BIP39 mnemonic checker: words, length and checksum
A BIP-39 mnemonic — the 12 or 24 “recovery words” behind most crypto wallets — is not just a list of random words. The words come from a fixed 2,048-word list, each encodes 11 bits, and the last word carries a few bits of checksum derived from SHA-256 of the rest. This BIP39 mnemonic checker verifies all three: that the phrase has a legal length, that every word is in the English list (with suggestions for typos), and that the checksum matches. It also lets you look up any word’s number, autocomplete a word from its first letters, and generate a new random phrase for testing.
Please read the red notice above the input. The words of a funded wallet are the wallet. This page is built to be safe — it runs entirely in your browser, makes no network requests, does not save anything and never turns words into keys or addresses — but you cannot verify that by looking at a web page, and neither can anyone else. Use it with test phrases, with the public examples from the BIP-39 specification, or to learn how the checksum works.
It is informational only and not investment or security advice.
How to use the BIP39 Mnemonic Checker
- Read the warning. Then type or paste a test phrase into Mnemonic words, or press Use the public test phrase.
- The result updates as you type: a badge for valid or invalid, and notes explaining the length, any unknown words (with suggestions) and the checksum.
- Turn on Hide words while typing if someone can see your screen; the word table is hidden too.
- Use Word lookup to autocomplete from a prefix, find a word’s number (1–2048) or find the word for a number.
- To create a phrase for a test wallet, tick the acknowledgement, choose a length and press Generate. Nothing is stored; the phrase disappears when you leave the page.
BIP39 Mnemonic Checker features
Full checksum verification
Entropy is rebuilt from the word indexes and hashed with SHA-256 via Web Crypto; the first ENT/32 bits must match the end of the last word.
Embedded English wordlist
All 2,048 words of the official BIP-39 English list are inside the page, so no download is needed.
Typo suggestions
An unknown word is matched by its first four letters and by one-letter edits — “abandn” suggests “abandon”.
Bit-level table
Each word’s index and 11 bits are shown, with the checksum bits highlighted in the final word.
Gated test-phrase generator
128 to 256 bits of entropy from crypto.getRandomValues, enabled only after you confirm the phrase is for testing.
Nothing derived, nothing stored
No seed, private key, xpub or address is ever computed, and no localStorage is used.
How the BIP-39 checksum works
A wallet starts with random entropy: 128 bits for 12 words up to 256 bits for 24. It appends the first ENT ÷ 32 bits of SHA-256(entropy) as a checksum, then cuts the result into 11-bit chunks; each chunk (0–2047) picks a word from the list. For 12 words that is 128 + 4 = 132 bits = 12 × 11.
Checking reverses the process: map each word to its index, join the bits, split off the checksum, hash the entropy and compare. The famous test phrase “abandon” × 11 + “about” is all-zero entropy: SHA-256 of 16 zero bytes begins with 0x37 = 0011…, so the checksum is 0011 = 3, and word number 4 in the list (index 3) is “about”.
| Words | Entropy | Checksum | Possible last words |
|---|---|---|---|
| 12 | 128 bits | 4 bits | 128 |
| 15 | 160 bits | 5 bits | 64 |
| 18 | 192 bits | 6 bits | 32 |
| 21 | 224 bits | 7 bits | 16 |
| 24 | 256 bits | 8 bits | 8 |
What the checksum can and cannot tell you
- It catches most single-word typos: a random wrong word passes the 4-bit check of a 12-word phrase only 1 time in 16, and the 8-bit check of a 24-word phrase 1 time in 256.
- It cannot tell you which word is wrong, and two swapped words can still pass by chance.
- A valid checksum says nothing about whether the phrase belongs to a wallet with funds. Optional BIP-39 passphrases (the “25th word”) are not covered at all.
- Because the last word is partly checksum, you cannot choose 12 words yourself; a hand-picked phrase is almost certainly invalid, and that is a good thing.
Safer ways to work with a real recovery phrase
Write it on paper or steel, never in a photo, a notes app or a password manager synced to the cloud. Check it by restoring on the hardware wallet that generated it — most offer a “check backup” function that verifies the words offline. Generate long passwords for everything else with the password generator or a passphrase generator, and check deposit addresses before sending with the wallet address validator.
BIP39 Mnemonic Checker FAQ
Is it safe to enter my seed phrase in a BIP39 mnemonic checker?
No — not here and not on any website. This page makes no network requests and stores nothing, but you cannot verify that, and malware or extensions can read what you type. Use test phrases only. If you have entered a real phrase online, move the funds to a new wallet.
Why does my phrase fail the checksum when every word is in the list?
At least one word is wrong or two are swapped. The last word carries checksum bits, so even a correct-looking list of real words will usually fail if any word differs from the original.
How many valid last words are there for a 12-word phrase?
128. The last word holds 7 bits of entropy and 4 checksum bits, so for any 11 words exactly 128 of the 2,048 words complete a valid phrase.
Do I need the whole word or just the first four letters?
Every word in the English BIP-39 list is uniquely identified by its first four letters, and many hardware wallets accept just those. This checker expects full words but suggests the match when you type a prefix.
Can I use the generated phrase for a real wallet?
You should not. The entropy is cryptographically random, but a phrase that has been displayed in a web browser could have been observed. Generate real phrases on a hardware wallet or an air-gapped device.
Does it support other languages?
Only the English list is embedded. BIP-39 also defines Japanese, Spanish, French, Chinese, Korean, Italian, Czech and Portuguese lists; their words are not interchangeable with English ones.