Glossary / Transactions & fees
Output index
Also known as vout.
- Definition
- An output index is the position of one output inside a transaction, counted from zero, and it is the number after the colon when you see a coin written as txid:1.
Transactions can pay dozens of recipients at once, so the identifier alone never says which payment is yours. The index is stored as a four byte number, allowing values up to 4,294,967,295, far more outputs than any block could hold. Paired with the transaction ID it forms an outpoint, the permanent address of a coin inside the ledger.
How it works
Outputs are ordered, and the order is whatever the transaction's creator serialized. The first output is index 0, the second is index 1, and the numbering runs to the end of the list with no gaps. Nothing about the index describes the amount, the recipient, or the purpose of the output; it is a position and nothing more.
Every input in every transaction carries one of these numbers. An input begins with a 36 byte reference: 32 bytes of transaction ID followed by a four byte little-endian index. That is how a spend says which coin it is consuming, and it is why nodes can check a spend without searching the whole chain.
The coinbase transaction of each block breaks the pattern deliberately. Having no coin to spend, its single input points at a transaction ID of 32 zero bytes with the index set to 0xffffffff, the maximum value the field can hold. Anything referencing that outpoint is recognized as newly issued money rather than a transfer.
Position also leaks information, which is why wallets treat it as a privacy setting. If change always landed at index 1, every observer could label the change output of every transaction instantly. Bitcoin Core randomizes where the change output sits, and BIP69 proposed the opposite approach of sorting inputs and outputs into a deterministic lexicographic order so that nothing can be inferred from ordering at all.
Where you see it
Coin control screens are where most people meet the number without knowing its name. Sparrow, Electrum, and Bitcoin Core each list your coins as a transaction ID plus an index, because two outputs of the same transaction can both belong to you and only the index tells them apart.
Exchange batching makes the number visible. When Kraken or Binance pays out a hundred withdrawals in one transaction, your coin sits at whatever position the batching software gave it, so seeing your money at index 87 of 120 outputs is completely normal and is not a sign that anything went wrong.
Developer tools ask for it directly. The gettxout command takes a transaction ID and an index as separate arguments, PSBT files reference inputs by outpoint, and hardware wallets show "output 2 of 3" on screen while you approve a spend. Support desks that ask for "the txid and the vout" are asking for the same pair of values.
Output index vs outpoint
An output index on its own is meaningless, because index 0 exists in every transaction ever made. An outpoint is the pair, transaction ID and index together, and it identifies exactly one coin out of every coin that has ever existed. The distinction matters the moment you paste something into a coin control field or a support ticket: half the reference is not a reference.