Transaction inputs

An input points to an earlier output by txid and index, and supplies the signature that unlocks it. Learn what an input contains and why it has no amount.

What an input contains

An input doesn't contain an amount. It contains a pointer to an earlier output plus a key to unlock it.

• Previous txid + output index together are called an outpoint: "output number 1 of transaction abc…".
• Unlocking script (or witness) usually holds a signature and a public key.
• Sequence is used for features like Replace-by-fee (RBF) and relative timelocks.

Where does the amount come from?

From the output being spent. A node looks up the outpoint in its UTXO set to find the value and the locking script. This is why an explorer needs the previous transactions to show input values and the fee.

Example

Input 0
  previous output: 3f4a…9c2e : 1
  unlocking data:  <signature> <public key>
  sequence:        0xfffffffd

A worked example

Suppose last week you received 0.05 BTC in output 1 of transaction 9f2c…. Today you want to spend it. Your new transaction's input says:

• Previous output: 9f2c…:1
• Proof: your signature and public key (in the witness for SegWit)
• Sequence: a value that also allows a fee bump later

The node looks up 9f2c…:1 in its UTXO set, finds 0.05 BTC locked to your key hash, checks your signature, and marks that output as spent.

Inputs and fees

Each extra input adds roughly 58 to 148 virtual bytes, depending on the address type. Spending many small coins makes a transaction big and costly. See Transaction size, weight and vbytes.