Password Entropy Calculator
Type a password below to see its entropy in bits, character set size, and estimated time to crack.
Reviewed by the ToolNestr Editorial Team — July 2026
How password entropy is calculated
Password entropy is a quantitative measure of a password's unpredictability, expressed in bits. The concept comes from information theory, where each bit represents a binary decision that halves the search space. The entropy calculation follows a simple formula: E = L × log₂(C), where L is the password's length and C is the number of possible characters at each position.
The character set size depends on which character types the password uses. Lowercase letters alone give C = 26 (∼4.7 bits per character). Adding uppercase doubles it to 52 (∼5.7 bits per character). Including digits brings it to 62 (∼5.95 bits per character). With all printable ASCII symbols included, C = 95 (∼6.57 bits per character). Each additional character type increases the entropy per position, but the most dramatic gains come from increasing password length.
For example, a 12-character password using all 95 printable characters has an entropy of 12 × log₂(95) = 12 × 6.57 = 78.8 bits. This means an attacker would need to try approximately 2^78.8 ≈ 4 × 10^23 combinations in the worst case. Even at 1 billion guesses per second, this would take more than 10 million years to exhaust all possibilities.
Worked example
Understanding entropy bits
Entropy bits represent the number of binary decisions needed to identify a specific password from all possible combinations. A password with 40 bits of entropy has 2^40 possible combinations. Each additional bit doubles the search space. This exponential relationship means that adding a single character to a password can increase its entropy by 5–7 bits, multiplying the cracking difficulty by 32–128 times.
Security researchers generally agree on these entropy benchmarks: fewer than 30 bits is trivially crackable in seconds; 30–50 bits provides fair protection against casual attacks but is vulnerable to determined adversaries; 50–70 bits is strong enough for most purposes; and 70+ bits provides very strong security that would take centuries to crack even with massive computational resources.
It is important to note that entropy calculations assume the password was randomly generated. Human-chosen passwords often have much lower effective entropy because they follow predictable patterns — dictionary words, keyboard walks, dates, names, and common substitutions. An entropy calculator shows the theoretical maximum, but actual security depends on how the password was created. This is why randomly generated passwords are always recommended over user-created ones.
Use cases
Security audits
Evaluate password policies across your organisation by calculating the entropy of sample passwords. Ensure that all accounts meet a minimum entropy threshold of 60 bits or higher.
Account registration
Test new passwords before using them for account sign-ups. Use the live feedback to adjust length and character variety until you reach your target entropy level.
Password policy design
Determine the optimal minimum length and character requirements for your application's password policy by analysing how different rules affect entropy.
Security awareness training
Demonstrate to users why longer passwords matter. Show them in real time how adding characters or character types dramatically increases crack times from seconds to centuries.
Tips for password entropy
Length beats complexity
A 20-character password using only lowercase letters has 94 bits of entropy. An 8-character password using all 95 characters has only 52 bits. Prioritise length even if it means simpler characters. A long passphrase of random words is both stronger and more memorable than a short jumble of symbols.
Avoid patterns and substitutions
Common substitutions like p„„ssw0rd or keyboard patterns like qwerty add very little real entropy because attackers include these patterns in their dictionaries. The entropy formula assumes random selection, not human-chosen variations.
Use a password manager
Password managers can generate and store truly random passwords with 80–128+ bits of entropy for every account. You only need to remember one strong master password. This is the single most effective way to improve your password security posture.
Consider passphrases
A passphrase made of 5–7 random words (like "correct horse battery staple") can achieve 60–80 bits of entropy while being far easier to type and remember than a random string of symbols. The key is randomness — the words must be chosen randomly, not a meaningful sentence.
Online vs offline cracking speeds
The time required to crack a password depends heavily on whether the attacker is attacking the service directly (online) or has obtained a copy of the password hash (offline). Online attacks are severely rate-limited by the server — typically 1–10 login attempts per second before the account is locked. At 1,000 guesses per second, even a modest password with 40 bits of entropy would take over 30,000 years to crack online.
Offline attacks are a completely different story. Once an attacker has the password hash file (through a data breach, SQL injection, or insider threat), they can attempt billions of guesses per second on their own hardware. Modern GPUs can compute MD5 or NTLM hashes at over 100 billion hashes per second. Slow hash functions like bcrypt (10,000 guesses/sec) or argon2 are designed specifically to mitigate offline attacks by making each guess computationally expensive.
This is why this calculator shows three different crack time estimates. The online estimate reflects real-world login attempts. The offline fast hash estimate represents the worst-case scenario where the attacker has breached a server using a fast hashing algorithm (like many legacy systems). The offline slow hash estimate shows the protection provided by modern key-derivation functions. For maximum security, use a password with 70+ bits of entropy and ensure the service uses slow hashing.
| Strength Level | Entropy (bits) | Online Crack Time | Offline Fast Hash |
|---|---|---|---|
| Weak | < 30 | Minutes to hours | Seconds |
| Fair | 30–50 | Days to years | Minutes to weeks |
| Strong | 50–70 | Centuries+ | Years to millennia |
| Very Strong | 70+ | Millions of years | Centuries+ |
Related tools
Generate secure passwords with our Password Generator, check password strength at a glance with the Password Strength Tester, test PIN code security with the PIN Code Strength Checker, or encrypt sensitive data with AES Encrypt & Decrypt.
Frequently asked questions
What is password entropy?
Entropy is a measure of a password's unpredictability, expressed in bits. Each bit of entropy doubles the number of guesses needed to crack it. Higher entropy means stronger security.
How is entropy calculated?
Entropy is calculated using the formula E = L × log₂(C), where L is the password length and C is the size of the character set used (lowercase, uppercase, digits, symbols).
What is a good entropy score?
50+ bits is fair, 60+ bits is strong, and 80+ bits is very strong. Most security standards recommend at least 64 bits of entropy for important accounts.
Why do online and offline crack times differ?
Online attacks are rate-limited by the server (∼1,000 guesses/sec), making them slow. Offline attacks happen on the attacker's hardware (∼1 billion guesses/sec for fast hashes), making them millions of times faster.
Does complexity matter more than length?
No — length matters more. A 20-character password with only lowercase letters has ∼94 bits of entropy. An 8-character password with all character types has only ∼52 bits. Prioritize length.
What character sets are included?
The calculator detects lowercase (a–z, 26 chars), uppercase (A–Z, 26 chars), digits (0–9, 10 chars), and symbols (33 chars) used in your password to determine the effective character set size.
Is my password sent anywhere?
No. All calculation happens entirely in your browser. The password never leaves your device.
What crack speed assumptions are used?
Online attack assumes 1,000 guesses per second (typical rate-limited login). Offline fast hash assumes 1 billion guesses per second (GPU cracking MD5/NTLM). Offline slow hash assumes 10,000 guesses per second (bcrypt/argon2).