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Blocks & network · 2 min read

What is a block?

By Oliver James Whitmore · Updated 20 October 2026

In one sentence

A block is a batch of transactions bundled together with a header and added to a blockchain as one unit, once the network accepts it as valid.

Each block stores the hash of the block before it, so the blocks form a chain.Block 100prev hash …7e1Merkle rootBlock 101prev hash …a40Merkle rootBlock 102prev hash …c9bMerkle rootAlter block 100 and its hash changes: block 101 no longer points to it
Each block stores the hash of the block before it, so the blocks form a chain.

How it works

Every block has two parts. The body is the list of transactions. The header is a compact summary that includes, among other things, the hash of the previous block and a fingerprint of all the transactions in this one.

That reference to the previous block is what makes a chain. Block 101 contains the hash of block 100; block 102 contains the hash of block 101; and so on back to the first block. If someone altered a transaction in an old block, that block's hash would change, the next block would no longer point to it, and every later block would have to be rebuilt — together with the consensus work or votes behind each of them.

Blocks have limits. In Bitcoin, a block may weigh at most four million "weight units", a measure introduced with the SegWit upgrade. In Ethereum, the limit is expressed in gas, the unit of computational work. Timing differs too: Bitcoin adjusts its difficulty so that a block appears about every ten minutes on average, while Ethereum divides time into fixed twelve-second slots, each of which can hold one block.

The first transaction in a Bitcoin block is special. Created by the block producer, it issues the new units the rules allow and collects the fees from the other transactions.

A simple example

Imagine a numbered ledger in which the top line of every page is a fingerprint of the page before it. Tear out page 50 and rewrite it, and the fingerprint at the top of page 51 no longer fits. To hide the change, you would have to rewrite page 51, then 52, and every page after it — while everyone else keeps adding new pages.

Common misconception

Misconception: A new block appears exactly every ten minutes.

In fact: Ten minutes is Bitcoin's long-run average, not a timetable. Finding a block is random: some arrive seconds apart, others take over an hour. Every 2,016 blocks the difficulty is adjusted to keep the average on target.

Where you meet it

  • Block explorers, which show each block's height, time, size and transactions.
  • Confirmations, which simply count blocks added on top.
  • Upgrade announcements, which often name the block height at which new rules start.

Sources

This card explains a technical concept for general education. It is not a recommendation to acquire, sell or hold any crypto-asset, and it is not financial advice.