What is a block in crypto?
A block is a batch of crypto transactions that has been checked by the network and permanently added to a blockchain. Each block also contains a fingerprint of the block before it, which links all blocks together into one chain. When an exchange says your deposit needs “3 confirmations”, it means your transaction must be in a block with at least two more blocks added on top.
What is a block in crypto?
A block in crypto is one page of a blockchain’s shared record. It holds a list of validated transactions, plus a block header with the information that ties it to the rest of the chain. Once a block is accepted by the blockchain network, changing it would mean redoing that block and every block after it, which is what makes the record so hard to tamper with.
Think of a blockchain as a notebook that thousands of people keep identical copies of. Every few minutes or seconds, a new page is filled with recent transactions and added to the end. Each page starts with a code made from the page before it, so nobody can quietly rewrite an old page without everyone noticing.
What’s inside a block
Every block has two parts: the block header and the transaction data. The header is small, but it does most of the security work.
| Part | What it contains | Why it matters |
|---|---|---|
| Previous block hash | The cryptographic hash of the block before it | Links the blocks into a chain |
| Merkle root | One hash that summarises all the transactions included | Any change to a transaction changes this hash |
| Timestamp | Roughly when the block was created | Puts blocks in chronological order |
| Difficulty and nonce | The target and the number miners vary to hit it | Proof that work was done (proof of work chains) |
| Transactions | All the transactions in the block, starting with the reward | The actual record of who sent what |
In Bitcoin, the first transaction in every block is the coinbase transaction: the one that pays the miner the block reward. It has nothing to do with the exchange of the same name.
The merkle tree is a clever shortcut. Transactions are hashed in pairs, then those hashes are hashed in pairs, until a single hash remains: the merkle root. That lets a wallet check that one transaction is in a particular block without downloading every transaction.
How new blocks are created
New blocks are created through a consensus mechanism, the set of consensus rules that decide who may add the next block. The two main types are proof of work and proof of stake.
- Transactions wait. When you send crypto, your transaction goes into a waiting area with other new transactions.
- A block producer picks transactions. On proof of work chains such as the Bitcoin blockchain, miners do this; on proof of stake chains, validators do. They usually pick transactions paying higher fees first.
- The block is sealed. In proof of work, miners race to solve cryptographic puzzles by trying huge numbers of guesses; Bitcoin mining as a whole uses as much energy as some entire countries, by most estimates. In proof of stake, a validator chosen by the protocol proposes the block.
- Nodes check it. Other nodes verify that every transaction and the header follow the rules. Only a valid block is accepted.
- The chain grows. The block is added, and subsequent blocks build on top of it.
Sometimes two miners find a valid block at almost the same moment. The network briefly has two versions, and the one that gets the next block wins. The losing one is called an orphan block (or stale block), and its transactions go back into the waiting area.
Block height, block time and block size
Block height, block time and block size are the three numbers you’ll see most often on block explorers and exchange pages. They describe where a block sits, how fast blocks arrive and how much fits in each one.
Block height
Block height is a block’s position in the chain. The genesis block, the very first block, has height 0, and each new block adds one. Bitcoin’s genesis block was created on 3 January 2009. Block height works as a unique identifier and a clock: some network upgrades and the Bitcoin halving are scheduled at a specific block height rather than a date.
Block time
Block time is the average time between one block and the next. On Bitcoin it is about 10 minutes, because the network adjusts mining difficulty to keep that average steady. Ethereum produces a block slot every 12 seconds. Shorter block times mean faster first confirmations, but not always faster final settlement.
Block size
Block size is how much transaction data one block may hold. Because space is limited, users compete for it with fees, so fees rise when many people want to send at once. Disagreement over larger blocks led to a hard fork in 2017, when Bitcoin Cash split off from Bitcoin under new rules while Bitcoin kept the old rules.
Why blocks matter to you
Blocks matter to you mainly in two places: deposits and withdrawals. They decide how long you wait and what you pay.
Confirmations. An exchange only credits a crypto deposit after it has a set number of confirmations, which differs per coin and per platform. If an exchange asks for 3 Bitcoin confirmations, expect around 30 minutes on average, and longer when blocks are slow. On faster chains it can take less than a minute.
Fees. Network fees depend on how much block space your transaction uses and how busy the network is, not on the amount you send. Sending €50 or €5,000 in a simple Bitcoin transaction costs roughly the same network fee. That’s why small transfers on busy chains can be expensive, and why many people withdraw less often in larger amounts. Exchanges also add their own withdrawal fee; see our page on network fees.
Mistakes are final. Once your transaction is in a block with a few blocks on top, it can’t be reversed. Always check the address and the network before you send.
Blocks and EU rules
In the EU, MiCA doesn’t regulate blocks or blockchains themselves; it regulates the companies that provide crypto services. Licensed exchanges must publish their fees, and they must disclose the climate and environmental impact of the consensus mechanism behind each coin they offer. That’s why you may see energy-use information next to proof of work coins on a MiCA-licensed platform.
Public and private blockchains
Blocks work the same way on public and private chains, but who may create them differs. On a public blockchain, anyone can run a node and, in principle, help produce blocks. On a private blockchain, only approved parties can add blocks. Businesses use private versions for things like supply chain management and sharing records between financial institutions, without a central authority holding the only copy.
The bottom line
A block is a sealed batch of transactions, linked to the one before it, that together form the blockchain’s immutable record. You don’t need to know how hashes work to use crypto, but knowing about block time, confirmations and fees helps you understand why deposits take time and why some transfers cost more. Read on about the blockchain, mining, validators and consensus mechanisms.
Questions people ask
What is a block height?
Block height is the position of a block in the chain, counted from the first block. The genesis block has height 0, the next one height 1, and so on, so the current block height tells you how many blocks have been added since the chain started.
How long does 1 block take on Bitcoin?
A new Bitcoin block is found about every 10 minutes on average. Individual blocks can take a few seconds or much longer, because finding one depends on luck as well as computing power.
How many bitcoins are in a block?
A block doesn't hold a set amount of bitcoin; it holds transactions of any size. The miner who finds a block receives a reward, which has been 3.125 BTC per block since the April 2024 halving, plus the transaction fees in that block.
What is the first block of a blockchain called?
The first block is called the genesis block. Bitcoin's genesis block was created on 3 January 2009, and every later block links back to it.
Is Bitcoin a block chain?
Bitcoin runs on a blockchain: a chain of blocks in which each block contains the hash of the one before it. Bitcoin is the currency; the blockchain is the ledger that records who owns it.