BIP39 Mnemonic Seed Generator

Overview

Generates BIP39 mnemonic seed phrases of 12 to 24 words from cryptographically random entropy, computing the checksum exactly as the Bitcoin BIP 39 specification defines it and indexing the embedded 2048-word English wordlist. Everything runs locally, and the tool itself warns that a browser is not the place to create a seed phrase you will actually use with funds.

How It Works

Pick a word count - 12, 15, 18, 21 or 24 - and the phrase regenerates immediately into the read-only output box. Click the box to copy the words. Each change of word count draws entirely new entropy, and the words come from the standard BIP39 English list, so the phrase you see uses the same vocabulary your hardware wallet does. There is no save, no network call, and no passphrase option.

Step-by-Step Usage Guide

1. Choose the word count. Twelve words (128 bits of entropy) is the common wallet default; twenty-four (256 bits) is the maximum.
2. Read the phrase from the output box. Words are space-separated and drawn from the official 2048-word English list.
3. Click the box to copy the phrase to the clipboard, then paste it only where you intend it to live.
4. Change the word count to draw a completely new phrase - there is no reuse of entropy between generations.

Technical Specifications & Standards

The construction follows BIP 39 precisely. For an n-word phrase, entropy is (n / 3) x 32 bits - 128 bits for 12 words up to 256 bits for 24 - drawn from crypto.getRandomValues. A checksum of exactly entropy/32 bits is taken from the SHA-256 of the entropy, so 12 words carry a 4-bit checksum. The concatenation of entropy and checksum, 132 bits for 12 words, is split into eleven-bit groups, and each group indexes the 2048-word list, which is why eleven bits (log2 of 2048) is the word size and why only lengths divisible by three exist. This tool embeds the official English wordlist verbatim, in the standard order that begins abandon, ability, able. What it deliberately does not do is take the next BIP 39 step: stretching the mnemonic into a 512-bit seed with PBKDF2-HMAC-SHA-512 over 2048 iterations with the passphrase as salt. No passphrase support means no 25th-word protection either. The practical security note is the one the UI prints: a phrase generated in a browser tab has unknown exposure to screen capture, extensions and compromised machines. Generate here to study the mechanism or to produce test fixtures; generate real seeds on a hardware wallet.

Targeted Use Cases

• Studying how entropy, checksum and wordlist indices combine in BIP 39, with a live phrase that changes as you alter the word count.
• Producing syntactically valid seed phrases as test fixtures for wallet software, address derivations and unit tests.
• Verifying that a wallet's wordlist matches the standard English list by comparing a few indices.
• Checking that a validator correctly accepts phrases of all five standard lengths and rejects checksum failures.

Notes & Gotchas

• Never use a browser-generated phrase with real funds. Screen capture, malicious extensions and clipboard monitors are all realistic threats that a hardware wallet removes.
• Treat any phrase you generate here as public test data. If you have typed one you care about, generate a replacement on trusted hardware and discard this one.
• Back up real phrases by writing them down in order, on paper, away from cameras - not by screenshots or cloud notes.
• Remember the words encode entropy plus a checksum: one mistyped word usually fails validation, which is a feature worth knowing about.

Frequently Asked Questions