A Bitcoin block is a batch of verified transactions bundled together and added to the blockchain approximately every 10 minutes. Each block contains a header (with the previous block's hash, a timestamp, and a nonce) and a body (the transactions themselves), forming a tamper-evident chain.
What Is a Bitcoin Block?
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The short version
Think of a block as one page in a ledger book. Every 10 minutes, a new page is filled with transactions, stamped with a unique fingerprint that links it to the previous page, and permanently added to the book. Tearing out or altering any page would break all the fingerprints that follow.
How It Works
A block has two parts: the header (80 bytes) and the transaction list. The header contains: version number, previous block hash, Merkle root (a hash summarizing all transactions), timestamp, difficulty target (nBits), and nonce. The transaction list includes the coinbase transaction first, followed by all included user transactions. Blocks have a maximum weight of 4 million weight units (roughly 1-4 MB of data depending on transaction types). Miners select which transactions to include, typically prioritizing those with higher fee rates (satoshis per virtual byte).
Anatomy of block 840,000 (the 2024 halving block)
Block 840,000 was mined on April 19, 2024. It contained 3,050 transactions, weighed 1.61 MB, and collected 37.6 BTC in transaction fees (unusually high due to Runes protocol launch). Its header hash starts with 19 zeros. The previous block hash links it irrevocably to block 839,999. Computing this block's hash takes milliseconds to verify but required the entire network's combined ~600 EH/s working for approximately 10 minutes to produce.
What People Get Wrong
Blocks are always 1 MB
Since SegWit (2017), Bitcoin uses block weight (4M weight units max). Actual block sizes now commonly reach 1.5-3.5 MB depending on transaction mix.
Every block takes exactly 10 minutes
10 minutes is the average. Individual blocks can take 1 minute or 60 minutes, it's probabilistic. Difficulty adjusts every 2,016 blocks to maintain the average.
More transactions make blocks slower
Block time is independent of transaction count. More transactions just mean a fuller block and potentially higher fees for inclusion.
Keep Reading
Sources & Further Reading
- Bitcoin Block Structure (Developer Docs)
Official specification of block headers, Merkle roots, and transaction format
- Mempool.space Block Explorer
Browse real Bitcoin blocks with transaction details, weight, and fee data
Questions People Also Ask
- How many transactions fit in a block?
- Typically 2,000-4,000 transactions depending on transaction size. Simple transfers are smaller; complex multi-input transactions take more space.
- What happens if two miners find a block at the same time?
- A temporary fork occurs. The network follows whichever chain gets the next block added first, orphaning the other. This resolves naturally within one block.
- Can a block be empty?
- Yes, miners can produce blocks with only the coinbase transaction. This occasionally happens when a miner finds a block very quickly and hasn't yet received new transactions.