The blockchain

The blockchain is the record of every bitcoin transaction that has been confirmed. It is a list of blocks, and each block points back to the one before it. Every node keeps a copy of that list, and the copies agree with each other.

What is the blockchain?

The blockchain is a set of files stored on each node's disk. Together, the files hold every block in order, from the first one in January 2009 up to the newest. A new block arrives about every 10 minutes, so the list keeps growing.

Think of it as a ledger, a book where every entry is kept in order. Each entry is a transaction, and changing an old entry is very hard.

Why does the chain matter?

The blockchain is the one shared record of who spent which coins. It is not a list of accounts with balances. It lists each transaction, the coins it spent, and the new coins it created.

Your wallet adds up the coins that are still unspent and locked to your addresses. That total is your balance. The outputs page explains how coins pass from one entry to the next.

Why is it called a chain?

Transactions are not added one at a time. They are grouped into blocks, and each block stores the hash of the block before it. That stored link turns a plain list of blocks into a chain.

If you change one block, its hash changes. The next block no longer matches it, and anyone can check for that mismatch. To hide a change, someone would have to redo the work for that block and every block after it. They would also have to do it faster than the rest of the network adds new blocks. That is why the chain is so hard to alter.

Grouping transactions into blocks also makes the record easier to pass around. Updates that arrive once every 10 minutes are simpler to share than constant small changes.

How do nodes share the blockchain?

Nodes share the chain with each other. When a miner finds a new block, it tells its peers. They check the block and pass it along to their own peers. A node that has fallen behind asks its peers for the blocks it is missing, and so it catches up. It works a bit like a BitTorrent file, where each copy passes along the parts the others lack.

How big is the blockchain?

The full chain takes up hundreds of gigabytes, and it grows every day as new blocks arrive. A new node must download all of it once, and that first download takes a long time. After that, the node only adds the new blocks as they arrive.

Disk space can be a constraint. Bitcoin Core offers pruning, which keeps the chain small. A pruned node still checks every block as it downloads it. Then it deletes the oldest block files and keeps only the newest ones. You set the size limit with the prune option, in MiB, and the minimum is 550. The node keeps checking each new block it receives.

Where is the blockchain on my computer?

Bitcoin Core keeps the blocks in a folder called blocks, inside its data folder. The location depends on your system:

• Linux: ~/.bitcoin/blocks/
• macOS: ~/Library/Application Support/Bitcoin/blocks/
• Windows: C:\Users\<you>\AppData\Local\Bitcoin\blocks\ in version 28.0 and later. Older versions used AppData\Roaming.

The files are named blk00000.dat, blk00001.dat, and so on. They store data in a form that computers read. Opening one in a text editor shows only unreadable bytes. To read the chain properly, use a block explorer. Our explorer shows each block on its own page, such as block 0.

What should you read next?

Next, read blocks. It explains what a block contains, and how its 80-byte header is built from fixed fields.