The Coinbase Transaction: Core Mechanics and Block Rewards

Every block added to the Bitcoin blockchain contains a unique, mandatory transaction at index 0 called the Coinbase Transaction. This transaction serves a critical role in the system: it is the sole mechanism by which new bitcoins are minted, and it is how miners collect transaction fees from user transactions.

This guide details the core mechanics of coinbase transactions, the block reward halving schedule, and the consensus-enforced maturity rules.

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🪙 1. The Block Reward Equation

Miners do not earn a static income. The total payout in a coinbase transaction—called the Block Reward—is dynamically calculated for each block as the sum of two variables:

$$\text{Block Reward} = \text{Block Subsidy} + \sum \text{Transaction Fees}$$

                           THE COINBASE REWARD PIPELINE

    ┌───────────────────────────┐
    │  Block Subsidy (New Coins)│ ──┐
    │  Currently 3.125 BTC      │   │
    └───────────────────────────┘   │
                                    ├─► [ Coinbase Payout Output ]
    ┌───────────────────────────┐   │
    │  Sum of Block Tx Fees     │ ──┘
    │  e.g. 0.125 BTC           │
    └───────────────────────────┘

• Block Subsidy: The newly minted coins generated out of thin air. This value decreases over time according to a strict mathematical schedule.
• Transaction Fees: The difference between the inputs and outputs of all transactions included in the block: $$\sum \text{Fees} = \sum \text{Inputs} - \sum \text{Outputs}$$ These fees are claimed by the miner and combined with the subsidy inside the coinbase payout outputs.

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📉 2. The Halving Schedule and Supply Cap

To enforce a finite supply of $21,000,000\text{ BTC}$, the block subsidy is designed to decay. Every 210,000 blocks (approximately $4\text{ years}$), the subsidy is cut exactly in half.

The Subsidy Epochs

$$\text{Subsidy} = \frac{50 \text{ BTC}}{2^{\text{Epoch}}}$$

Epoch	Block Range	Approximate Years	Subsidy Per Block	Total Coins Created
0	0 - 209,999	2009 - 2012	50.0 BTC	$10,500,000\text{ BTC}$
1	210,000 - 419,999	2102 - 2016	25.0 BTC	$5,250,000\text{ BTC}$
2	420,000 - 629,999	2016 - 2020	12.5 BTC	$2,625,000\text{ BTC}$
3	630,000 - 839,999	2020 - 2024	6.25 BTC	$1,312,500\text{ BTC}$
4	840,000 - 1,049,999	2024 - 2028	3.125 BTC	$656,250\text{ BTC}$

Once block height $6,930,000$ is reached (around year 2140), the subsidy shifts down to 0 satoshis. From that point on, miners will be funded entirely by transaction fees.

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🔒 3. The 100-Block Maturity Rule

Because a block could be orphaned during a temporary blockchain split (reorganization), coinbase outputs carry a highly strict security constraint known as the Coinbase Maturity Rule.

The Consensus Rule

A coinbase output cannot be spent as an input in a subsequent transaction until it has been buried by at least 100 blocks on the active chain:

$$\text{Maturity Age} \ge 100 \text{ Blocks}$$

• Why? If a miner spent their $3.125\text{ BTC}$ block reward immediately in Block $N+1$, and Block $N$ was subsequently orphaned due to a chain reorg, Block $N$'s coinbase transaction would cease to exist. This would cause all subsequent transactions spending those newly minted coins to become invalid, causing massive cascading transaction failures across the network.
• The Constraint: Validating nodes enforce this strictly; any transaction attempting to spend a coinbase output with less than 100 confirmations is rejected.