Free Tool
Password Entropy Calculator
Entropy is the number that measures how hard a password is to guess, in bits. Enter a password — or build one from a length and character sets — and this calculator works out its entropy, explains what the number means, and shows why the commonly quoted targets of 50, 60 and 80 bits disagree.
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What this tool does
Entropy is measured in bits, and each bit doubles the number of possible passwords. A password with 40 bits has about a trillion possibilities; one with 80 bits has about a million trillion trillion. The number comes from two things: how many characters the password has, and how many different characters each position could be. Ten digits give about 3.3 bits per character; all 95 printable keyboard characters give about 6.6 bits per character.
This calculator lets you work it out two ways. Type a password and it estimates the entropy from its actual length and character mix. Or pick a length and tick the character sets — lower-case, upper-case, digits, symbols — and it shows the entropy of a random password of that shape, which is the number that matters when a password is truly random. It also explains the three targets you will see quoted online — around 50, 60 and 80 bits — and why they disagree: they assume different attackers, different password storage and different time horizons.
Entropy is a property of how a password was chosen, not just of the text itself. A password like password123 looks like it has lots of characters, but because it is one of the first things attackers try, its real entropy is close to zero. Randomness is what entropy measures, and a calculator can only estimate it from the characters it sees — which is why the result is always an estimate, and why patterns matter as much as length.
Try it
Either type a password, or build one from a length and character sets.
Waiting. Enter a password, or a length and character sets, and press Calculate entropy.
How it works
The maths is simple. If each character can be one of N possibilities and the password has L characters, the number of possible passwords is N raised to the power of L, and the entropy in bits is L times log base 2 of N. A 15-character password drawn from all 95 printable characters has about 15 times 6.6, or roughly 99 bits. A 15-character password using only lower-case letters has about 15 times 4.7, or roughly 70 bits. The same length, very different strength — the character set matters as much as the length.
When you type a real password, the calculator estimates N from the character groups it actually contains, and counts L from its length. That estimate assumes the characters were chosen randomly. If they were not — if the password is a word, a name or a pattern — the real entropy is lower than the estimate, sometimes far lower. That is why the calculator warns you about patterns instead of trusting the number alone, and why the strength checker on this site looks at patterns too.
The three targets come from different threat models. Around 50 bits is enough against an online attacker who can only try a few guesses before being locked out. Around 60 bits is a common recommendation for passwords that might be stolen and attacked offline at high speed. Around 80 bits is the cautious target for a password that must survive years of improving hardware. None of them is wrong — they answer different questions, which is why the guide on password entropy reconciles them instead of picking one. For everyday use, a 15-character random password or a four-word random passphrase clears the 60-bit bar comfortably.
Limitations
Entropy is an estimate of guessing difficulty, not a guarantee of safety. It assumes the attacker guesses at random; real attackers try common passwords, words and patterns first, so a familiar password has far less real entropy than this calculator shows. The calculator also cannot know whether your password was already exposed in a breach — a leaked password has zero effective entropy, however many bits it appears to have.
- The calculation treats every character as equally likely. Human-chosen passwords are not random, so real entropy is usually lower than the estimate.
- Crack times depend on the attacker's hardware and on how the password was stored — both vary enormously and neither is visible to this tool.
- Entropy says nothing about reuse: a high-entropy password used on ten accounts is one breach away from ten compromised accounts.
Examples
Example 1 — 8 digits, a PIN. Eight digits, each one of ten possibilities: 8 times log2(10), about 27 bits. That is roughly 100 million possibilities — enough against an online login with lockouts, but an offline attacker trying ten billion guesses a second would finish in a hundredth of a second. This is why a short PIN is fine for a phone lock screen and useless as a website password.
Example 2 — 12 characters, mixed case and digits. Twelve characters from 62 possibilities: about 71 bits, or roughly two thousand trillion trillion possibilities. That clears the 60-bit target and would take an offline attacker centuries at ten billion guesses a second. It is a solid password — provided it was chosen randomly and not built from a word plus a number.
Example 3 — four random words. Four words drawn at random from a list of about 7,700 words: each word adds about 13 bits, for about 52 bits total. That looks lower than the mixed 12-character password, but it clears the 50-bit bar, is far easier to remember, and is exactly the passphrase approach modern guidance recommends. Add a fifth word and you are near 65 bits.
Frequently asked questions
How many bits of entropy should a password have?
It depends on the threat model. Around 50 bits is a common floor for online-only attacks, 60 bits is a widely quoted recommendation for passwords that might be attacked offline, and 80 bits is the cautious target. A 15-character random password or a four- to five-word random passphrase clears the 60-bit bar.
What is a bit of entropy?
One bit doubles the number of possibilities. A password with 40 bits has about a trillion possible values; with 50 bits, about a thousand trillion. Each extra bit makes guessing twice as hard.
Does a long password always have high entropy?
Only if it was chosen randomly. A long password made of a common phrase or a repeated pattern has far less real entropy than its length suggests, because attackers try familiar text first.
Why do sources disagree about 50, 60 or 80 bits?
Because they assume different attackers: a slow online login, a fast offline attack on a stolen password file, or a password that must survive years of faster hardware. The number depends on the threat, not just the password.
Is entropy the same as password strength?
Almost, but not quite. Entropy measures guessing difficulty for a randomly chosen password. Real strength also depends on whether the password was leaked, reused, or built from a predictable pattern — none of which entropy alone can see.