Block Candidate: Definition and How Miners Build a Candidate Block in Bitcoin
A Block Candidate (or Candidate Block) is a proposed Bitcoin block that has been assembled by a miner but has not yet been successfully mined. It contains a block header, a coinbase transaction, and a set of selected transactions from the mempool. The candidate becomes a valid Bitcoin block only after the miner finds a Proof of Work that satisfies the network's difficulty target.
Block Candidate Explained in Simple Terms
Before a new Bitcoin block can be added to the blockchain, a miner must first create a draft version of that block.
This draft is called a block candidate.
The miner fills it with:
pending transactions from the mempool
a coinbase transaction that pays the block reward
the previous block's hash
other required block information
Once the candidate block is complete, the miner begins searching for a valid block hash by repeatedly changing the nonce and calculating SHA-256 hashes.
If a valid hash is found, the candidate block becomes an official Bitcoin block and is broadcast to the network.
Think of a block candidate as an unsigned contract. The document is complete, but it only becomes legally valid after the required signature is added. In Bitcoin, the "signature" is the valid Proof of Work.
How Block Candidate Works
Every miner continuously builds and updates candidate blocks.
The process works as follows:
Transactions Selected
The miner chooses transactions from the mempool, usually prioritizing those with the highest fee rates.Candidate Block Constructed
The miner creates a block containing the selected transactions, a coinbase transaction, and the previous block hash.Proof of Work Begins
Mining hardware repeatedly hashes the block header using different nonce values.Valid Hash Found
The block hash becomes less than or equal to the current target threshold.Block Broadcast
The candidate block becomes a valid Bitcoin block and is shared with the network.
The process can be summarized as:
Mempool Transactions → Candidate Block → Proof of Work → Valid Block
Example of Block Candidate in Practice
A mining pool creates a candidate block using:
several thousand pending Bitcoin transactions
the latest previous block hash
a coinbase transaction paying the block subsidy and transaction fees
Thousands of ASIC miners begin hashing the candidate block.
After trillions of hash attempts, one miner discovers a valid Proof of Work.
The mining pool broadcasts the block.
Bitcoin nodes validate it, and the candidate block officially becomes the newest block in the blockchain.
Why Block Candidates Matter
Block candidates are essential because they:
organize pending transactions into a proposed block
allow miners to begin Proof of Work
determine which transactions may be confirmed next
include the coinbase transaction that creates new bitcoins
form the starting point for every mining cycle
Without candidate blocks, miners would have nothing to hash.
What Does a Block Candidate Contain?
A typical candidate block includes:
block header
previous block hash
Merkle root
timestamp
difficulty target
nonce
coinbase transaction
selected mempool transactions
These components are continuously updated as network conditions change.
Block Candidate vs Valid Block
Block Candidate
Represents:
a proposed block that has not yet been successfully mined.
Characteristics:
not part of the blockchain
still undergoing Proof of Work
may be replaced if another miner finds a block first
Valid Block
Represents:
a block that satisfies the Proof of Work requirement and all Bitcoin consensus rules.
Characteristics:
accepted by Bitcoin nodes
added to the blockchain
used as the foundation for the next block
Every valid block begins as a candidate block.
Why Do Miners Update Candidate Blocks?
Candidate blocks are not static.
Miners frequently rebuild them because:
new transactions enter the mempool
higher-fee transactions become available
another miner finds a new block
the timestamp changes
mining pools issue new job assignments
These updates help maximize transaction fee revenue and ensure miners work on the most recent version of the blockchain.
Can Different Miners Have Different Block Candidates?
Yes.
Because each miner chooses transactions independently, two miners may create different candidate blocks even while referencing the same previous block.
Differences may include:
selected transactions
transaction order
coinbase transaction contents
total transaction fees
Despite these differences, any candidate block that satisfies Bitcoin's consensus rules and Proof of Work can become the next valid block.