What is block?

A batch of transactions recorded together on a blockchain.

Each block references the previous one, forming the chain. Block times vary: roughly ten minutes on Bitcoin, about twelve seconds on Ethereum.

Blocks are why crypto settlement is not instant. Your transaction is not recorded when you send it; it is recorded when someone includes it in a block, and it becomes hard to reverse only as further blocks are built on top.

How it works

A block has a header and a body. The body holds transactions. The header holds a timestamp, a reference to the previous block's hash, and a commitment to the transactions inside, usually the root of a merkle tree built over them.

Linking by hash is what makes history tamper-evident. Changing any transaction changes the merkle root, which changes the header hash, which breaks the reference held by every later block.

Who may produce a block depends on the consensus rule. Under proof of work a miner must find a header hash below a target, which costs energy. Under proof of stake a validator is selected for a slot and simply proposes.

Capacity is bounded. Bitcoin limits block weight; Ethereum limits total gas per block. That bound is what creates a fee market, because when demand exceeds capacity, inclusion goes to whoever pays more.

Settlement certainty accumulates with depth. Each confirmation is one more block on top, and exchanges pick a number of confirmations per asset before crediting a deposit.

Example

Illustrative: you withdraw Bitcoin at 14:00 and the fee you paid puts you in the next block, mined at 14:07. That is one confirmation. Your exchange requires three, so it credits at roughly 14:27 after two more blocks. Had you underpaid the fee rate, your transaction would have sat in the mempool through several blocks until fee rates fell. Ten minutes is an average, not a schedule; individual gaps of two minutes and forty minutes are both ordinary.

Why it matters when you buy

Block time and confirmation requirements decide how long a deposit or withdrawal takes, and they differ by orders of magnitude between chains. A transfer that clears in seconds on one network takes an hour on another, and exchanges add their own confirmation thresholds on top. Check the network options for your asset before withdrawing, and see the guide on using a block explorer.

Related terms

• blockchain — the chain of linked blocks
• confirmation — depth of a transaction below the tip
• block height — the position of a block in the chain
• mempool — where transactions wait for inclusion
• merkle tree — how a block commits to its transactions
• block reward — what the producer is paid

Questions

Why does my transaction show zero confirmations?

Because it has been broadcast but not yet included in a block. It will confirm when a producer picks it up, which depends on the fee you paid relative to others waiting.

Are faster blocks better?

Not straightforwardly. Shorter block times give quicker first confirmations but each one carries less weight, so chains with fast blocks require more of them for equivalent certainty.

Can a block be undone?

Yes, near the tip. A chain reorganization can replace recent blocks, which is why exchanges wait for several confirmations before treating a deposit as final.