about 122 bits of entropy
About strong passwords
What makes a password hard to guess is how many equally likely possibilities it was drawn from, which is what the entropy figure above counts. Length buys far more of it than variety does: adding characters multiplies the possibilities, while adding a symbol to a short password barely moves the number. A twenty character password of lower case letters is harder to break than an eight character one using everything on the keyboard.
That figure is only honest if the password really was drawn at random. A word with letters swapped for lookalikes has almost none of the entropy its length suggests, because the guessing starts from the word. Everything here is drawn character by character from the sets you choose, with at least one from each so a password that claims to contain a digit does, and the result is shuffled so the first characters do not always follow the same pattern.
Lookalike characters are left out by default, since a password you may have to read aloud or copy by hand should not depend on telling a capital I from a lower case l. Turn them back on if the password only ever gets pasted.
Where the randomness comes from
This tool runs entirely in your browser, using its built in cryptographic random number generator. Nothing you type is sent to us, nothing is stored, and each result is thrown away the moment you leave the page. Numbers are drawn without the rounding bias that the usual shortcut introduces, so every outcome is as likely as every other.
Because nothing is recorded, a result here proves nothing to anyone afterwards. If you need a drawing that other people have to accept, for a prize, a campaign or an audit, use Winner Picker, which publishes each result on a permanent link with a signed record anyone can check.