Capacity

Capacity is the storage limit of a Cell in a transaction. It represents both the amount of CKB tokens stored and the maximum space available for the Cell's content—including its data, lock, type, and capacity fields.

When a transaction creates an output Cell, its occupied capacity must not exceed the capacity of the original input Cell:

Occupied (Cell) ≤ Cell's capacity

One of the transaction validation rules is:

capacity_in_bytes >= len(capacity) + len(data) + len(type) + len(lock)

This ensures the Cell has enough capacity to store all its fields. The capacity also represents the balance of CKB tokens, similar to the nValue field in Bitcoin's CTxOut.

For instance, if Alice owns 100 CKB, she can unlock a group of Cells with a total quantity of 100 bytes.

Minimum Cell Capacity (61 Bytes)

Each Cell must reserve a minimum of 61 CKBytes to store its essential fields, including:

• capacity: 8 bytes
• lock: 32 bytes
• args: 20 bytes
• hash_type: 1 byte

These add up to 61 bytes. Since 1 CKByte grants the right to store 1 byte on-chain, a Cell must hold at least 61 CKBytes to cover this base storage.

tip

Allocating 62 CKBytes or more is recommended in practice to accommodate transaction fees and ensure smooth inclusion in blocks.