# 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.

Canonical page: https://mybitcoinexplorer.com/learn/transaction-inputs
Topic: Transactions
Published: 2026-10-29
Updated: 2026-10-30


## 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.

> Diagram: The fields of one input — Previous txid:32,Output index:4,Unlocking script:var,Sequence:4

- **Previous txid + output index** together are called an **outpoint**: "output number 1 of transaction abc…".
- **Unlocking script** (or [witness](https://mybitcoinexplorer.com/learn/witness-data)) usually holds a [signature](https://mybitcoinexplorer.com/learn/digital-signatures) and a [public key](https://mybitcoinexplorer.com/learn/public-keys).
- **[Sequence](https://mybitcoinexplorer.com/learn/locktime-and-sequence)** is used for features like [Replace-by-fee (RBF)](https://mybitcoinexplorer.com/learn/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.

> Diagram: An input unlocks a previous output — Previous output: 0.5 BTC + lock → (points to)Input: txid:index + signature → *Node: checks the unlock

## 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](https://mybitcoinexplorer.com/learn/transaction-size-and-weight).


## Key takeaways

- An input references a previous output by txid and index (an outpoint).
- It carries the data that satisfies that output's lock, usually a signature.
- Input amounts come from the outputs being spent.

## Common questions

### Why doesn't an input list its amount?

Because the amount is already recorded in the output being spent. Repeating it would waste space and could be inconsistent.

### What is a coinbase input?

The first transaction in a block has a special input that doesn't spend anything. See coinbase transaction.

### What is an outpoint?

The pair of a transaction ID and an output number, written txid:index. It names exactly one output.

### Can one transaction have many inputs?

Yes. The pizza transaction from 2010 had 131 inputs.

## Related lessons

- [Transaction outputs](https://mybitcoinexplorer.com/learn/transaction-outputs)
- [Transaction IDs (txid)](https://mybitcoinexplorer.com/learn/transaction-id-txid)
- [Witness data](https://mybitcoinexplorer.com/learn/witness-data)
- [Anatomy of a raw transaction](https://mybitcoinexplorer.com/learn/anatomy-of-a-raw-transaction)