Solidity Storage Slot Calculator — Free & Private

A free edge API by formatho.com, privacy-first developer tools.

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Free, no signup, runs entirely in your browser — your data never leaves your device.

🔓 Free🔒 Zero tracking🚫 No data collection⚡ Edge-fast

Usage

# mapping at slot 1, uint key 42
curl "https://storage-slot-calculator-formatho.filesformatho.workers.dev/api?type=mapping&slot=1&key=42"

# mapping with an address key at slot 0x5
curl "https://storage-slot-calculator-formatho.filesformatho.workers.dev/api?type=mapping&slot=0x5&key=0x90F8bf6A479F320ead074411a4B0e7944Ea8c9C1&key_type=address"

# dynamic array: length slot 2, element 7 slot
curl "https://storage-slot-calculator-formatho.filesformatho.workers.dev/api?type=array&slot=2&index=7"

# bytes/string data slot at slot 3
curl "https://storage-slot-calculator-formatho.filesformatho.workers.dev/api?type=bytes&slot=3"

Returns the exact 32-byte storage slot (hex + decimal) and the keccak-256 preimage, per the official Solidity storage layout rules. Key types: uint, int, address, bool, bytes32, string, bytes.

Full parameter reference: /api endpoint.

Privacy-first: slots are computed in-memory at the edge and never logged or stored.

Full browser tool

Calculate slots entirely client-side: Storage Slot Calculator on formatho.com.

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