How to test your seed phrase without moving your coins

You have the words written down. Complete, legible, stored safely. And you have no way to know whether they work — until the day you need them, which is exactly the worst day to find out.

A backup is not tested because it looks tidy. It is tested when someone has checked that it rebuilds your wallet. Those are two different things, and almost everybody does the first one and believes they did the second.

Why "the wallet says it's valid" proves nothing

When you type twelve words and the software accepts them, all that happened is that the checksum matched. In BIP-39 that checksum is four bits over one hundred and twenty-eight bits of entropy: roughly one in every sixteen random twelve-word combinations from the list passes that check.

Which means you can write down twelve random words, have them accepted, have them open a real wallet, and find that wallet empty — because it was never yours. The checksum verifies the shape, not the ownership. It is a typo detector, and an excellent one. It is not proof that the paper is yours.

The method everyone recommends, and what's wrong with it

The usual advice is to restore the phrase into another wallet and look at the balance. It works, but it has two serious problems.

You type your words into one more device. Every place you enter the phrase is a new surface: a keyboard, a clipboard, an app that may keep more than it claims. If the goal was not to expose the backup, the method exposes it.
If it shows zero, you learned nothing. One seed opens four different and equally valid wallets, depending on the address format the program uses. Restoring into a wallet that derives one format and seeing no funds is perfectly consistent with your backup being correct. That is the case that frightens people for no reason — and also the one that sometimes hides a real problem.

The check that actually settles it, using only the public side

There is an asymmetry that makes all of this possible: your phrase produces an extended public key, and that key produces all your addresses. The public key cannot sign, cannot spend, and is useless for stealing. It is only good for computing addresses.

So the question "do these words build my wallet?" can be answered without touching the wallet:

Get the extended public key from your wallet (xpub, ypub or zpub, depending on the format).
Take an address you know for certain is yours — one you have used to receive.
Derive the addresses from that key and check whether any of them is that one.

If it shows up, it is proven that what you wrote down is what built your wallet. If it shows up in none of the four formats, you have a concrete, located problem — which is infinitely better than a vague doubt.

Why this runs in your browser and not on a server. The extended public key cannot spend, but it does reveal every address and every balance you have, permanently. Sending it to someone else's site means handing over your complete financial history. That is why the computation runs on your machine and the page declares connect-src 'none': it cannot send a single byte anywhere even if it wanted to, and you can confirm that yourself in the browser's network panel.

What is NOT proven, even on a match

Being precise here is worth more than celebrating.

The passphrase. If you use an extra word on top of the twelve, note that a passphrase has no checksum: any passphrase is valid and opens a real, different wallet. If you ran the check without it, that piece remains unproven.
That the paper survives. Ink, humidity, moves, fires. No calculation proves that: the medium and the number of locations do.
That someone else could use it. You know which wallet it was and whether there is a passphrase. Whoever comes after you does not. That is the dependency behind most permanently lost coins, and no technical check resolves it.

Do it now

The check takes a minute and asks for neither your phrase, nor your private key, nor a password. Only the extended public key and one of your addresses.

Test my backup →