Bitcoin's 5 levels of scalability

With the growing number of Bitcoin L2s, it’s crucial to have a framework to track their progress.

As part of the Bitcoin Heartbeats project, we propose the following standard to classify them.

LEVEL 1

Bitcoin L2s at Level 1 do not utilize Bitcoin as the base layer, do not use Bitcoin for data availability (DA), and lack state verification.

Examples: Most Bitcoin sidechains fall into this category. They operate their own consensus mechanisms, store data independently, and establish bridges to Bitcoin without leveraging Bitcoin’s infrastructure for state or data.

LEVEL 2

Bitcoin L2s at Level 2 use Bitcoin as the base layer for storing proofs while relying on external DAs (such as Celestia, Avail DA, or Filecoin) for transaction data storage.

They do not perform state verification. Current Bitcoin rollups predominantly fit into this level. Due to their relatively low user base, these solutions often choose external DAs to minimize costs—a practical approach at this stage.

LEVEL 3

Bitcoin L2s at Level 3 use Bitcoin for storing all data, but do not include state verification.

Trustless Computer and perhaps Ordinals serve as examples here.

LEVEL 4

Bitcoin L2s at Level 4 store all data on Bitcoin and include state verification performed by external light clients.

Supersonic, a ZK rollup on Bitcoin, is an example of this level.

LEVEL 5

Bitcoin L2s at Level 5 store everything on Bitcoin and incorporate native state verification directly on Bitcoin, like with BitVM or OP_CAT in the future.