depends on throttling, not length. It directs verifiers to rate-limit failed authentication attempts and references keeping the probability of a successful online guess over the life of the authenticator below roughly 1 in 1,000,000. In other words, a 6-digit code is acceptable only because the verifier limits attempts and expires the code quickly — the length alone is not what makes it secure. The 'very strong' tier in this tool is aligned with that 1-in-1,000,000 target; the tier labels themselves are our own heuristic, not a NIST classification. ### Common OTP schemes compared Exact figures for typical configurations, assuming a 5-attempt lockout. Longer or larger-alphabet codes buy headroom, but the attempt limit does most of the work. | Scheme | Combinations | Entropy | Chance in 5 attempts | |---|---|---|---| | 4-digit numeric | 10,000 | 13.3 bits | 1 in 2,000 | | 6-digit numeric | 1,000,000 | 19.9 bits | 1 in 200,000 | | 8-digit numeric | 100,000,000 | 26.6 bits | 1 in 20,000,000 | | 6-char alphanumeric | 2,176,782,336 | 31.0 bits | 1 in 435,356,467 | | 8-char case-sensitive | 218,340,105,584,896 | 47.6 bits | 1 in 43,668,021,116,979 | ## Frequently asked questions **Is a 6-digit OTP secure?** On its own, no — a 6-digit numeric code has only one million combinations and about 19.9 bits of entropy. It becomes secure in practice through a strict attempt limit and a short expiry: with a 5-attempt lockout the odds of guessing it are about 1 in 200,000. Remove the lockout and it is brute-forceable in seconds. Security comes from the controls, not the digits. **How is OTP entropy calculated?** Entropy in bits equals the code length multiplied by the base-2 logarithm of the alphabet size. A 6-digit numeric code is 6 × log₂(10) ≈ 19.9 bits; an 8-character case-sensitive alphanumeric code is 8 × log₂(62) ≈ 47.6 bits. Each added character contributes log₂(alphabet) more bits. **Why does the attempt limit matter more than length?** Because online OTPs are guessed one request at a time, not computed offline. If the verifier locks the account after 5 wrong tries, an attacker gets at most 5 chances against the whole space regardless of how fast they can send requests. That single control turns a one-million-combination code into roughly 1-in-200,000 odds. Without it, the same code offers almost no protection. **Does a longer or alphanumeric code help?** It raises entropy and shrinks the guess odds, which helps for high-value flows or when rate-limiting is weak. But it also hurts usability — longer and mixed-case codes are harder to type and more error-prone over SMS or voice. For most flows, a 6-digit numeric code with a tight lockout and short expiry is the pragmatic balance; reserve longer codes for high-risk actions. **What does the strength rating mean?** It is this tool's own heuristic, not an external standard. It scores the chance of a successful guess within your configured attempt limit: 1 in 1,000,000 or better is 'very strong' (aligned with the NIST-referenced target), better than 1 in 100,000 is 'strong', better than 1 in 10,000 is 'fair', and anything weaker is 'weak'. The underlying combination space, entropy and probabilities are exact math. **Does this tool send my configuration anywhere?** No. Every calculation runs in your browser using big-integer arithmetic for the combination space. Nothing is transmitted, logged or stored. You can disconnect from the network and it still works. [OTP One-time passcodes over SMS and voice, with throttling and expiry built in.](https://www.broadnet.me/products/otp) [MyOTP Turnkey verification with delivery, retries and rate-limiting handled for you.](https://www.broadnet.me/products/myotp) [Answers Straight answers on OTP, 2FA and messaging security.](https://www.broadnet.me/answers) [Developer hub Quickstart, TypeScript SDK, references and tools.](https://www.broadnet.me/developers) ## Building OTP into your product? BroadNet delivers one-time passcodes over SMS and voice with the throttling, expiry and delivery controls that make short codes safe. Talk to our team about your verification flow. [Request test access](https://www.broadnet.me/signup) [Contact sales](https://www.broadnet.me/contact)