A transaction block is a batch of transactions that a blockchain network has accepted, plus a small header that holds the hash of the block before it. That link puts blocks in a fixed order. Changing an old block would break every block after it. On Bitcoin, miners produce a new block about every 10 minutes on average. Each block can hold up to 4,000,000 weight units of data. Once a block is accepted, its transactions count as confirmed. Each later block makes them harder to reverse.
What a block contains
A Bitcoin block has two parts: an 80-byte header and a list of transactions.
The header: 80 bytes, six fields
Bitcoin's developer reference lists these header fields:
| Field | Size | What it does |
|---|---|---|
| Version | 4 bytes | Says which block validation rules apply |
| Previous block header hash | 32 bytes | SHA256(SHA256) hash of the previous block's header |
| Merkle root hash | 32 bytes | One hash computed from every transaction in the block |
| Time | 4 bytes | Unix time when the miner started hashing; must be later than the median time of the previous 11 blocks |
| nBits | 4 bytes | Encoded target threshold that the header hash must not exceed |
| Nonce | 4 bytes | The number miners change to find a valid header hash |
Two fields make the chain work. The previous block header hash links each block to the one before it. The merkle root commits to every transaction in the block. To build it, transaction IDs are paired and hashed, round after round, until one hash remains. If you change any transaction, the merkle root changes. That changes the header hash, which breaks the link from the next block.
The transaction list
The first transaction in every block is the generation transaction. The miner creates it to pay itself. It can claim two things:
- the block subsidy, which is new bitcoin created by the protocol, and
- the transaction fees from every other transaction in the block.
The Bitcoin whitepaper describes it as "a special transaction that starts a new coin owned by the creator of the block". A generation transaction is invalid if it claims more than the subsidy plus fees. Its outputs cannot be spent until at least 100 more blocks have been added.
How a transaction gets into a block
- Signing. The sender signs the transaction with the private key that controls the coins being spent. The signature proves ownership without revealing the key. Coins are usually received at a Bitcoin address, which is an encoded hash of a public key.
- Broadcast. The signed transaction goes out to the network's nodes and waits in their pools of unconfirmed transactions.
- Selection. Miners choose which transactions to include. Bitcoin's developer guide says transactions are prioritised by fee per byte. A transaction that pays too little can wait a long time.
- Proof of work. The miner keeps changing the nonce and other variable data and re-hashing the header. It stops when the hash falls below the target set by nBits. The whitepaper calls this "incrementing a nonce in the block until a value is found".
- Verification. The miner broadcasts the block. Nodes accept it only if every transaction in it is valid and none of its coins are already spent. Checking a valid hash takes one calculation. Finding one takes enormous computing work.
- Building on it. Nodes accept the block by mining the next block on top of it.
Where fees come from
There is no separate fee field. Under the whitepaper's rule, if a transaction's outputs add up to less than its inputs, "the difference is a transaction fee". The block's miner collects it. Fees depend on the transaction's size in bytes, not on the amount sent, and they rise and fall with network demand.
Why blocks arrive about every 10 minutes
Bitcoin does not fix the time between blocks. It sets how hard the proof of work is. Every 2,016 blocks, nodes compare how long those blocks actually took with the ideal of 1,209,600 seconds (two weeks). If the blocks came faster, the target gets harder. If they came slower, it gets easier. Two weeks divided by 2,016 blocks is the 10-minute average. Single blocks still vary a lot, and some arrive within seconds of each other.
The block subsidy and halvings
In Bitcoin Core, the subsidy starts at 50 BTC and halves every 210,000 blocks. The code comments that this happens "approximately every 4 years". The most recent halving came at block 840,000, mined on 20 April 2024 (UTC). The subsidy fell from 6.25 BTC to 3.125 BTC. At the current pace, the next halving is due at block 1,050,000, when the subsidy falls to 1.5625 BTC. After that, fees make up a growing share of what miners earn.
How much block weight does your data use?
Enter the base size (no witness data) and the witness (signature) size in bytes to see the weight under Bitcoin's SegWit rule.
| Base size: bytes without witness (signature) data | Witness (signature) data | Weight in weight units (a block may not exceed 4,000,000) |
|---|---|---|
| 250 bytes | 150 bytes | 1,150 WU |
| 1,000,000 bytes | 0 bytes | 4,000,000 WU |
| 500,000 bytes | 2,000,000 bytes | 4,000,000 WU |
- Both sizes are your own inputs, not figures from any source.
- Total size is taken as base size plus witness size, so witness bytes count once and base bytes count four times.
- The first example shows that 1,000,000 bytes with no witness data already reach the 4,000,000 limit. The second shows that 2,500,000 bytes, mostly signatures, also fit.
This is arithmetic on the BIP 141 weight formula only: it does not measure a real transaction's size, estimate fees or say whether miners will include it.
Confirmations: why waiting adds security
A transaction has one confirmation once it is in a block. Each later block adds another. To rewrite a buried transaction, an attacker would have to redo the proof of work for that block and every block after it, then overtake the honest chain. The whitepaper describes "the longest chain" as proof of the sequence of events.
The bitcoin.org FAQ calls six confirmations a common benchmark. It says this is "often considered to be as safe as waiting 6 months on a credit card transaction". This is a convention, not a protocol rule. Each recipient chooses how many confirmations they need.
Two blocks can occasionally be mined at the same height. Nodes usually keep the first one they see. The network then settles on the chain with the most cumulative work. That is why one confirmation is weaker than several.
Block size limits
Since the SegWit soft fork (BIP 141), Bitcoin limits blocks by weight, not raw bytes. Weight = base size × 3 + total size. Base size leaves out witness (signature) data, and total size includes it. A block may not exceed 4,000,000 weight units. Signature-heavy transactions therefore fit more bytes per block than the old 1 MB rule allowed.
How other blockchains differ
Ethereum also chains blocks by the hash of the previous block, but it does not use proof of work. Time is divided into 12-second slots, according to ethereum.org. For each slot, the network randomly selects a validator with 32 ETH staked to propose a block, and other validators re-execute its transactions to verify it. A slot can go empty if the chosen validator is offline. Ethereum limits blocks by gas, a measure of computation, not by bytes. Validators can move the gas limit up or down a little with each block.
Key points
- A block = an 80-byte header + a list of transactions (Bitcoin).
- The previous-block hash and the merkle root make history tamper-evident.
- The first transaction, the generation transaction, pays the miner the subsidy plus fees.
- Difficulty is adjusted every 2,016 blocks to keep the average near 10 minutes.
- Subsidy since 20 April 2024: 3.125 BTC per block.
- More confirmations make a payment harder to reverse; six is a convention, not a rule.
Related reading: What Are Blockchains? · What is Bitcoin Mining?
Limitations: this article describes Bitcoin's consensus rules and Ethereum's documentation as of October 2026; details such as gas limits and fee levels change over time.
