Bitcoin Data Model

This is a high-level overview of Bitcoin’s data model. For the full details, refer to

Block Headers

The 80 bytes block header hash encodes the following information:

Bytes Parameter Type Description
4 version i32 The block version to follow.
32 hashPrevBlock char[32] The double sha256 hash of the previous block header.
32 merkleRoot char[32] The double sha256 hash of the Merkle root of all transaction hashes in this block.
4 time u32 The block timestamp included by the miner.
4 nBits u32 The target difficulty threshold, see also the Bitcoin documentation.
4 nonce u32 The nonce chosen by the miner to meet the target difficulty threshold.


A transaction is broadcasted in a serialized byte format (also called raw format). It consists of a variable size of bytes and has the following format.

Bytes Parameter Type Description
4 version i32 Transaction version number.
var tx_in count uint Number of transaction inputs.
var tx_in txIn Transaction inputs.
var tx_out count uint The number of transaction outputs.
var tx_out txOut Transaction outputs.
4 lock_time u32 A Unix timestamp OR block number.


Bitcoin uses the term “CompactSize Unsigned Integers” to refer to variable-length integers, which are used to indicate the number of bytes representing transaction inputs and outputs. See the Developer Reference for more details.


Bitcoin’s UTXO model requires a new transaction to spend at least one existing and unspent transaction output as a transaction input. The txIn type consists of the following bytes. See the reference for further details.

Bytes Parameter Type Description
36 previous_output outpoint The output to be spent consisting of the transaction hash (32 bytes) and the output index (4 bytes).
var script bytes uint Number of bytes in the signature script (max 10,000 bytes).
var signature script char[] The script satisfying the output’s script.
4 sequence u32 Sequence number (default 0xffffffff).


The transaction output has the following format according to the reference.

Bytes Parameter Type Description
8 value i64 Number of satoshis to be spent.
1+ pk_script bytes uint Number of bytes in the script.
var pk_script char[] Spending condition as script.