Password Entropy
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Entropy, measured in bits, quantifies how unpredictable a password is based on the size of the character set used (lowercase, uppercase, numbers, symbols) and the password's length. Higher entropy means more possible combinations an attacker would have to try, making the password harder to guess or brute-force.

Entropy grows exponentially with length but only linearly-ish with character set size, so a longer password using a smaller character set can have more entropy than a shorter, more complex-looking one. This is why security guidance has shifted toward encouraging longer passphrases over short, symbol-heavy passwords.

No, standard entropy calculations assume the password is chosen uniformly at random from the given character set. Real passwords using dictionary words, keyboard patterns, or predictable substitutions (like '@' for 'a') have far less effective entropy than the raw calculation suggests, because attackers check these patterns first.

As a general guideline, 60+ bits is reasonable for most personal accounts, while 80+ bits is recommended for high-value or sensitive accounts, though the right threshold also depends on how the service stores and protects passwords against attack.

For the same entropy value, they offer equivalent theoretical security, but a truly random password is harder to memorize, which often leads to unsafe practices like reuse or writing it down. A sufficiently long random passphrase of unrelated words can offer comparable entropy while remaining memorable.