# The nonce

The nonce is a 4-byte number in the block header that miners change on every attempt. Learn why it runs out so fast and what miners change next.

Canonical page: https://mybitcoinexplorer.com/learn/nonce
Topic: Mining
Published: 2026-10-29
Updated: 2026-10-30


## A number to change
The [The block header](https://mybitcoinexplorer.com/learn/block-header) has a 4-byte field called the **nonce** ("number used once"). Miners set it to 0, hash the header, check the result, then try 1, 2, 3… Each change gives a completely different hash.

> Diagram: Changing only the nonce gives a completely new hash each time — 3

## When the nonce runs out
4 bytes allow about 4.3 billion values. Modern machines try that many in well under a second. When they run out, miners change something else:
- the **extra nonce**: spare bytes in the [The coinbase transaction](https://mybitcoinexplorer.com/learn/coinbase-transaction), which changes the [Merkle trees and the merkle root](https://mybitcoinexplorer.com/learn/merkle-root);
- the **timestamp**, within the allowed range;
- some **version bits**.

Then the nonce search starts again.

### Worked example: The genesis block's nonce
[Block 0](/block/0) has nonce **2,083,236,893**. Its target was the easiest Bitcoin has ever had, yet it still took about two billion tries.



## How fast do miners use up the nonce?
| Machine speed | Time to try all 4.3 billion nonces |
|---|---|
| 1 GH/s (an early GPU) | about 4 seconds |
| 100 TH/s (one modern ASIC) | about 0.00004 seconds |
| 800 EH/s (the whole network) | far less than a nanosecond |

That's why the extra nonce in the coinbase transaction is essential.

## Is the winning nonce special?
No. It's just the number that happened to produce a low enough hash with that exact header. Change any other field and a different nonce would be needed.


## Key takeaways

- The nonce is a 4-byte header field miners change on each attempt.
- Its 4.3 billion values run out quickly on modern hardware.
- Miners then change the extra nonce, time or version bits.

## Common questions

### Is there a correct nonce?

Many nonces might work, and often none does for a given header. Miners just need any combination that produces a hash below the target.

### Why is it called a nonce?

It's short for 'number used once', a common cryptography term.

### What does nonce stand for?

Number used once.

### Where can I see a block's nonce?

It's in the block header; MyBitcoinExplorer's API returns it in a block's full details.

## Related lessons

- [The block header](https://mybitcoinexplorer.com/learn/block-header)
- [Proof of work](https://mybitcoinexplorer.com/learn/proof-of-work)
- [Target and difficulty](https://mybitcoinexplorer.com/learn/target-and-difficulty)
- [The coinbase transaction](https://mybitcoinexplorer.com/learn/coinbase-transaction)