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What is password entropy? How to measure password strength in bits

Password entropy measures password strength in bits, representing the logarithm (base 2) of the number of possible combinations. A truly random 8-character password from a 95-character set has log2(95^8) ≈ 52 bits of entropy. Adding one bit of entropy doubles the attacker's work. 2^28 (28 bits) is trivial. 2^50 (50 bits) is moderate. 2^80 (80 bits) is strong against any attacker.

Security domain
Fundamentals
How passwords and hashing work
Topic focus
Password entropy
password-entropy

How this is calculated

Entropy calculation assumes the password is truly random. Human-chosen passwords have far less entropy than their length suggests because humans are bad at randomness. The password 'Tr0ub4dor&3' looks strong but was generated by a human following predictable patterns (capitalize first letter, common substitutions 0→o, 4→a, & for a, append a symbol and number). Its actual entropy against a targeted attack is much lower than the theoretical 52 bits. The only way to get full entropy is to generate passwords randomly, either from a password manager or with dice for passphrases. This is why password managers that generate random strings give you more security per character than human-chosen passwords of the same length.

Verdict

Entropy is the technically correct way to measure password strength, but remember that human-chosen passwords always have lower entropy than their length suggests. Randomness generators give you the entropy you paid for. Humans don't.

More Passwords scenarios

Password length guide
password-length
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Passphrase vs password
passphrase-vs-password
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Password manager guide
password-manager
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NIST guidelines
nist-guidelines
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Brute force resistance
brute-force
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2FA vs strong password
2fa-vs-password
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Password hashing
password-hashing
View details ➜
Password mistakes
password-mistakes
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Biometrics vs passwords
biometrics
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Password generators
generator-tools
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Password reuse risks
password-reuse
View details ➜

Frequently asked questions

How long should a secure password be?
16 characters or more, drawn from uppercase, lowercase, numbers, and symbols. A 16-character mixed password is currently impractical to brute-force with commodity hardware, while 8-character ones can be cracked in hours by modern GPUs.
Is the generated password actually random?
Yes. The generator uses the browser's crypto.getRandomValues(), a cryptographically secure random source backed by OS entropy. The output is suitable for production password managers, API keys, and seed phrases.
Is my password saved anywhere?
No. The entire generator runs in your browser, with no network requests, no logging, and nothing stored. Close the tab and the password is gone from memory. Check your browser's Network tab to verify there are no outbound calls when a new password is generated.
What's the difference between a passphrase and a password?
A passphrase is a string of dictionary words (like "correct-horse-battery-staple"), long but memorable. A password is usually shorter with mixed character classes (like "Kx9$mQ2!pR"). Passphrases are typically stronger per character of memory effort, and this tool generates both styles.
Should I use the same password everywhere?
Never. Use a password manager (Bitwarden, 1Password, Apple Passwords) and generate a unique strong password for every site. Password reuse is the single biggest cause of account takeover. One breached service leaks the credential, and every other account using it is automatically compromised.
How often should I change my passwords?
Modern guidance (NIST SP 800-63B) says don't rotate strong unique passwords on a schedule. Only change them if you suspect compromise. Forced rotation encourages weak, incremented passwords. Strong password + password manager + 2FA is safer than any rotation policy.

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