Definition
The coinbase transaction (also called the generation transaction) is the special first transaction in every Bitcoin block. It has no spending inputs from prior transactions; instead it mints the block subsidy and sweeps up every transaction fee in the block, paying the total to one or more outputs the miner chooses. It is the only mechanism by which new bitcoin enters circulation.
Do not confuse it with the exchange of a similar name. In protocol terms, “coinbase” refers both to this transaction and to the arbitrary data its single input is allowed to carry. Satoshi Nakamoto embedded a now-famous newspaper headline about bank bailouts in the coinbase of the genesis block, and miners still encode pool tags, signatures, and the block height (required by BIP34) in that same field today.
Why the coinbase transaction is central to ASIC mining
For anyone running hardware, the coinbase is not an abstraction. It is the part of the block that an ASIC and its pool actually manipulate millions of times per second. The amount paid out is the block reward: the fixed block subsidy plus all collected transaction fees. The subsidy is cut in half roughly every four years at each halving, which steadily shifts miner economics toward fee revenue and makes the coinbase output the single most important number in any profitability model.
Crucially, the coinbase transaction is hashed into the block. Its hash becomes a leaf in the Merkle tree, and every change to the coinbase produces a different Merkle root, which is part of the 80-byte block header your chips grind. That property is exactly what makes the coinbase the engine room of pooled mining.
The coinbase as a search-space expander
A Bitcoin block header contains only a 32-bit nonce. Modern ASICs exhaust all four billion of those values in a fraction of a second, far faster than a new block arrives. The coinbase transaction solves this by giving the miner extra mutable bytes. Inside the coinbase input sits an extranonce field, conventionally split into two parts:
- extranonce1 — assigned by the pool to each connection so two miners never grind identical work.
- extranonce2 — rolled freely by the miner; its size is negotiated when the connection subscribes (commonly several bytes).
Each new extranonce value changes the coinbase, which changes the Merkle root, which gives the chips an entirely fresh header to search. This is why pools can keep a machine busy on a single job for far longer than the header nonce alone would allow.
How pools deliver and reconstruct the coinbase
Under Stratum V1, the pool does not send the full coinbase. In a mining.notify message it sends two halves, coinb1 and coinb2, along with the Merkle branches. The miner builds the complete coinbase by concatenating coinb1 + extranonce1 + extranonce2 + coinb2, hashes it, folds that result up through the branches to recompute the Merkle root, and only then assembles the header for hashing. When a share or block is found, the miner returns the job ID, its extranonce2, the timestamp, and the winning nonce so the pool can verify the work.
The coinbase outputs are constructed by the pool from a getblocktemplate response, which supplies the available coinbasevalue. A typical pool directs that value to its own address and takes its fee by adjusting the coinbase output split; solo pools instead pay the block straight to the miner’s address and tag the coinbase with an identifying string. This is the literal mechanism by which mining rewards and pool fees are paid.
Coinbase control under Stratum V2
Stratum V2 changes who owns the coinbase. On a Standard Channel, the pool assembles the coinbase and the miner only rolls the header nonce, which can starve very fast machines of work. On an Extended Channel, the miner controls the coinbase and extranonce again, restoring the large search space older Stratum V1 miners depend on. Template-negotiation approaches such as OCEAN’s DATUM go further: the miner builds its own block template from its own node and dictates the coinbase, while the pool only sets difficulty. This pushes block construction back toward the individual operator and is a meaningful step in decentralizing who decides what goes into a block.
If you are setting up your own machine, the practical takeaway is that the coinbase ties hardware, pool protocol, and economics together in one structure. Whether you are running a Bitaxe at home or a fleet in a mining pool, understanding it makes share and payout behaviour far less mysterious. If you would rather go further toward template sovereignty, solo mining with your own node gives you full control of the coinbase you mine. Browse our ASIC miners to find hardware that fits the setup you have in mind.
In Simple Terms
The special first transaction in each block that creates new Bitcoin and pays the miner.
The coinbase transaction (also called the generation transaction) is the special first transaction in every Bitcoin block. It has no spending inputs from prior transactions; instead it mints the block subsidy and sweeps up every transaction fee in the block, paying the total to one or more outputs the miner chooses. It is the only mechanism by which new bitcoin enters circulation.
Do not confuse it with the exchange of a similar name. In protocol terms, "coinbase" refers both to this transaction and to the arbitrary data its single input is allowed to carry. Satoshi Nakamoto embedded a now-famous newspaper headline about bank bailouts in the coinbase of the genesis block, and miners still encode pool tags, signatures, and the block height (required by BIP34) in that same field today.
Why the coinbase transaction is central to ASIC mining
For anyone running hardware, the coinbase is not an abstraction. It is the part of the block that an ASIC and its pool actually manipulate millions of times per second. The amount paid out is the block reward: the fixed block subsidy plus all collected transaction fees. The subsidy is cut in half roughly every four years at each halving, which steadily shifts miner economics toward fee revenue and makes the coinbase output the single most important number in any profitability model.
Crucially, the coinbase transaction is hashed into the block. Its hash becomes a leaf in the Merkle tree, and every change to the coinbase produces a different Merkle root, which is part of the 80-byte block header your chips grind. That property is exactly what makes the coinbase the engine room of pooled mining.
The coinbase as a search-space expander
A Bitcoin block header contains only a 32-bit nonce. Modern ASICs exhaust all four billion of those values in a fraction of a second, far faster than a new block arrives. The coinbase transaction solves this by giving the miner extra mutable bytes. Inside the coinbase input sits an extranonce field, conventionally split into two parts:
- extranonce1 — assigned by the pool to each connection so two miners never grind identical work.
- extranonce2 — rolled freely by the miner; its size is negotiated when the connection subscribes (commonly several bytes).
Each new extranonce value changes the coinbase, which changes the Merkle root, which gives the chips an entirely fresh header to search. This is why pools can keep a machine busy on a single job for far longer than the header nonce alone would allow.
How pools deliver and reconstruct the coinbase
Under Stratum V1, the pool does not send the full coinbase. In a mining.notify message it sends two halves, coinb1 and coinb2, along with the Merkle branches. The miner builds the complete coinbase by concatenating coinb1 + extranonce1 + extranonce2 + coinb2, hashes it, folds that result up through the branches to recompute the Merkle root, and only then assembles the header for hashing. When a share or block is found, the miner returns the job ID, its extranonce2, the timestamp, and the winning nonce so the pool can verify the work.
The coinbase outputs are constructed by the pool from a getblocktemplate response, which supplies the available coinbasevalue. A typical pool directs that value to its own address and takes its fee by adjusting the coinbase output split; solo pools instead pay the block straight to the miner's address and tag the coinbase with an identifying string. This is the literal mechanism by which mining rewards and pool fees are paid.
Coinbase control under Stratum V2
Stratum V2 changes who owns the coinbase. On a Standard Channel, the pool assembles the coinbase and the miner only rolls the header nonce, which can starve very fast machines of work. On an Extended Channel, the miner controls the coinbase and extranonce again, restoring the large search space older Stratum V1 miners depend on. Template-negotiation approaches such as OCEAN's DATUM go further: the miner builds its own block template from its own node and dictates the coinbase, while the pool only sets difficulty. This pushes block construction back toward the individual operator and is a meaningful step in decentralizing who decides what goes into a block.
If you are setting up your own machine, the practical takeaway is that the coinbase ties hardware, pool protocol, and economics together in one structure. Whether you are running a Bitaxe at home or a fleet in a mining pool, understanding it makes share and payout behaviour far less mysterious. If you would rather go further toward template sovereignty, solo mining with your own node gives you full control of the coinbase you mine. Browse our ASIC miners to find hardware that fits the setup you have in mind.