Contract Storage Slot Calculator

Calculate storage slots for mappings and arrays in Solidity smart contracts

Mapping Key (address or uint256)Storage Slot Number

1. What is a storage slot?

In Ethereum smart contracts, storage is organized into 32-byte slots. Each state variable occupies a specific slot, and mappings use a calculated slot based on the key and base slot number.

2. How does it work?

For mappings, the storage slot is calculated as: keccak256(key . slot)

Where key is the mapping key (padded to 32 bytes) and slot is the base slot number of the mapping.

solidity

1// Example: Calculate slot for balances[user]
2contract ERC20 {
3    mapping(address => uint256) public balances; // slot 0
4
5    function getBalanceSlot(address user) public pure returns (bytes32) {
6        return keccak256(abi.encodePacked(user, uint256(0)));
7    }
8}

Use cases

Reading storage directly with eth_getStorageAt
Storage proofs for L2 bridges
Off-chain verification of contract state
Advanced contract analysis and debugging

3. Examples

ERC-20 balance mapping

Key: 0xd8dA6BF26964aF9D7eEd9e03E53415D37aA96045
Slot: 0
→ Storage Slot: 0xfca351f4d96129454cfc8ef7930b638ac71fea35eb69ee3b8d959496beb04a33

NFT owner mapping (uint256 key)

Key: 1 (token ID)
Slot: 2
→ Use for mapping(uint256 => address) ownerOf

Nested mapping

First calculate outer mapping slot, then use result as slot for inner key

4. Source Code

typescript

1// npm install viem
2
3import { keccak256, encodePacked, pad, toHex } from 'viem';
4
5// Calculate storage slot for mapping
6function getMappingSlot(key: string, slotNumber: number): string {
7  let keyBytes: `0x${string}`;
8
9  // Handle address (20 bytes) or uint256 (32 bytes)
10  if (key.startsWith('0x')) {