How Strong Is My Password?

Why most strength meters are wrong

The bar you see under a password box on most websites is counting character classes. Is there a capital? A digit? A symbol? Is it more than eight long? Four ticks, green bar, off you go.

By that measure Password1! is excellent. It is also somewhere in the first few thousand guesses of any real attack, because attackers do not work alphabetically — they work from lists of passwords people actually used, and from dictionaries with the obvious substitutions already applied.

A meter that calls that password strong is worse than no meter at all. It sends somebody away reassured.

What this one does instead

It tries to describe your password the way a guessing program would, and prices each description.

A word from a dictionary is one guess to software that has the dictionary, times the number of ways it might have been capitalised. A four-digit year is about 130 possibilities, not ten thousand. qwerty is a handful. Only the parts where no pattern is found are priced as though every character had to be tried.

Then it finds the cheapest way of cutting the password into those pieces, because an attacker gets to use the cheapest one. That is the number reported, rather than the flattering one you get by pretending the whole thing was random.

What the substitutions really cost

That last row is the point. The rules a form enforces and the thing that makes a password hard to guess are only loosely related.

The number is an upper bound

Worth stating plainly, because it cuts the other way from how these tools usually shade things.

This knows a list of common passwords and a list of ordinary English words. It does not know your dog's name, your street, the football club on your wall, or the password you also used in 2019 that is now in a breach file somebody has. A real attacker targeting you specifically may know all of those.

So where it finds no pattern, it assumes the characters were chosen unpredictably — which is the best case. A password can be weaker than this says. It is rarely stronger. Treat a good score as "no obvious problem found", not as a guarantee.

Here is that limitation with a number on it. The dictionary behind this page holds 1,252 words. Type correcthorsebatterystaple — the famous example — and it scores 107 bits, because only one of those four words is in the list and the rest is priced as unpredictable letters. Against a cracker carrying an ordinary twenty-thousand-word English vocabulary, four words run together is nearer 57 bits, and against the two-thousand-word list the example was drawn from, 44. All three are strong enough in practice; the point is that the number here is the generous end of the range, and a bigger dictionary would only ever move it down.

Why the generator on this site quotes a lower number

Paste a passphrase from our password generator into this page and the two will disagree. Venture-Shine-Desk-Castle-Snow is 51 bits according to the generator and 65 here. Both are right, and the difference is worth understanding.

The generator knows exactly how it built that password: five words drawn from a known list, always capitalised, always joined with the separator you picked. Nothing else was ever a possibility, so 51 bits is what it is.

This page is looking at a string with no idea where it came from, so it has to allow that you might have capitalised differently (5 bits across five words), that the separator was one of about fifteen plausible ones (3.9 bits), and that the pieces could have been arranged another way (4.6 bits). Thirteen and a half bits of not knowing.

Trust the lower figure. An attacker who works out that you generate passphrases stops paying for any of that, and planning for the attacker who has already guessed your method is the only safe way round.

What the time to guess assumes

A trillion guesses a second, against a stolen database, offline. That is the pessimistic case and it is the one worth planning for, because it is what happens when a company is breached.

Against a live login form nobody gets more than a few tries a second before being locked out, so the times would be astronomically longer. The offline number is the one that matters, since you do not get to choose how the site you signed up to stores your password.

The advice that survives all of this

Length beats complexity: each extra character multiplies the work by the size of the alphabet, while a symbol added to a short password multiplies it by rather less.

Unrelated words beat clever substitutions, as long as something else chose the words.

And the thing that actually loses accounts is not guessing at all — it is reuse. A password that appears in one breach opens every other account it was used on, whatever it scores here. A different password everywhere, kept in a password manager, matters more than any single password being magnificent.

Nothing is transmitted

Typing a real password into a website is normally a bad idea, and you should be suspicious of any page that asks you to. The reason it is safe here is structural rather than a promise about our conduct: there is no server. The page loads, the checking runs in your browser, and no request is made with anything you typed. Turn off your connection after the page loads and it still works.