How Gas Gets Spent, Opcode by Opcode

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Why this matters

Gas is the EVM's resource meter — every opcode has a fixed or computed cost, and a transaction that runs out reverts entirely (but still pays for what was used). Understanding how gas is debited per-opcode is the foundation of all on-chain optimization work, and it's the only way to predict whether your contract will hit the block gas limit at scale.

Demo

Trace the gas debited by a single SSTORE and a single MSTORE.

Try it yourself

Calculate the gas for storing 5 new uint256 values in a struct: 5 * 22100 + overhead. Compare to repacking into 2 storage slots (2 * 22100).
Run cast estimate <addr> <calldata> for a transaction that writes one new storage slot and confirm the estimate is ≥22100 + base.
Identify which opcode dominates the cost of a basic transfer(uint256) call on an ERC20 — it's the SSTORE that updates the recipient balance.

References

// Gas costs (post-Berlin, Berlin/London/Shanghai):
ADD            // 3
MLOAD          // 3 + memory expansion
MSTORE         // 3 + memory expansion
SLOAD cold     // 2100 (EIP-2929)
SLOAD warm     // 100
SSTORE new     // 22100 (zero -> non-zero)
SSTORE update  // 5000 (non-zero -> non-zero)
SSTORE clear   // refund: 4800 (non-zero -> zero), capped
CREATE         // 32000 + bytecode cost
CALL           // 100 base + EIP-2929 access + 2300 stipend if value transfer
LOG3           // 1500 + 256*topics + 8*databytes