Password Strength Calculator
Calculate Password Strength values quickly with a free online utility tool.
This password strength calculator estimates a password's entropy (a measure of unpredictability) based on its length and character variety, and translates that into an approximate time a brute-force attack would take to guess it.
Enter a password (evaluated locally, never transmitted), and the calculator returns its estimated entropy and crack-time range.
Worked Calculation Examples
| Scenario | Result | Calculation Step |
|---|---|---|
| "Tr0ub4dor&3" (11 chars, full character set) | ≈72.1 bits, centuries to crack offline | log2(94) ≈ 6.55. 11 × 6.55 ≈ 72.1 bits - strong against brute force, though this ignores dictionary-based attacks against recognizable word substitutions. |
| "correcthorsebatterystaple" (25 lowercase letters) | ≈117.5 bits, effectively uncrackable by brute force | log2(26) ≈ 4.7. 25 × 4.7 ≈ 117.5 bits - despite a smaller character set, the extra length produces far higher entropy than the shorter, more complex password above. |
How Password Entropy Is Calculated
Entropy (in bits) = length x log2(character set size). A password using only lowercase letters has a character set of 26, while one mixing uppercase, lowercase, numbers, and symbols has a much larger character set - both length and character variety increase entropy, but length has an outsized effect since it multiplies rather than just adding to the exponent.
Why Length Matters More Than Complexity Rules
A long password made of random words (a passphrase) often has higher entropy, and is easier to remember, than a short password crammed with mandatory symbols and numbers - modern security guidance increasingly favors encouraging length over rigid complexity requirements for exactly this reason.
How to Use the Password Strength Calculator
- Open the Password Strength Calculator and enter the values requested in the input fields.
- Check the units, percentages, dates, or time periods before reading the answer.
- Review the instant result and adjust any value to compare another scenario.
- Use the formula, example, and FAQs below to understand how the password strength calculator works.
Frequently Asked Questions
How is password entropy calculated?
Entropy (bits) = password length x log2(character set size), where character set size depends on which types of characters (lowercase, uppercase, numbers, symbols) are used.
Does a longer password matter more than a complex one?
Length has an outsized effect on entropy compared to added complexity rules, which is why a longer passphrase of random words is often both stronger and easier to remember than a short, complex password.
Does high entropy mean a password is safe from all attacks?
No - entropy only measures resistance to brute-force guessing. A password can have decent entropy but still be unsafe if it was exposed in a data breach or matches a commonly used password pattern, which attackers check first.
How do I use this password strength calculator?
Enter the known values, review the units or settings, and the calculator updates the result instantly. The formula and example on this page show how the answer is produced.
What does the Password Strength Calculator calculate?
Calculate Password Strength values quickly with a free online utility tool. It is designed for fast browser-based calculations without sign-up, downloads, or manual spreadsheet setup.
What formula does this password strength use?
The formula is: Entropy (bits) = Length × log₂(Character Set Size). This entropy formula is a direct application of Claude Shannon's 1948 information theory, and its practical password-security implications were popularized far beyond cryptography circles by a widely shared 2011 webcomic ('correct horse battery staple') from xkcd creator Randall Munroe, which illustrated - correctly, per the math - that a long passphrase of random common words can have higher entropy, and be far easier to remember, than a short password crammed with mandated symbols and numbers.
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Last updated: September 27, 2026.