Unspent Transaction Output (UTXO)

Overview

Every UTXO is identified by the transaction that created it and its output index. It contains an amount and a locking condition that defines how it can be spent. Until a valid transaction consumes it, the UTXO remains part of the network’s active spendable set.

Suppose a wallet controls outputs worth 0.4 BTC and 0.7 BTC and needs to send 0.8 BTC. The wallet may consume both inputs, create one output for the recipient, another change output for itself, and leave the difference as the transaction fee. The original outputs then cease to be spendable.

UTXO selection affects fees and privacy. Using many small outputs increases transaction size. Combining outputs associated with different addresses can reveal that they are controlled by the same wallet. Wallet software uses coin-selection strategies to balance fee efficiency, privacy, and change management.

The UTXO model prevents double spending by allowing each output to be consumed only once in canonical history. Nodes maintain the current UTXO set so they can validate new transactions without recalculating every historical balance.

UTXOs can also carry script conditions, maturity requirements, or token-related data depending on the protocol. They should not be confused with transaction outputs that have already been spent. A wallet balance is usually the sum of the spendable UTXOs it can authorize.

Wallet recovery and accounting also depend on complete UTXO discovery. A wallet can control an address yet miss an output if its derivation scan or index is incomplete. Watch-only systems should preserve descriptor or derivation information, not only visible addresses. Businesses should label UTXOs by source and purpose so later consolidation or spending does not destroy important accounting or privacy boundaries.