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What Is a Seed Phrase? 12 vs 24 Words and How to Store It Safely

A seed phrase is 12 or 24 words that encode your crypto wallet's master secret. Anyone with the words can restore the wallet and move all of its funds.

By TradingDots Editorial 9 min read

Silver metal key lying in front of a stack of gold Bitcoin coins on a white background
In this guide
  1. What Is a Seed Phrase and How Does It Work?
  2. Seed Phrase vs Private Key
  3. The BIP-39 Word List: 2,048 Words and a Checksum
  4. 12 vs 24 Word Seed Phrase: Which Is More Secure?
  5. The Passphrase: Your Optional “25th Word”
  6. Seed Phrase Storage: Paper, Metal or Split?
  7. What Never to Do With Your Seed Phrase
  8. How to Restore a Wallet From a Seed Phrase
  9. Bottom Line
  10. Sources

A seed phrase is a list of 12 or 24 ordinary words that encodes the master secret of a crypto wallet. Anyone who has those words, in the right order, can rebuild your wallet on any compatible device and move every coin in it, so the seed phrase is effectively the wallet itself.

You may also see it called a recovery phrase, secret recovery phrase, mnemonic or backup phrase. They all mean the same thing. The words are generated by your wallet when you first set it up, and the app or device asks you to write them down because it is the only way to get your funds back if the phone breaks or the hardware wallet is lost.

This guide explains where the words come from, how the built-in checksum works, the real difference between 12 and 24 words, what the optional passphrase (the “25th word”) does, how to store your phrase safely and the mistakes that empty wallets every day.

What Is a Seed Phrase and How Does It Work?

Crypto ownership comes down to private keys: secret numbers that let you sign transactions. A modern wallet holds many private keys, one or more for each address and coin. Writing all of them down would be impractical, so wallets generate them from a single master secret instead.

That master secret starts as entropy, which simply means random data. Your wallet produces 128 or 256 random bits (1s and 0s), then converts them into words so a human can copy them accurately. When you restore a wallet, the software turns the words back into the same bits, derives the same master key and regenerates every private key and address in the same order. That is why a phrase written down years ago still restores today’s balances.

Most wallets follow a standard called BIP-39 (Bitcoin Improvement Proposal 39), published in 2013. Despite the name, it is used by wallets for Ethereum, Solana and most other coins too. If you are new to how wallets hold keys, start with our guide to what a crypto wallet is and how it works.

Seed Phrase vs Private Key

People often use the terms interchangeably, but they are different layers of the same system.

  • A private key controls one address. If it leaks, the funds at that address are at risk.
  • A seed phrase generates all the private keys in a wallet, across every account and supported coin. If it leaks, everything is at risk.

The process is one-way. From the seed phrase, the wallet runs a key-stretching function called PBKDF2 (2,048 rounds of HMAC-SHA512) to produce a 512-bit seed. From that seed, a second standard, BIP-32, derives a tree of keys, and “derivation paths” (defined in BIP-44 and related standards) decide which branch is used for which coin. You can go from phrase to keys, but not from a key back to the phrase.

The practical lesson: guard the seed phrase even more carefully than any single private key. Exporting one private key to another app exposes one address; typing your seed phrase anywhere exposes the whole wallet.

The BIP-39 Word List: 2,048 Words and a Checksum

BIP-39 defines a fixed list of exactly 2,048 words. The number is not arbitrary: 2,048 = 2^11, so each word represents exactly 11 bits of data. The English list was chosen so that:

  • The first four letters of every word are unique. “Abandon” and “ability” both start with “ab”, but no two words share the same first four letters, so you can store just four letters per word if space is tight.
  • Similar-looking words are avoided where possible to reduce copying errors.
  • Words are common and easy to spell.

Lists also exist in other languages, including Spanish, French, Japanese and Chinese, but English is by far the most widely supported. Stick with English unless you have a strong reason not to.

How the checksum works

The last word is partly a checksum: a few bits calculated from the rest of the phrase, used to catch mistakes. The wallet takes the SHA-256 hash of your random entropy and appends its first few bits before splitting everything into 11-bit chunks.

  • For 12 words: 128 bits of entropy + 4 checksum bits = 132 bits = 12 × 11.
  • For 24 words: 256 bits of entropy + 8 checksum bits = 264 bits = 24 × 11.

If you mistype a word or swap two words during recovery, the checksum almost always fails and the wallet tells you the phrase is invalid instead of opening an empty wallet. It also means you cannot just pick 12 words you like: only about 1 in 16 random 12-word combinations (and 1 in 256 random 24-word combinations) has a valid checksum. Never create a phrase yourself; let the wallet’s random number generator do it.

12 vs 24 Word Seed Phrase: Which Is More Secure?

A 24-word phrase has 256 bits of entropy; a 12-word phrase has 128 bits. On paper, 24 words is astronomically stronger. In practice, both are far beyond the reach of brute-force guessing, as the worked example below shows.

12 words 24 words
Random entropy 128 bits 256 bits
Checksum bits 4 8
Possible valid phrases 2^128 ≈ 3.4 × 10^38 2^256 ≈ 1.2 × 10^77
Brute-force risk today Not practical Not practical
Ease of backup Faster to write, fits small plates Twice the writing, more room for copying errors
Typical use Many mobile and browser wallets Many hardware wallets by default

BIP-39 also allows 15, 18 and 21 words, which some wallets offer. The extra security of 24 words mainly matters as a margin against unknown future weaknesses. The far bigger risks for real people are theft, phishing and losing the backup, none of which word count changes.

Example: How long would it take to guess a 12-word seed phrase by brute force?

  • Possible phrases: 2^128 ≈ 340,000,000,000,000,000,000,000,000,000,000,000,000 (3.4 × 10^38).
  • Suppose an attacker could test a hypothetical 1 trillion (10^12) phrases per second, far faster than real hardware because each guess needs 2,048 rounds of hashing plus address checks.
  • Time to try them all: 3.4 × 10^38 ÷ 10^12 = 3.4 × 10^26 seconds.
  • One year is about 3.15 × 10^7 seconds, so that is roughly 1.1 × 10^19 years. On average, they would find it halfway through: about 5 × 10^18 years.
  • The universe is about 1.38 × 10^10 years old, so the average search would take around 390 million times the age of the universe.

Now suppose you split a 12-word phrase into two halves and a thief finds the half with the first 6 words. They need 6 more words: 2,048^6 ≈ 7.4 × 10^19 combinations, and the checksum cuts the valid ones to about 4.6 × 10^18. At the same 10^12 guesses per second, that is about 4.6 million seconds, or roughly 53 days. That is why naive splitting is a bad idea.

The Passphrase: Your Optional “25th Word”

BIP-39 supports an optional passphrase, often called the 25th word (or 13th word for a 12-word phrase). It is any extra text you choose, and it is mixed into the seed calculation. The same 24 words with a different passphrase produce a completely different wallet.

What it is good for:

  • Protection if your words are found. A thief who finds your written phrase gets only the wallet with no passphrase, which you can leave empty or nearly empty as a decoy.
  • Several hidden wallets from one seed, each opened by a different passphrase.

The catch is serious. There is no “wrong passphrase” error. Type it with one letter off, a different capital or an extra space and the wallet will simply open a different, empty wallet. If you forget the passphrase, your funds are gone just as surely as if you lost the words. Store the passphrase separately from the seed phrase, and make sure the backup for one does not sit next to the backup for the other.

Do not confuse this passphrase with the PIN or password that opens your device or app. Those protect the device; the BIP-39 passphrase is part of the wallet itself.

Seed Phrase Storage: Paper, Metal or Split?

A good backup has to survive three threats: theft, physical damage (fire, flood, fading ink) and your own memory. Here is how the common options compare.

Method Pros Cons Best for
Paper card (from the wallet box) Free, quick, offline Burns, tears, fades and can be water-damaged Small balances or a temporary backup
Metal seed phrase plate (stamped or tile-based steel or titanium) Survives typical house fires and floods; lasts decades Costs money; anyone who finds it can read it Long-term storage of meaningful amounts
Two identical copies in separate secure places Protects against losing one location Doubles the theft surface Most people, paired with metal
Shamir backup (SLIP-39) or multisig Any 2 of 3 (for example) shares restore the wallet; one share alone reveals nothing More complex; needs supporting wallets Larger holdings, inheritance planning
Naive split (words 1–12 here, 13–24 there) Feels safer Each half leaks a lot; losing either half can mean losing everything Not recommended

Practical storage tips

  • Write clearly in pencil or permanent ink, number each word and double-check the order against the wallet’s verification step.
  • For a seed phrase metal plate, choose stainless steel or titanium and look for independent fire and corrosion tests.
  • Keep at least one copy somewhere other than your home, such as a safe deposit box or a trusted family member’s safe.
  • Do a test recovery with a small amount before you rely on the backup.
  • Leave instructions for heirs on where the backup is and how to use it, without writing the words in the instructions themselves.

Our comparison of hot wallets vs cold wallets and our guide to hardware vs software wallets cover the device side of keeping keys offline.

What Never to Do With Your Seed Phrase

Warning: No legitimate wallet maker, exchange, support agent or app will ever ask for your seed phrase. Anyone who asks for it, by message, website, phone call or a “wallet validation” form, is trying to steal from you.

  • Do not type it into any website or app other than the official wallet during a restore you started yourself. Fake wallet sites and browser extensions are a top source of theft; see our list of common crypto scams.
  • Do not photograph it or store it in notes apps, email, cloud drives or password managers that sync online. Phones get hacked and cloud accounts get breached.
  • Do not use a phrase that came pre-printed with a device or was given to you by someone else. Whoever generated it knows it.
  • Do not invent your own words or choose a phrase from a book or song. Human-chosen phrases are guessable.
  • Do not rely on memory alone. A “brain wallet” fails after an accident, illness or simply time.
  • Do not keep the only copy in one place. One fire or move can wipe it out.

If you think your phrase has been exposed, create a brand-new wallet with a fresh phrase on a clean device and move your funds there immediately. Changing the device PIN does nothing, because the old words still control the old wallet. Our guide to how people lose crypto covers more of the failure modes.

How to Restore a Wallet From a Seed Phrase

  1. Install the official wallet app or reset your hardware wallet, downloading software only from the maker’s verified site.
  2. Choose “restore” or “recover existing wallet,” not “create new.”
  3. Enter the words in order. Most wallets autocomplete after a few letters, which helps catch typos.
  4. If you used a passphrase, enter it exactly.
  5. Check that your addresses and balances appear. If they do not, the wallet may use a different derivation path; many wallets let you select the account type or path manually.

A few wallets, such as Electrum’s native format, use their own seed systems instead of BIP-39, so a phrase from one may not restore in another. Note the wallet name and any derivation path alongside your backup instructions.

Bottom Line

A seed phrase is the human-readable form of your wallet’s master secret: 12 or 24 words from a fixed list of 2,048, with a checksum to catch typos. Both lengths are safe against brute force; the real threats are phishing, digital copies and losing the backup. Store it offline, ideally on metal, in more than one secure place, consider a passphrase only if you will reliably remember it, and never type it anywhere you did not deliberately choose. More wallet and security guides are in our crypto hub.

Sources

Frequently asked questions

What is the difference between a seed phrase and a private key?

A private key controls a single address, while a seed phrase generates all the private keys in a wallet. Exposing a seed phrase puts every account and coin in that wallet at risk, so it needs even stronger protection.

Is a 12 or 24 word seed phrase better?

A 24-word phrase has 256 bits of entropy versus 128 bits for 12 words, but both are effectively impossible to brute-force with current technology. The bigger risks are theft and losing your backup, so choose the length your wallet supports and focus on storing it well.

Can someone guess my seed phrase?

Not by random guessing if your wallet generated it. A 12-word phrase has about 3.4 x 10^38 possibilities, which would take billions of times longer than the age of the universe to search. Phrases are stolen through phishing, malware, photos and found backups, not guessing.

Where is the safest place to store a seed phrase?

Offline, on a fire- and water-resistant medium such as a stainless steel or titanium seed phrase plate, kept in a secure place like a home safe or safe deposit box. Many people keep two copies in different locations and never store the words digitally.

Is a seed phrase metal plate worth it?

For meaningful amounts, usually yes. Paper can burn, tear or get wet, while a good metal plate is designed to survive typical house fires and flooding for decades. Check for independent stress tests before buying.

What happens if I lose my seed phrase?

If your wallet still works, create a new wallet with a fresh phrase, back it up properly and move your funds to it right away. If you lose both the device and the phrase, there is no reset option and the funds cannot be recovered.

Can I change my seed phrase?

You cannot change the words of an existing wallet. Instead, you create a new wallet with a new phrase and transfer your funds to it, which is what you should do if you think the old phrase was exposed.

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