What Is a Block?
A block is a batch of transactions recorded on a blockchain, together with the data needed to verify them. Each block contains the hash of the previous one, which links blocks into a chain.
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A block is a batch of transactions recorded on a blockchain, bundled with the information needed to verify them. Each block contains the hash of the block before it, and those links are what turn individual blocks into a chain. When your transfer is “confirmed”, it means it has been included in a block on the valid chain.
What’s inside a block?
A block has two parts: a header and a list of transactions. In Bitcoin, the block header is just 80 bytes and contains:
- Version: which set of rules the block follows.
- Previous block hash: the field that links the block to the chain.
- Merkle root: a single hash that summarizes every transaction in the block.
- Timestamp: the approximate time the block was created.
- Target and nonce: the difficulty target and the value the miner varied to meet it.
The first transaction in every Bitcoin block is the coinbase transaction, which pays the miner the block subsidy plus the fees from the included transactions. Since the April 2024 halving, the subsidy has been 3.125 BTC per block.
How is a block created and added to the chain?
On Bitcoin, miners pick pending transactions from the mempool, assemble a candidate block and compete through proof of work. The miner who finds a valid hash broadcasts the block. Full nodes then check every transaction, every signature and the size of the miner’s reward. A single invalid transaction is enough for the entire block to be rejected, no matter how much work went into it.
On proof-of-stake networks such as Ethereum, a block is proposed by the validator whose turn it is and accepted through votes from other validators. Ethereum divides time into 12-second slots, with one block proposal per slot.
Block height and block time
Block height is a block’s position in the chain. Bitcoin’s first block, the genesis block, was mined on January 3, 2009 and sits at height 0. Block explorers show the height of the block that contains your transaction, which makes it easy to count how many blocks have been added since. Clicking a block number also shows who produced it, how many transactions it holds and how much it collected in fees; our guide on tracking a blockchain transaction walks through the steps.
Bitcoin targets an average of about one block every ten minutes, but that is only an average. Sometimes two blocks arrive a minute apart; sometimes the network waits more than an hour. That is why you can’t assume three confirmations will take exactly thirty minutes.
Why is block size limited?
If blocks could be any size, the storage and bandwidth needed to run a node would grow quickly, and fewer people could independently verify the chain. Since SegWit, Bitcoin measures capacity as a limit of 4 million weight units per block. When demand exceeds that space, users bid against each other with fees, which is why transfer costs climb during busy periods. Ethereum measures block capacity in gas rather than bytes, with a cap on the total gas each block can use.
Once your transaction lands in a block it has one confirmation. Every block added on top increases that count and makes the transaction harder to reverse.
Frequently asked questions
What’s the difference between a block and a transaction?
A transaction is a single instruction to move value from one address to another. A block is the package that records many transactions at once.
How many transactions fit in a block?
There is no fixed number. On Bitcoin it depends on how large each transaction is; a typical block holds a few thousand.
What is an orphan block?
When two valid blocks are found at nearly the same time, the one that ends up outside the winning chain is commonly called an orphan or stale block. Its transactions usually return to the mempool and are picked up by later blocks.