What Is a Bitcoin Block? Size, Time and What's Inside
A Bitcoin block bundles confirmed transactions and links to the block before it. See what's inside, how often blocks arrive and why their size is limited.
By Christopher Cannucciari · Published
Key takeaways
- A block is a bundle of confirmed transactions, plus a header that links it to the block before it. Chained together, the blocks form the blockchain.
- A new block is added about every ten minutes on average. Blocks are found by chance, so single gaps vary a lot.
- The header holds the previous block’s hash, a summary of the transactions, a timestamp, the difficulty target and a number miners vary. The first transaction in the block pays the miner.
- Block height counts the blocks before a given block. The genesis block is height 0.
- Block space is limited to 4 million weight units since 2017, so transactions compete for it. That competition is the fee market.
What is a Bitcoin block?
A Bitcoin block is a bundle of transactions that a miner has checked and packaged, plus a small header that ties it to the block before it. Once the network accepts the block, every transaction inside counts as confirmed.
Think of a numbered, sealed batch of payments. The seal carries a fingerprint of the previous batch, so the batches can only be arranged in one order. What Is a Blockchain? shows how the links make old blocks hard to change, and How Does Bitcoin Work? places blocks among keys, miners and nodes.
What is inside a Bitcoin block?
A block has two parts: a header of 80 bytes and a list of transactions. The header is the part that does the linking and carries the proof of work.
| Header field | What it holds | Why it is there |
|---|---|---|
| Version | A number that can signal support for upgrades | Lets miners show which rule changes they back |
| Previous block hash | The fingerprint of the block before | Links each block to the one before |
| Merkle root | One hash computed from all transactions in the block | Any change to a transaction changes it |
| Timestamp | The time the miner put in the block | A loose record of when, not a precise clock |
| Difficulty target | The target the block’s hash had to beat | Lets every node check the work |
| Nonce | A number miners change on each attempt | The thing miners vary while searching |
The Merkle root is built by hashing the transactions in pairs, then hashing those results in pairs, until one hash remains. Change a single character in one transaction and the root changes, so the header commits to every transaction without listing them.
A block’s own hash is not stored in it. It is computed by hashing the header, and it must fall below the target. When the nonce alone runs out, miners vary other details, such as part of the first transaction. Bitcoin Hash Rate and Mining Difficulty Explained shows how the search works.
What is the first transaction in a block?
The first transaction in every block is the coinbase transaction, which creates the new bitcoin that pays the miner. It has no inputs from earlier coins. Its output pays the block subsidy plus all the fees from the other transactions in the block to an address the miner chooses.
The subsidy is the part cut at each halving, as What Is the Bitcoin Halving? explains. A miner cannot claim more than the rules allow, because nodes reject a block that does.
Every other transaction is an ordinary payment that the miner picked from the waiting pool. Each one spends outputs from earlier transactions and creates new ones, as What Is a UTXO? describes.
What is block height?
Block height is the number of blocks that come before a given block in the chain. The genesis block, the first one, has height 0, the next has height 1, and the count goes up by one with every new block. The Bitcoin Genesis Block tells the story of block 0.
Height is how people and the protocol point at a block. The subsidy halves at multiples of 210,000 and difficulty is recalculated every 2,016 blocks, both counted in height, not dates. Height also gives confirmations a simple meaning. A transaction in the newest block has one confirmation, and if five more blocks arrive on top of it, it has six. How Long Does a Bitcoin Transaction Take? uses this to explain waiting times.
Height is not time. It rises by one per block, while the timestamp is only the miner’s claim, so height is the exact reference.
How often is a new block added?
A new block is added about every ten minutes on average, but any single gap can be much shorter or much longer.
Finding a block is like a lottery in which machines buy tickets at enormous speed. Every attempt has a tiny chance of winning, and nobody can schedule the winning draw. So one block may follow another within seconds, and the next may take an hour. What keeps the long-run average near ten minutes is the difficulty adjustment, which makes the target harder when blocks come too quickly and easier when they come too slowly. Inside a Bitcoin Mining Operation shows who is racing and why.
How big is a Bitcoin block?
A Bitcoin block is limited to 4 million weight units, which allows a size of a few megabytes at most. That limit came with the SegWit upgrade in 2017. Before then, the rule was simpler: a block could hold at most 1 megabyte of data.
Weight is not the same as raw size, because signature data is cheaper to include than other data. That made room for more transactions in each block.
| Measure | What it counts |
|---|---|
| Size | The raw number of bytes in the block |
| Weight | Each byte counts four units, except signature (witness) data, which counts one |
| Virtual size (vbytes) | Weight divided by four, the unit fee rates are quoted in |
Real blocks are usually smaller than the ceiling, and their size changes with the mix of transactions in them. One reason for a limit is to keep running a node affordable, so more people can check the chain for themselves. The size limit was also at the center of the scaling debate behind the August 2017 split that created Bitcoin Cash, which Bitcoin Forks Explained covers.
Why does limited block space create a fee market?
Limited block space creates a fee market because users compete for room in a block that arrives about every ten minutes. When more transactions are waiting than fit, miners fill the block with the ones that pay the most per unit of space, and the rest wait for a later block.
Picture a delivery van with a fixed cargo volume that leaves about every ten minutes. Parcels are charged by the space they take, not by what is inside. A transaction works the same way: its fee depends on its size in vbytes, not on how much bitcoin it moves. When demand is low, a small fee gets a transaction into the next block, and when demand is high, it takes a bigger one.
Fees are also part of what miners earn, next to the subsidy. What Is the Bitcoin Mempool? explains the waiting pool and how miners choose, and Bitcoin Fees Explained covers what you pay and why.
What are stale blocks and temporary forks?
A stale block is a valid block that lost a race and did not stay in the chain. It happens when two miners find a block at nearly the same moment. For a short time, different nodes see different newest blocks, which is a temporary fork.
It resolves when the next block is built on one of the two. Nodes follow the chain with the most accumulated work, so they all switch to that branch, and the other block is dropped. Transactions that appeared only in the dropped block go back to waiting and are usually included in a later block. The miner of the stale block earns no reward for it.
This is routine and different from a dispute over the rules, which Bitcoin Forks Explained covers. It is also why a payment under several blocks is safer than one in a single block.
How can you look at a block yourself?
You can look at a block on a block explorer, a website where anyone can search the public chain by height, hash or transaction. Enter a height such as 0 to see the genesis block. A block page lists the height, hash, time, size, weight, number of transactions, the miner’s reward and the full list of payments.
A node you run yourself can show the same data from your own copy of the chain, with no third party. What Is a Bitcoin Node? explains what running one involves. Never type a seed phrase or private key into an explorer. Anyone who asks you to is attempting theft.
If you are new to all this, the Start Here path lists the core guides in order.
Where to go next
- What Is a Blockchain? Bitcoin’s Ledger Explained: how blocks link into a tamper-evident record.
- The Bitcoin Genesis Block: The First Block and Its Hidden Message: block 0 and why every other block leads back to it.
- What Is the Bitcoin Mempool? Why Transactions Wait: where transactions wait before a miner picks them.
- How Long Does a Bitcoin Transaction Take? Confirmations and Fees Explained: what block timing means for your payment.
Frequently asked questions
What is a Bitcoin block?
A block is a bundle of Bitcoin transactions that a miner has checked and added to the chain, together with a small header that links it to the previous block. The first transaction in a block pays the miner, and the rest are ordinary payments from users.
How often is a new Bitcoin block added?
About every ten minutes on average. The average is steered by the difficulty adjustment, but each block is found by chance, so single gaps can be a few seconds or more than an hour.
How big is a Bitcoin block?
A block is limited by weight, not by raw size. Since the SegWit upgrade in 2017 the limit is 4 million weight units, which allows a block of a few megabytes at most. Before that, the limit was 1 megabyte of block data. Real blocks vary with the mix of transactions.
What is block height?
Block height is the number of blocks that come before a given block in the chain. The first block, the genesis block, has height 0, the next has height 1, and so on. Height is how people and the protocol rules refer to a particular block.
What is inside a Bitcoin block?
A block holds a header and a list of transactions. The header contains a version number, the hash of the previous block, a summary hash of the transactions, a timestamp, the difficulty target and a number that miners change. The first transaction creates the new bitcoin that pays the miner.
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