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/ Contract Storage Slot Calculator

Mapping Key (address or uint256)

Storage Slot Number

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Contract Storage Slot Calculator

Calculate storage slots for mappings and arrays in Solidity smart contracts

■ 01

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.

■ 02

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. // Example: Calculate slot for balances[user] con…

■ 03

Sample Cases

Covers ERC-20 balance mapping, NFT owner mapping (uint256 key), and Nested mapping so you can compare common inputs and outputs quickly.

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/ Integration

Source code from the selected tool, shown here alongside the live version on the right.

/ Source Code

TypeScript

typescript

// npm install viem

import { keccak256, encodePacked, pad, toHex } from 'viem';

// Calculate storage slot for mapping
function getMappingSlot(key: string, slotNumber: number): string {
  let keyBytes: `0x${string}`;

  // Handle address (20 bytes) or uint256 (32 bytes)
  if (key.startsWith('0x')) {