Most password advice is a list of rules with no numbers attached, which is why nobody follows it. This guide attaches the numbers. Everything below comes out of the same entropy calculation the Password Generator shows you as you move the sliders, so you can check any of it live.
What "bits" actually mean
Entropy in bits answers one question: how many guesses would it take to be sure of finding this? Each additional bit doubles that number. Thirty bits is about a billion possibilities; 40 bits is about a trillion.
The useful frame is time. Assume an attacker who has stolen a password database and is guessing offline at 10 billion guesses per second — a reasonable order of magnitude for a decent GPU rig against a fast hash. Then:
- 47 bits (10 random lowercase letters) — under 4 hours
- 52 bits (8 random characters, all four character sets) — about a week
- 78 bits (12 random characters, all sets) — 1.5 million years
- 105 bits (16 random characters, all sets) — longer than the universe has existed, by a factor of about ten thousand
Notice where the cliff is. Eight random characters — which satisfies most websites' rules and which a lot of people consider a strong password — falls in about a week. Twelve is fine. Sixteen is over-engineered in a way that costs you nothing, since you aren't typing it from memory anyway.
Why "P@ssw0rd!" is worse than it looks
Character-substitution tricks feel like they add strength. They add almost none, because the entropy figures above assume random characters, and a mangled dictionary word isn't random.
An attacker's first pass isn't brute force through all 94 characters. It's a wordlist of common passwords, then the same wordlist with predictable substitutions applied — a for @, o for 0, i for 1, s for $, a capital at the front, a digit and a bang at the end. Those transformations are in every cracking tool by default, in a rules file that has been tuned for twenty years. "P@ssw0rd!" is not a 9-character random string worth 59 bits; against a real attack it is a common word plus a handful of predictable rules, and it falls essentially instantly.
This is the single most important thing to understand about password strength: entropy is a property of how the password was generated, not of how it looks. A password that looks messy but came out of your head is weak, because heads are predictable. A password that came out of a cryptographic random number generator is exactly as strong as the arithmetic says.
Passphrases, and the honest numbers
Switch the generator to passphrase mode and you get something like hurry-unify-enamel-rebel-daily-luncheon. Words are drawn from the EFF's 7,772-word list using the browser's cryptographic RNG, so each word contributes log2(7,772) = 12.92 bits:
- 3 words — 38.8 bits — 47 seconds. Not enough. Don't.
- 4 words — 51.7 bits — about 4 days. Still not enough on its own.
- 6 words — 77.5 bits — roughly 700,000 years. This is the default, and it's the right default.
- 8 words — 103.4 bits — comfortably beyond any physical attack.
Two things worth being straight about here, because plenty of sites aren't.
First: a passphrase's strength comes from the number of words and the size of the list, not from its length in characters. Six lowercase words is around 39 characters, and if you scored that by counting characters you'd get about 141 bits. The honest figure is 77.5. An attacker who knows you generated a passphrase — and you should assume they do, because these generators are public — guesses words, not letters. The generator on this site measures it the honest way; a lot of strength meters don't, and will happily tell you a four-word phrase is "very strong".
Second: capitalising the words or changing the separator adds essentially nothing. They're single choices applied to the whole phrase, not independent random choices per word, so they contribute a couple of bits at most. Use them if they make the phrase easier to type. Don't count on them.
When a passphrase is the right tool: any password you have to type by hand or say to someone — your device login, your password manager's master password, WiFi, a disk encryption key. Six random characters are miserable to type on a TV remote; six words are fine. For everything that lives in a password manager and gets pasted, use 16 random characters instead. There's no reason to make it memorable.
The five accounts to fix first
Unique passwords everywhere is the correct answer and it's also why people give up. If you're going to fix five accounts this week, fix these, in this order:
- Your email. It is the master key to everything else — every "forgot password" link on every other service ends up there. An attacker with your email doesn't need any of your other passwords. This one gets a unique passphrase and two-factor authentication before anything else on the list.
- Your password manager, if you have one. Same logic. A six-word passphrase you can actually remember, written down on paper and kept somewhere physical if you need to.
- Your phone account. The overlooked one. Whoever controls your mobile number can intercept SMS codes and talk a carrier into a SIM swap, which unwinds most two-factor setups.
- Banking and anything holding a card. Obvious, but note it ranks below email — because email can reset banking, and banks tend to have fraud reversal that email providers don't.
- Anything reusing an old password. Credential stuffing — taking a password leaked from one breach and trying it everywhere — is the most common way ordinary accounts get taken, and it costs an attacker nothing. Reuse is the actual vulnerability; complexity is a distant second.
The bulk generation mode is genuinely useful here: generate ten at once, paste them into your manager as you work down the list, and close the tab. Nothing is stored, and there is no history to clear because nothing left your browser.
Handing out WiFi without reading it aloud
A strong WiFi password is a social problem. Nobody wants to dictate hurry-unify-enamel-rebel-daily-luncheon across a room, and the temptation is to set something short instead.
The QR Code Generator has a WiFi mode that solves it. Enter the network name, the password and the security type, and it produces a QR code that both iOS and Android cameras recognise natively — point, tap, connected. No app, no typing.
Print it and stick it inside a kitchen cupboard, or on the back of a guest room door. Practically, it means you can run a genuinely strong WiFi password and a guest network without anyone ever needing to know the string. It's a small thing that removes the main reason people keep weak WiFi passwords.
Both tools run entirely in your browser. The QR code is drawn on a canvas on your device, and the password is generated by crypto.getRandomValues() locally — neither is transmitted anywhere, which is a property you'd want from a password generator and which is worth checking in your browser's network tab rather than taking on faith.
What none of this protects you from
Entropy measures the generator. It cannot see the rest of your life. A 105-bit password typed into a convincing fake login page is gone. One reused across two sites is only as strong as the weaker site's security. One stored in a note on your desktop is a filing problem, not a cryptography problem.
Two-factor authentication does more for your actual safety than any amount of extra password length, because it defeats the attack that actually happens to ordinary people — someone using a password that leaked somewhere else. Get email and banking onto an authenticator app, and you have done more than upgrading every password on your list from 12 to 16 characters.
The exact entropy formulas and the crack-time assumptions used above are written out on the methodology page.