I believe it would depend upon the entropy used for the seed, as that would
affect how many bits the checksum represents.
https://github.com/bitcoin/bips/blob/master/bip-0039.mediawiki#Generating_the_mnemonic

So for a 24 word / 256 bit mnemonic the checksum is 8 bits, thus there are
8 valid checksums and if you picked a random checksum from the wordlist of
2048 words you'd have a 1 in 256 chance of picking a valid one.

On Sun, Dec 23, 2018 at 1:44 PM Aymeric Vitte via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> Has anybody already looked at this: given N randomly chosen words
> belonging to a BIP39 2048 words dictionary, what is the probability to
> get a "valid" BIP39 seed (ie with the right checksum)?
>
> The result looks (very) surprising to me and might have some use cases,
> just would like to know if this topic has already been discussed before
> going further
>
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