The Role of blk.dat Files

When you run a Bitcoin full node, the blockchain is stored on your hard drive in a series of files named blk00000.dat, blk00001.dat, and so on. These files are the "ground truth" of the Bitcoin network.

1. What are blk.dat files?

The blk.dat files are flat binary files that contain raw block data. Instead of using a traditional relational database (like SQL) to store blocks, Bitcoin Core simply appends new blocks to the end of the current .dat file.

2. The 128MB Limit

To prevent files from becoming too large and unwieldy for the operating system to handle, Bitcoin Core caps each blk.dat file at 128 MiB.

• Once blk00000.dat reaches ~128MB, the node creates blk00001.dat.

• This makes it easier to move, copy, or verify the blockchain data in chunks.

3. Raw Serialization

The data in these files is stored in its raw serialized format. This is the exact same byte-sequence that is transmitted over the P2P network.

• The file contains the Block Header.

• The file contains the full list of Transactions.

• The file contains the Witness Data (if applicable).

4. Location on Disk

By default, these files are located in the blocks/ subdirectory of your Bitcoin data directory:

• Linux: ~/.bitcoin/blocks/

• Windows: %APPDATA%\Bitcoin\blocks\

• macOS: ~/Library/Application Support/Bitcoin/blocks/

Property	Value
File Format	Binary (Raw)
Max Size	128 MiB per file
Content	Serialized Blocks
Dependency	Requires index (LevelDB) to find specific blocks quickly.

In the next section, we will look at the Magic Bytes and the internal structure of these files.