Calculate solidity storage slot

Derive an EVM storage slot for a mapping key, dynamic-array element, or simply laid-out struct field.

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ToolSolidity Storage Slot Calculator
Input
Output
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Preview

How it works

A mapping entry hashes the 32-byte key encoding followed by the 32-byte mapping slot. A dynamic-array element starts at keccak256 of the array slot and adds its index; a struct field uses the supplied slot offset. The struct mode assumes one complete slot per field and does not model Solidity packing.

  • Slot zero and index zero make the first declaration easy to inspect.
  • Decimal and hexadecimal entry match compiler and debugger notation.

Worked example

A mapping(address => uint256) at slot 0
keccak256(pad32(key) ++ pad32(0)) is the slot the value lives at
Input
											Kind: mapping
Base slot: 0
Mapping key: 0x0000000000000000000000000000000000000001
Mapping key type: address
Array index: 0
Struct field slot offset: 0
Output
												Slot hex: 0xada5013122d395ba3c54772283fb069b10426056ef8ca54750cb9bb552a59e7d
Slot decimal: 78541660797044910968829902406342334108369226379826116161446442989268089806461
Explanation: Mapping slot: keccak256(pad32(key) . pad32(baseSlot)).

When to use this

Proxy upgrade reviews inspect occupied slots, Foundry traces debug mapping reads, and inline assembly uses sload for array elements.

Edge cases

  • Packed uint128 fields share a slot, so the simple struct offset is not a replacement for compiler layout output.
  • A UTF-8 string key and a zero-padded address key have different byte encodings before hashing.
  • Index arithmetic wraps at the 256-bit EVM word size and is displayed masked to 32 bytes.

References