What Is a Bitcoin Block?

When you send Bitcoin - whether it is 0.001 BTC to a friend or a company moving millions - your transaction first lands in a waiting area called the mempool (the pool of transactions that have been broadcast but not yet recorded). Roughly every 10 minutes a miner (a computer competing to add the next block) scoops up a batch of those waiting transactions, checks that each one is valid, and packages them into a single block.

A block is typically between 1 and 4 MB in size, which holds anywhere from a few hundred to several thousand transactions. Once a block is built and accepted by the network, the transactions inside it are confirmed and written down for good. For you, that is the moment a payment stops being "pending" and becomes settled.

Picture a page in a ledger. Each page records a set of transactions, and pages are added in order, one after another, building an unbroken record of every Bitcoin transaction ever made. That running record is the blockchain.

Every block header carries the hash of the block before it. A hash is a unique fingerprint of the block's data - a fixed-length string of characters that stands in for everything inside. Change even one character in a block and its hash changes completely, so a hash quietly proves nothing has been touched.

That is what creates the chain. Block 800,000 points to block 799,999, which points to block 799,998, all the way back to block 0. If someone tried to alter a transaction in block 799,999, its hash would change, block 800,000 would no longer point to the right place, the chain would break, and every node on the network would reject it.

This is why the system is called a blockchain. That chain of hashes is what makes Bitcoin's history tamper-proof, and it is why you can trust an old transaction without trusting any single company. Rewriting any past block would mean redoing all the mining work for every block that came after it - a task that is computationally impossible at scale.

Block 0 - the very first Bitcoin block - was mined by Satoshi Nakamoto on January 3, 2009. It is called the genesis block. Unlike every block that followed, it points to no previous block, because none existed yet. It is where the whole chain begins.

Satoshi tucked a message into the genesis block's coinbase transaction, and it is still readable today:

The first transaction in every block is a special one. It is called the coinbase transaction - not to be confused with the company of the same name. This is the transaction that creates brand-new Bitcoin and pays it to the miner who solved the block.

That coinbase transaction bundles two things: the block reward (currently 3.125 BTC, cut in half roughly every four years) and all the transaction fees paid by the people whose transactions made it into the block. As the reward keeps halving, those fees will slowly become the main reason miners keep working. You can see how that fits together on our mining page.

Every other transaction in the block is an ordinary transfer - one or more inputs (Bitcoin being spent) mapped to one or more outputs (Bitcoin being received). The miner checks each one: do the inputs exist, has the sender already spent them, is the digital signature valid? Anything that fails gets thrown out, which is part of why you can trust what lands in a block.

Miners tend to pick transactions that pay a higher fee per byte of data. When the network is busy, people compete for the next block by attaching larger fees; when things are quiet, even a small fee will get you confirmed quickly. In practice, the fee you choose is a nudge for how soon you want in.

Bitcoin aims to produce a new block about every 10 minutes. No clock enforces this. It is a target the network holds steady through something called the difficulty adjustment (an automatic tuning of how hard the mining puzzle is).

Every 2,016 blocks - roughly two weeks - the network looks back and recalculates how hard that puzzle should be. If the previous 2,016 blocks came in faster than 10 minutes on average (which means more computing power joined), the difficulty goes up. If blocks were slower, it comes back down.

This self-correcting loop is one of Bitcoin's quietly clever ideas. However much mining power is added or pulled away, the network keeps nudging itself back toward a steady pace. It has done this reliably since 2009 with no one at the wheel, which is part of why the steady rhythm you rely on stays so consistent.

The 10-minute target was a deliberate choice by Satoshi. It balances speed (faster feels better for you as a user) against security (too fast, and blocks might not reach every node before the next one is found, causing conflicts). It also keeps the blockchain from growing faster than ordinary computers can handle.

Bitcoin started with a 1 MB block size limit. As it caught on, blocks began filling up. More transactions competed for the same small space, fees climbed, and confirmation times got hard to predict. That tension set off the block size debate - one of the most heated stretches in Bitcoin's history.

One camp simply wanted to raise the block size limit. Bigger blocks would fit more transactions and push fees down. The other camp warned that bigger blocks would make it harder for everyday people to run a full node, which would chip away at decentralization.

The compromise that won out was Segregated Witness (SegWit), switched on in August 2017. SegWit reorganized how transaction data is stored, which lifted capacity to about 4 MB of "block weight" without touching the base block size limit. It also fixed a long-standing bug called transaction malleability and opened the door to the Lightning Network.

Those who disagreed split off to create Bitcoin Cash (BCH) with an 8 MB block limit. Bitcoin itself stuck with SegWit, and its capacity has held up well alongside second-layer solutions like Lightning.

The moment your transaction lands in a block, it has one confirmation. When the next block is mined on top of that one, it has two. Each block stacked above yours makes it dramatically harder for anyone to reverse it, so confirmations are really just a measure of how settled your payment is.

Here is roughly how much patience different situations call for:

Because Bitcoin's blockchain is public, you can look up any block or transaction yourself using a block explorer (a website that reads the raw blockchain and shows it back in plain, human-friendly form). No account, no permission needed.

With a block explorer, you can:

• Look up any transaction by its transaction ID (txid)
• View all transactions in a specific block
• Check how many confirmations a transaction has
• See the balance of any public address
• Monitor the mempool to see pending transactions

Popular block explorers include mempool.space (open-source and privacy-respecting), blockstream.info, and blockchain.com/explorer. Any of them is a fine place to watch your own transaction move along. And if you run your own Bitcoin node, you can ask the blockchain directly and skip the third-party site entirely.

For live network data - including the current block height and hashrate - see our Bitcoin stats page.