Multi-Input Transaction

Overview

Meaning and scope

A multi-input transaction is a blockchain transaction that consumes value or authorization from more than one input source. In UTXO systems, it spends several previously unspent outputs and proves control over each one. Wallets use multi-input transactions to combine smaller balances into a larger payment, pay fees, or consolidate funds.

Mechanism and operation

A multi-input transaction contains two or more sources that jointly fund or authorize one transaction. In a UTXO model, each input references a specific unspent output from an earlier transaction and supplies the unlocking data required to spend it. The values of all valid inputs are added together, then allocated across the outputs and the transaction fee. Wallets create multi-input transactions when no single UTXO is large enough for the intended payment. They also use them for consolidation, batching, coin selection, and fee management. The transaction is atomic: nodes either accept all inputs together or reject the transaction.

Practical payment example

A Bitcoin wallet combines several unspent outputs to fund one purchase, then tracks both recipient output and change output.

Operational checks

A wallet should also avoid selecting an input already used by another pending transaction, because that creates a double-spend conflict. Replacing or rebuilding the transaction can change its transaction ID. Payment and treasury systems should distinguish multi-input structure from a Multi-Output Transaction. Multiple inputs describe funding sources; multiple outputs describe destinations.

Distinctions and limits

Each referenced output can have a different locking condition, so the transaction may require several Digital Signatures or scripts. Combining outputs associated with different addresses can reveal that one entity probably controls them, while a larger input count increases serialized size and usually raises fees.

Related reading

Related definitions: transaction, Digital Signatures.