A Transaction ID (TXID) is a unique 64-character hexadecimal hash that serves as the permanent, immutable identifier for a specific Bitcoin transaction. Derived from double-hashing the transaction's serialized data, it enables anyone to locate, verify, and reference the transaction on the blockchain or via APIs, ensuring transparency and auditability.

Practical Examples

• After sending bitcoin from your wallet, copy the TXID and paste it into a blockchain explorer like Blockstream.info to view details, including inputs, outputs, fees, and confirmations.
• In a multisig setup, share the TXID with co-signers to confirm the transaction has been broadcast and included in a block, verifying shared custody.
• Use the TXID in API calls to Blockstream's explorer service to automate balance checks for an address in a custom app, tracking incoming payments without running a full node.
• During disputes, provide the TXID to a support team; they can instantly inspect the on-chain record to resolve issues like double-spends or fee disputes.

Key Takeaways

• TXIDs are generated via SHA-256 double hash of the transaction's binary representation, making them collision-resistant and unique forever.
• They are case-insensitive but conventionally lowercase; any alteration to the transaction (e.g., fee change) produces a new TXID.
• TXIDs facilitate broadcast transaction tracking and are essential for tools like fee estimates and Replace-By-Fee (RBF).
• Public and verifiable by all, TXIDs underscore Bitcoin's open ledger, unlike proprietary transaction IDs in centralized systems.

In-Depth Explanation

When constructing a transaction, wallet software assembles inputs (references to unspent outputs via previous TXID and output index), outputs (amounts locked to addresses via scripts), and metadata like fees and locktime. This structure is serialized into a binary format, then double-hashed with SHA-256 (first hash of the data, then hash of that result) to yield the 256-bit (32-byte) TXID, typically displayed as 64 hex characters. For SegWit transactions, the witness data is excluded from the TXID calculation to prevent malleability, ensuring stability for replacement mechanisms like RBF (BIP125). Upon broadcast transaction, the TXID propagates through the network; once mined into a block, it becomes part of the blockchain at a specific block height. Explorers index TXIDs for quick retrieval, displaying decoded elements like scriptPubKey, signatures, and UTXO changes. This self-verifying nature means TXIDs are tamper-evident: even a single bit flip alters the hash entirely, providing cryptographic proof of integrity without trusted third parties.

Why Bitcoiners Care

TXIDs make Bitcoin auditable and trustless. Anyone can independently verify payment history and confirmation status on a blockchain explorer like blockstream.info without asking a bank or custodian.