About this tool
The Shannon Entropy Calculator measures the information content of text. It builds a frequency map over the exact characters you enter, computes each symbol's probability p = count / length, and evaluates the Shannon formula H = −Σ p·log2(p) in bits per character. Multiplying by the length gives the total information in bits. It also reports the number of distinct symbols and the theoretical maximum entropy log2(distinct), which the actual entropy approaches only when every symbol is equally likely.
This actual (observed) entropy is distinct from a password-strength estimate, so the tool computes both. For the strength side it uses the charset-pool model: it detects which character classes are present — lowercase (26), uppercase (26), digits (10), common symbols (32), space (1), and other (100) — sums the pool size P, and reports pool entropy = length · log2(P) bits. A qualitative label follows the usual bands: under 28 bits is very weak, 28–35 weak, 36–59 reasonable, 60–127 strong, and 128+ very strong.
All logarithms use log2(x) = Math.log(x) / Math.LN2, and every calculation runs locally in your browser — nothing you type is stored or transmitted. Note the two numbers answer different questions: observed entropy tells you how unpredictable this specific string is given its own character distribution, while pool entropy estimates how hard the string would be to brute-force assuming each position is drawn uniformly from the detected character pool.