Question

When optimizing gas costs in Solidity, why is packing multiple small variables into a single 32-byte storage slot more efficient than storing them individually?

Accepted Answer

In the Ethereum Virtual Machine, storage is organized into slots that are exactly 32 bytes wide. Every time the EVM performs a storage operation, it reads or writes an entire 32-byte slot at once. If you store variables individually, each variable occupies its own 32-byte slot, even if the data inside only requires a fraction of that space. For example, a boolean takes 1 byte, but if stored alone, it occupies a full 32-byte slot, wasting 31 bytes of allocated space. Variable packing involves declaring small types, such as uint8 or bool, consecutively in the contract code so the compiler can fit multiple variables into the same 32-byte slot. When you access these packed variables, the EVM retrieves the single slot containing all of them. This is more efficient because it reduces the number of expensive SLOAD and SSTORE operations. An SLOAD operation loads a value from storage into memory, and an SSTORE operation saves a value to storage; both are high-gas cost operations. By grouping multiple variables into one slot, you reduce the total number of storage slots the contract must interact with, thereby minimizing the number of expensive operations required to retrieve or update your data.

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When optimizing gas costs in Solidity, why is packing multiple small variables into a single 32-byte storage slot more efficient than storing them individually?