Bitcoin Coinbase Structure Guide for Solo Miners

A Bitcoin coinbase structure guide starts with one useful distinction: this is not Coinbase the company. The coinbase transaction is the first transaction in every Bitcoin block. It creates the block subsidy and collects that block's transaction fees. If your solo rig finds a valid block, this transaction is where the reward is assigned.

That makes the coinbase transaction worth reading, even if you never plan to build one by hand. It answers the custody question in a form Bitcoin nodes can enforce: which address receives the money, how much it receives, and whether the block follows consensus rules.

What the Bitcoin coinbase transaction does

A normal Bitcoin transaction spends earlier outputs. It names one or more inputs, proves authorization with signatures, and creates new outputs. A coinbase transaction has no ordinary input to spend. Instead, its one input identifies no previous transaction and carries special data called the coinbase script, or scriptSig.

Bitcoin permits this transaction because each valid block may create new bitcoin under fixed rules. The amount is not arbitrary. The total value of its outputs must be no more than the block subsidy plus the fees from transactions included in that block.

At the current stage of Bitcoin's issuance schedule, the subsidy is 3.125 BTC per block. Fees vary from block to block. A block with more fee-paying transactions can support a larger coinbase output. A miner or pool may claim less than the permitted amount, but it cannot claim more. Nodes reject a block that overclaims by one satoshi.

The term coinbase reward usually means both parts together: the subsidy and the transaction fees. They are created and assigned by one transaction, but they come from different places. The subsidy is new issuance. Fees were already left behind by the transactions in the block.

Bitcoin coinbase structure, field by field

The coinbase transaction resembles a normal transaction at the outside. It has a version, an input count, an input, an output count, one or more outputs, and a locktime. Its special behavior sits in the input and in the consensus rules around it.

The special input

The input points to a previous transaction hash of all zeroes and uses an output index of ffffffff. These values mean there is no prior output. The input sequence is normally ffffffff as well, although miners can set it differently where consensus permits.

Its scriptSig must be from 2 to 100 bytes long. Since BIP34, it must begin with the encoded height of the block being mined. This lets nodes confirm that the coinbase belongs at that height.

The remaining space can carry data chosen by the block builder. That commonly includes an extranonce. The extranonce is not a separate header field. It is data inside the coinbase scriptSig that changes the coinbase transaction ID.

Why does that matter? The coinbase transaction is one leaf in the block's Merkle tree. Change its extranonce, and its transaction ID changes. That changes the Merkle root in the block header. A miner then has a fresh header space to hash after it has worked through simpler header changes such as the nonce.

A text tag may also appear in the scriptSig. It is not a payment instruction and it does not prove a claim about who made the block. It is just data. Read it with the same caution you would apply to any label someone chose for themselves.

The payout outputs

The outputs say where the reward can be spent after maturity. A simple solo block can have one output that pays the full permitted reward to one Bitcoin address. More complicated construction can split the amount across several outputs, but the output values must still fit under the subsidy-plus-fees limit.

For a home miner, the essential check is plain: does the constructed coinbase pay your address, and does it pay the expected total? The address is not an account balance held by a pool. It is a locking condition written into the transaction. Once the block confirms, the output is visible on-chain.

NexusPool (nexuspool.io) builds a Bitcoin solo payout to the address supplied by the miner. Its Payout Preflight shows that constructed payout for the current block before the rig finds anything. It is a useful check because a promise about a future payout is weaker than the output script you can inspect now.

A payout output may use a modern SegWit address type, an older address type, or another standard script the miner selected. The address format affects the locking script, not the reward amount. Use an address you control. If you cannot restore the wallet that controls its keys, a correctly built coinbase transaction will not fix that problem.

The SegWit commitment

Blocks containing SegWit transactions need another coinbase detail. The coinbase input includes a witness with one 32-byte reserved value. The coinbase outputs also include an OP_RETURN output containing the witness commitment.

That commitment links the block to the witness data used by SegWit transactions. It is not a payment output. It cannot be spent, and it should not be mistaken for an extra fee or a destination for the block reward.

A block can include several outputs. When more than one output looks like a witness commitment, Bitcoin uses the one with the highest output index. This is a narrow rule, but it matters to software that validates or constructs blocks.

How fees reach the coinbase transaction

Fees do not move into the coinbase through a separate transaction. Each ordinary transaction has an input value and an output value. The difference is its fee. A block builder adds the fees from the transactions it includes, then sets the maximum permitted coinbase value to that fee total plus the subsidy.

This is why a payout estimate needs a specific block template. Fees can change as transactions enter, leave, or are replaced in the candidate block. A new template can also contain no transactions at first. In that case, the coinbase can claim the subsidy but no transaction fees.

The valid amount is also not the same thing as a guaranteed amount. A rig must first find a header below the network target. Network difficulty sets those odds at every pool. Solo mining is a lottery, and an average wait is not a schedule.

A found block still has a maturity period

Coinbase outputs cannot be spent immediately. Bitcoin requires 100 further blocks before a coinbase output becomes spendable. This is called coinbase maturity.

The rule gives the network time to settle the chain around the new block. It also means a wallet can show the reward as received while still marking it immature. That is expected behavior, not a stuck payout.

The block must also remain in the accepted chain. A valid-looking candidate block that loses a chain race is not a confirmed reward. This is why submission and propagation matter after a rig finds a share that meets the network target. They protect the luck the miner already had. They do not change the chance of getting that share.

What to check before you point a rig

For most home miners, the useful checks are short. Confirm the Bitcoin address is yours. Confirm the coinbase payout output uses that address. Confirm the displayed amount separates or accounts for the subsidy and the current template's fees. Then remember that a successful block reward becomes spendable only after 100 confirmations.

If you mine through a solo service, also ask what happens between your found share and the network. A clear answer includes how the pool reads a share, what it does when the share is a valid block, and where the block is sent. Vague language about luck does not answer any of those questions.

To connect a Bitcoin rig to NexusPool, use solo.nexuspool.io:3350 with your Bitcoin address as the mining identity. The endpoint accepts Stratum V1 and encrypted Stratum V2 on the same port. Your distance from its regions can affect how quickly work reaches your rig, but it does not affect the network difficulty that sets your odds.

The coinbase transaction is small compared with the machinery around it. Yet it is the part that states, in consensus-enforced bytes, where a winning block's money goes. Trust nothing. Verify the payout output before you mine.