What is coinbase transaction?

The first transaction in every block, created by the miner, which pays the block subsidy plus all fees in that block to an address of the miner's choosing.

The name predates the exchange of the same name and has nothing to do with it. It is the only transaction with no ordinary inputs, which is how new coins enter existence: the miner is not spending anything, it is claiming what the protocol permits.

It is also where a miner can place arbitrary bytes, which is how Bitcoin's first block carries a newspaper headline and how mining pools mark the blocks they find.

How it works

Every block contains exactly one coinbase transaction and it must come first. Its output equals the block subsidy, which halves on a fixed schedule, plus the total fees paid by every other transaction in the block. A miner that constructs a coinbase claiming more than that produces a block other nodes reject.

The output cannot be spent until 100 further blocks have been built on top (source: Bitcoin developer documentation). That maturity delay exists so a chain reorganization cannot leave spent coins that never existed: if the block is orphaned, the reward vanishes, and the delay ensures nobody has already spent it.

The coinbase field itself holds arbitrary data. Beyond the block height, which is required, miners use it for pool identifiers, messages, and extra entropy while searching for a valid hash.

Ethereum has no equivalent transaction. Since the transition to proof-of-stake, issuance and fee distribution are handled by the protocol at the consensus layer rather than by a transaction inside the block.

Example

Illustrative arithmetic using a 3.125 BTC subsidy. A block contains 2,800 transactions paying 0.18 BTC in fees in total. The coinbase transaction pays 3.125 plus 0.18, so 3.305 BTC, to the pool's address. That output sits unspendable until block height plus 100, roughly 16 hours later at ten-minute blocks. If a reorg removes the block during that window, the reward is simply gone, and the fees return to the mempool with their transactions.

Why it matters when you buy

Coinbase transactions are where new supply enters, so their size sets the ongoing issuance you are buying into, and the halving schedule is the one supply change on Bitcoin that is fully known in advance. As the subsidy shrinks, fees become a larger share of what secures the chain, which eventually shows up in what you pay to move coins. The chain pages cover network fee levels, and the coin pages show supply.

Related terms

block subsidy — the newly issued half of the reward; block reward — subsidy plus fees together; bitcoin halving — the schedule that shrinks the subsidy; mining pool — who usually claims it; chain reorganization — the reason for the 100-block delay.

Questions

Why can't miners spend the reward immediately?

Because the block might be orphaned by a reorg. Bitcoin requires 100 further blocks before a coinbase output is spendable, so a reward that turns out never to have existed cannot already have been spent.

What happens to fees if a block is reorganized out?

The transactions return to the mempool and their fees are claimed by whichever miner includes them next. The orphaned block's coinbase output simply ceases to exist.

Does Ethereum have a coinbase transaction?

Not since the move to proof-of-stake. Validator rewards and fee payments are handled by the protocol itself rather than by a special transaction inside the block, though the term survives in some older tooling.