Blocks

A Bitcoin block is a batch of transactions that the network has confirmed. Each block begins with an 80-byte header, a fixed-size summary of the block. The header links the block to the one before it. It also carries the proof of work, the hashing a miner did to earn the block. A new block arrives about every 10 minutes.

How are blocks made?

Blocks are built during mining. A transaction does not go straight into the blockchain. It waits in the mempool, the pool of transactions the nodes have seen. A miner gathers transactions from that pool into a candidate block, which is not yet valid. Then the miner tries to make it valid:

1. Build the header, with the nonce set to 0.
2. Hash the header with SHA-256, twice.
3. Compare the result with the target.
4. If the hash is too high, add 1 to the nonce and hash again.
5. When the hash is low enough, the block is solved. The miner then builds the next block on top of it.

Step 5 is the link in the chain. The next block's header holds the solved block's hash as its previous block hash. Every miner then builds on that block.

What is inside a block?

A block has two parts. The header comes first, followed by the list of transactions. The first transaction in that list is the coinbase transaction, and it pays the block reward to the miner.

Only the header goes into the hash. The transactions are tied to the header through the Merkle root. The Merkle root is one fingerprint made from every transaction in the block. If you change one transaction, the Merkle root changes, and so does the block hash.

What are the fields of the block header?

Each field has a fixed size, so the header is always exactly 80 bytes.

Field	Size	What it holds
Version	4 bytes	The block version, used to signal upgrades
Previous block hash	32 bytes	The hash of the block before this one
Merkle root	32 bytes	One fingerprint of all the transactions in the block
Time	4 bytes	When the miner started the block, as a Unix timestamp
Bits	4 bytes	The target, written in a compact form
Nonce	4 bytes	A number the miner changes while it searches

The previous block hash ties the block to the chain. The Merkle root ties it to its transactions. The bits set the target. The nonce is the number a miner changes until it finds a hash below that target.

How is a block found?

A miner hashes the header twice with SHA-256, a method called double SHA-256. The result is the block hash. If that hash is below the target, the block is valid, and the miner has done the work. If it is not, the miner changes the nonce and tries again.

The target works like the bar in a limbo game. The lower the bar sits, the harder it is to get under.

Block explorers show a hash with its zeros first. For example, block 100,000 has the hash 000000000003ba27…fd33e506, and all 64 digits appear in the diagram. Its target begins 000000000004864c, and the other 48 digits are zeros. The hash is smaller than the target, so the block is valid.

What is the first block?

The first block, called the genesis block, was mined by Satoshi Nakamoto on 3 January 2009. It has been part of the chain ever since. You can see it at block 0. Its nonce is 2,083,236,893. Its bits are 1d00ffff, which is the target for difficulty 1, the starting point of the difficulty scale.

How often are blocks made?

The network aims for one block about every 10 minutes. Some blocks come sooner and some take longer. Over a full day, the average is about 144 blocks. Every 2,016 blocks, the difficulty changes to keep the average close to 10 minutes.

Can I look at a block?

Yes. Each block has its own page on our explorer, at /block/ followed by its height. The latest blocks page lists the newest ones as our node receives them.

Try block 170. Its second transaction is the first payment between two people, sent from Satoshi Nakamoto to Hal Finney. You can open that transaction on its own page.

What should you read next?

Next, read difficulty. It explains how the network adjusts the target, so that blocks keep arriving about every 10 minutes, even as mining power comes and goes.