Block

A block bundles transactions that are added to the blockchainGlossaryBlockchainBitcoin’s public ledger: a chain of blocks in which each block refers to its predecessor. Anyone wanting to change an old entry would have to recreate all the blocks that follow it.On the learning path: Stage 1 · Step 3 – How does Bitcoin work? →In the glossary →× together. On average, a new one is added roughly every ten minutes. Miners assemble it; every nodeGlossaryNodeA computer that runs Bitcoin software and exchanges transactions and blocks with other nodes. Together, the nodes form the Bitcoin network; full nodes check every rule themselves.On the learning path: Stage 1 · Step 3 – How does Bitcoin work? →In the glossary →× checks it.

What a block consists of

• Block header (80 bytes): among other things, the hash of the previous block (this is what forms the chain), a fingerprint of all transactions (the Merkle root), a timestamp, the target for the mining puzzle and the nonceGlossaryNonceA 4-byte number in the block header that miners change over and over during proof of work until the hash of the block header is less than or equal to the target. Each new value is a new attempt.On the learning path: Stage 1 · Step 3 – How does Bitcoin work? →In the glossary →×, which miners vary as they try out solutions.
• Transactions: The first is always the coinbase transaction, with which the miner pays themselves the block rewardGlossaryBlock rewardWhat a miner receives for a valid block: newly created bitcoin (the subsidy) plus the fees of all transactions in the block. The subsidy halves every 210,000 blocks; on 20 April 2024 it fell to 3.125 BTC.On the learning path: Stage 2 · Step 2 – Halving & 21 million →In the glossary →×.^[1]

Space is limited: since SegWitGlossarySegWit (Segregated Witness)A 2017 soft fork that treats signature data (the ‘witness’) separately. It fixed transaction malleability, increased block capacity and makes payments with modern addresses (bc1q…) cheaper.In the glossary →×, a block may contain at most 4 million weight units.^[2] That’s why transactions in the mempoolGlossaryMempoolThe buffer in which a node collects valid but still unconfirmed transactions until a miner includes them in a block. There is no central mempool – every node keeps its own.On the learning path: Stage 5 · Step 4 – First withdrawal →In the glossary →× compete for a place through their fees.

Block height and confirmations

Blocks are numbered consecutively; the number is called the block height. The genesis blockGlossaryGenesis blockThe very first block of the Bitcoin blockchain, created on 3 January 2009. It contains a newspaper headline about bank bailouts – and its reward of 50 BTC can never be spent, for technical reasons.In the glossary →× has height 0, and the current height is 970,043. The halvingGlossaryHalvingEvery 210,000 blocks – roughly every four years – the number of new bitcoin per block is cut in half. The fourth halving, on 20 April 2024, reduced it from 6.25 to 3.125 BTC.On the learning path: Stage 2 · Step 2 – Halving & 21 million →In the glossary →× is also tied to block height: it happens every 210,000 blocks.^[1]

Once your transaction is in a block, it has one confirmationGlossaryConfirmationA transaction has one confirmation as soon as it is in a block; each further block adds one. With every confirmation, it becomes harder to displace the payment after the fact.On the learning path: Stage 5 · Step 4 – First withdrawal →In the glossary →×. Each further block on top adds another and makes a change less likely. Many providers only credit deposits after several confirmations.