Password length
What each password length is actually worth, in bits and in time, and where the useful range ends.
Eight characters is the answer to a 1990s question
It is still what most password policies ask for, and it has not been adequate for a long time. A random eight-character password is 51.5 bits and falls to a fast offline attack in 27 minutes. Sixteen characters is 103.1 bits and the same attack takes 171 billion years.
The gap is not a slope, it is a cliff, because entropy grows linearly with length and the search space grows exponentially with entropy. Four more characters multiplies the work by 57,289,761.
Every length
| Length | Entropy | Fast offline | Bcrypt | Rate limited | Verdict |
|---|---|---|---|---|---|
| 6 | 38.7 bits | instantly | 251 days | 69 years | Not usable. |
| 8 | 51.5 bits | 27 minutes | 5.2 thousand years | 520 thousand years | The old minimum, and no longer enough. |
| 10 | 64.4 bits | 144 days | 39 million years | 3.9 billion years | Borderline. |
| 12 | 77.3 bits | 3.0 thousand years | 298 billion years | longer than the universe has existed, by a wide margin | The practical minimum for anything that matters. |
| 14 | 90.2 bits | 23 million years | longer than the universe has existed, by a wide margin | longer than the universe has existed, by a wide margin | Comfortable. |
| 16 | 103.1 bits | 171 billion years | longer than the universe has existed, by a wide margin | longer than the universe has existed, by a wide margin | The sensible default. |
| 18 | 116.0 bits | longer than the universe has existed, by a wide margin | longer than the universe has existed, by a wide margin | longer than the universe has existed, by a wide margin | Beyond the useful range. |
| 20 | 128.9 bits | longer than the universe has existed, by a wide margin | longer than the universe has existed, by a wide margin | longer than the universe has existed, by a wide margin | Symbolic. |
| 24 | 154.6 bits | longer than the universe has existed, by a wide margin | longer than the universe has existed, by a wide margin | longer than the universe has existed, by a wide margin | Well past any threat. |
| 32 | 206.2 bits | longer than the universe has existed, by a wide margin | longer than the universe has existed, by a wide margin | longer than the universe has existed, by a wide margin | Arbitrary. |
Two characters equal one word
A keyboard character chosen at random from 87 possibilities carries 6.443 bits. A word chosen at random from a 7,776-word diceware list carries 12.925. Two characters is 12.89 bits and one word is 12.92 — within half a per cent.
So a 12-character password and a 6-word passphrase are the same strength, and the passphrase is enormously easier to type on a phone. Neither is better; they are the same thing measured differently.
The number that is not on this page
Every figure here assumes every character was chosen at random. A password a person invented carries a small fraction of the entropy its length implies, because people substitute predictably — a 3 for an E, an exclamation mark at the end, a year that is either this one or a birthday. Attackers model that, and the models are good.
Which makes the practical advice short: let a generator choose, and let a manager remember. The length only means what this page says it means if you did not pick the characters yourself.