What is the block consists of in the bitcoin blockchain?

6 minutes

How the new block is created?

Transactions, which have been created by users, wait in the mempool (memory pool) in order to be processed by a miner. Transactions with higher fees have priority to be authorized, so they are attached to the block first. Miners start looking for the appropriate SHA-256 hash by changing the nonce value (proof-of-work concept). Thereafter, when one of the nodes finds suitable hash, it broadcasts the block to the network. Rest of the nodes check if the transactions in the block are valid and not already spent. If the new block is accepted, nodes will link next block with the hash of previous one.

What data does each block contain?

We know that the bitcoin network consists of blocks. But what data do those blocks contain? Well, each block consists of:

  • header: 80-byte, more about it in a moment,
  • magic number: 4-byte, always 0xD9B4BEF9
  • size of the block: 4-byte, indicates the amount of data that block can handle. The block is limited to 1mb size,
  • transactions counter: 1 to 9-byte, number of transactions that block contains,
  • transactions: < 1mb, list of all transactions in the block

The header of the block consist of the block metadata:

  • version: 4-byte, the currently used bitcoin protocol version,
  • hash of the previous block: 32-byte, the hash of the previous block header,
  • merkle root hash: 32-byte, the hash of the Merkle tree root of all transactions, that this block contains,
  • timestamp: 4-byte, used to sort blocks chronologically,
  • target: 4-byte, determines the difficulty of finding the appropriate block hash,
  • nonce: 4-byte, miner increments this number to find correct hash

2 * 32 bytes + 4 * 4 bytes = 80 bytes


Transactions are the most valuable data in the block. The first transaction in each block is called the generation transaction. This is the payment, which the miner adds on the top of the transaction field. If the node manages to mine his block, he will receive bitcoins, that he sheer added! That's how new coins are genuinely created "from the air". Each transaction consists of:

  • Txins: list of all transaction's inputs,
  • Txouts: list of all transaction's outputs,
  • Txins count: amount of all transaction's inputs,
  • Txouts count: amount of all transaction's outputs,
  • witnesses: serialization of the witness data used in SegWit transactions,
  • lock time: indicates the block number or the timestamp until the transaction is locked. Usually, this is set to zero, so the transaction is valid immediately after the block is added to the blockchain.

The Segregated Witness (SegWit) is the feature in the bitcoin network, that enables transaction signature data to be segregated efficiently. So, there is more space for transaction data in the block, but the block size is the same. The following data is included in the SegWit transactions:

  • version: 4-byte, the currently used bitcoin protocol version,
  • marker: 1-byte 0x00, indicates whenever transaction uses SegWit or not,
  • flag: 1-byte 0x01, indicates whenever transaction uses SegWit or not

Transactions merkle tree

Merkle tree is used to compute the unique hash value of all transactions. Firstly, transactions are converted to the hash, by the SHA-256 algorithm one by one. Secondly, the tree is constructed on top of those transactions.

The merkle tree structure
The merkle tree structure

The root hash represents all of the transactions in the block. Therefore, if someone changed one of the transactions, it would cause changing all the hashes along the way! Moreover, the block hash would change too. And when the block hash changes, the consecutive block hash changes too, and so on. So, the whole blockchain from that particular moment is invalid! The most important takeaway is that we cannot change transaction data while it is already attached to the blockchain!

How would we verify if the transaction was attached to the block? Let's assume that we consider the hash4. To check if the hash is in the block, we need to obtain log2n transaction hashes (n = number of all transactions), which is called a Merkle path or the transaction proof. In this scenario, the Merkle path is simply: Hash3, Hash 1|2, Hash 5|6|7|8, Root Hash. We can simply query the network for that piece of information. So, we are starting at our hash4 and traversing upward calculating each hash on the current branch. Finally, we will get the root hash, and if the calculated root hash is equal to the root hash in the block, our transaction is embedded in the tree!

Transaction verification
Transaction verification

The bitcoin network obtains a special kind of node called Simplified Payment Verification (SPV). They are responsible for verifying if a transaction is in the block. Thanks to Merkle path, they don't need to download the whole blockchain, only a header of the block.

What are the transactions inputs and outputs?

Let's assume that Mark wants to send 1 BTC to Carl. Mark recently received 0.5 BTC and 0.7 BTC in different transactions. He is signing his transaction with those inputs: 0.5 BTC and 0.7 BTC. Because of that, he is eligible to get 0.2 BTC as a rest. In this particular case, he will sign a message with multiple inputs. Of course, if he had received more than 1 BTC in one transaction earlier, he would have included only one input in the transaction message. Besides inputs, there are also at most two outputs: one returning the change and one for the actual payment. There is no need to check the whole transaction history to sign a message.

Inputs and outputs in bitcoin transactions
Inputs and outputs in bitcoin transactions

<-- Return to all articles

Latest Posts

4 minutes

Aptos - is it the next 500x Ethereum killer?

-- Read --
5 minutes

How does the signing transaction works in the bitcoin blockchain?

-- Read --