What is transaction ID (TXID)?

The unique hash identifying a transaction on a blockchain, which is what you paste into an explorer to check whether a withdrawal has been sent and confirmed.

It is the only piece of evidence that settles an argument about a missing transfer. An exchange telling you a withdrawal was processed means nothing on its own. A transaction ID means the transfer exists on a public ledger where you, the receiving exchange, and anyone else can look at it independently.

The mistake people make is contacting support before checking. Most missing deposits are visible on an explorer in seconds, and the answer is usually that the transaction is fine and the receiving venue has not credited it yet.

How it works

A transaction ID is a cryptographic hash of the transaction's own contents, so it is derived from the data rather than assigned by anyone. On Bitcoin it is the double SHA-256 hash of the serialized transaction, as described in the Bitcoin developer documentation. On Ethereum it is the Keccak-256 hash of the signed transaction. Either way it is a long hexadecimal string, and any change to the transaction produces an entirely different one.

The sending exchange displays it once the transaction has been broadcast to the network. From that moment the transaction exists publicly whether or not it has been mined, and an explorer will show it as pending. Once included in a block it gains a block height and starts accumulating confirmations.

The identifier tells you three things: that the transfer was actually sent, which address it went to, and how many confirmations it has. The last is what determines when the receiving venue credits it, and that threshold is the receiver's policy rather than a property of the chain.

Example

Illustrative. You withdraw to another exchange and the deposit has not appeared after twenty minutes. The sending exchange shows a transaction ID, so you paste it into a block explorer.

The explorer shows the transaction in a block with 4 confirmations, sent to the address you were given. The receiving exchange's help center states it credits that asset after 6 confirmations. At roughly ten minutes a block, the deposit will credit in about twenty more minutes and there is nothing to report.

Change one detail and the answer changes completely. If the explorer showed the destination as an address you do not recognize, you copied the wrong one, and the transaction is final and unrecoverable.

Why it matters when you buy

Every transfer between venues is a step where funds are visibly in flight and neither side is responsible. Knowing how to read the identifier turns a frightening gap into a wait with a known end. Confirmation requirements and network options differ by chain, which the chain pages cover, and the guide on using a block explorer walks through reading one.

Related terms

• block explorer: where you look a transaction ID up
• confirmation: what the receiving venue counts
• deposit address: the destination the identifier records
• transaction receipt: whether execution succeeded or reverted
• memo destination tag: the extra field some chains require

Questions

My exchange has no transaction ID for my withdrawal. What does that mean?

The transaction has not been broadcast yet. Exchanges batch withdrawals and hold them for internal review, so the funds are still with the venue and the delay is theirs rather than the chain's.

Can two transactions have the same ID?

Not in practice. The identifier is a hash of the transaction contents, and finding a collision is computationally infeasible. Resending the identical transaction produces the identical hash, which is a duplicate rather than a second transfer.

The explorer says confirmed but my deposit is missing. Now what?

That is the case where you open a ticket with the receiving venue, and the identifier is the first thing they will ask for. Common causes are a missing destination tag, a deposit below the venue's minimum, or the wrong network for that address.