BIP39 Mnemonic Generator
Generate BIP39 seed phrases for cryptocurrency wallets. Entropy created entirely in your browser.
Do not put real funds behind a phrase generated in a web browser. The entropy here comes from crypto.getRandomValues and nothing is transmitted, but a browser tab is a far larger attack surface than a hardware wallet: extensions, other scripts and your clipboard all sit next to it. Use this to learn, test, and verify — and generate anything you will actually fund on a hardware wallet or air-gapped device. Never share a seed phrase with anyone.
Checked in your browser — nothing is sent anywhere. Never paste a phrase that holds real funds into any website, including this one.
What is a BIP39 mnemonic?
BIP-39 (Bitcoin Improvement Proposal 39) defines how to turn random entropy into a human-readable seed phrase. Take 128–256 bits of entropy, append the first ENT/32 bits of its SHA-256 hash as a checksum, then read the combined bits in 11-bit groups — each group indexes one of exactly 2048 words. A 12-word phrase carries 128 bits of entropy plus a 4-bit checksum; 24 words carry 256 bits plus 8.
That checksum is the part people overlook, and it is what makes a phrase verifiable: change one word and the last few bits no longer agree, which is how a wallet catches a transcription error instead of silently deriving an empty account. It also means a phrase cannot be checked by eye — the words can all be real BIP-39 words and the phrase still be invalid. That is what the checker above is for, and why this page re-derives the checksum of every phrase it generates and shows you the result rather than asking you to take it on trust.
The wordlist is deliberately built so that the first four letters of every word are unique, so a wallet can accept a phrase typed with only those four characters — and so that no two words are close enough to be confused when written by hand.
Built and maintained by Meet Shah · Last updated
What this tool is used for
- Understanding how a seed phrase is structured while learning about wallet backups.
- Producing a throwaway phrase for a testnet wallet or a development environment.
- Checking that a phrase's word count and checksum are consistent.
- Demonstrating the mnemonic format for a talk or documentation.
- Generating a phrase for a wallet you are deliberately treating as disposable.
Frequently Asked Questions
- Where does the randomness come from?
- `crypto.getRandomValues`, the browser's cryptographic generator, entirely in your browser with nothing transmitted. That is the same source used for key generation elsewhere in the platform.
- What is the checksum for?
- BIP-39 appends checksum bits taken from the SHA-256 hash of the entropy, so a mistyped or reordered word fails validation instead of silently deriving a different wallet. The phrase shown here is verified against its own checksum on screen.
- How many words should I use?
- Twelve words is 128 bits of entropy — far beyond brute force. Twenty-four is 256 bits and is what hardware wallets typically default to. The extra words guard against future cryptanalysis, not against today's attacker.
- Should I use a phrase generated in a browser?
- Not for real funds. A browser is a large attack surface — extensions, cached pages, malware — and this tool exists for learning and testing. Real value belongs behind a hardware wallet that generates its seed offline.
- What does the optional passphrase do?
- It is mixed into the seed derivation, so the same words plus a different passphrase produce a completely different wallet. It is genuine protection if the phrase is found, and total loss if you forget it — there is no recovery.
- Can I recover the words from an address?
- No. Derivation runs one way through PBKDF2 with 2,048 iterations. Anyone who has the words has the wallet, and anyone who loses them has lost it — which is why the phrase must never be photographed, typed into a website, or stored in a password manager sync.
Common errors and gotchas
- Using a browser-generated phrase for a wallet holding real value, where hardware generation is the standard advice.
- Photographing or typing the phrase into anything connected, which is how seed phrases are stolen.
- Assuming 12 and 24 words are equivalent, since they carry different entropy.
- Storing the phrase in a password manager whose recovery depends on something less secure.
- Treating the word order as unimportant, when the phrase is worthless scrambled.