Function Selector Calculator – 4-Byte Selector & Event Topic Hash

Q: Why must I use uint256 and not uint?

The canonical signature uses full type names. uint is an alias for uint256 in Solidity, and hashing the alias gives a wrong selector. This tool rewrites uint/int automatically and strips parameter names and data-location keywords.

Q: How are tuples (structs) written?

As parenthesised type lists: exactInputSingle((address,address,uint24,address,uint256,uint256,uint256,uint160)). Write the struct's fields in order; the struct name itself is not part of the signature.

Q: How many possible selectors are there?

4 bytes give 232, about 4.3 billion, possible values. With tens of thousands of known signatures collisions are rare but real; within one contract the Solidity compiler refuses two functions with the same selector.

What is a function selector?

A function selector is the first 4 bytes of the Keccak-256 hash of a function's canonical signature: the name followed by the parameter types in parentheses with no spaces or names, for example transfer(address,uint256) → 0xa9059cbb. It forms the first 4 bytes of every contract call's calldata and tells the contract which function to run; the remaining bytes are the ABI-encoded arguments.

Common selectors

The table is computed on this page with the same Keccak implementation used by the tool, so it doubles as a self-test. Paste any selector into the box above to look it up.

Worked example

Take function approve(address spender, uint256 amount) external returns (bool). Strip the keyword, parameter names, visibility and return type to get approve(address,uint256). Keccak-256 of that ASCII string starts 0x095ea7b3…, so the selector is 0x095ea7b3. The calldata for approving 1 USDT (6 decimals) is the selector, the spender address left-padded to 32 bytes, and 1000000 as a 32-byte word: 4 + 32 + 32 = 68 bytes.

ERC-20 selectors you will see most

Signature	Selector
`totalSupply()`	`0x18160ddd`
`balanceOf(address)`	`0x70a08231`
`transfer(address,uint256)`	`0xa9059cbb`
`transferFrom(address,address,uint256)`	`0x23b872dd`
`approve(address,uint256)`	`0x095ea7b3`
`allowance(address,address)`	`0xdd62ed3e`

Common mistakes

Hashing uint instead of uint256. transfer(address,uint) hashes to a different value; the canonical form always spells out the width.
Including parameter names or spaces. transfer(address to, uint256 amount) is not a canonical signature.
Using the struct name for tuples. Structs are written as their field types in parentheses, never as MyStruct.
Confusing selector and topic. Functions use 4 bytes; events use the full 32-byte hash.

Accepted input formats

transfer(address,uint256): canonical form.
function transfer(address to, uint256 amount) external returns (bool): Solidity declaration; names, visibility and returns are removed.
event Transfer(address indexed from, address indexed to, uint256 value): the indexed keyword is ignored for the topic hash.
0xa9059cbb or a 32-byte topic: reverse lookup against the built-in list.

To build or read full calldata, continue with the ABI encoder / decoder.

Frequently asked questions

What is a function selector?

The first four bytes of the Keccak-256 hash of the canonical function signature: name plus parameter types with no spaces or names, for example transfer(address,uint256) → 0xa9059cbb. The EVM uses it to route a call to the right function.

Why must I use uint256 and not uint?

The canonical signature uses full type names. uint is an alias for uint256 in Solidity, and hashing the alias gives a wrong selector. This tool rewrites uint/int automatically and strips parameter names and data-location keywords.

How are tuples (structs) written?

As parenthesised type lists: exactInputSingle((address,address,uint24,address,uint256,uint256,uint256,uint160)). Write the struct's fields in order; the struct name itself is not part of the signature.

What is an event topic?

The full 32-byte Keccak-256 of the event signature, stored as topics[0] in every log that event emits, for example Transfer(address,address,uint256) → 0xddf252ad…. Indexed parameters become topics[1..3].

How many possible selectors are there?

4 bytes give 2³², about 4.3 billion, possible values. With tens of thousands of known signatures collisions are rare but real; within one contract the Solidity compiler refuses two functions with the same selector.

Can two functions share a selector?

Yes, collisions exist because only 4 bytes are used; this is why the lookup can return several signatures and why the EVM cannot tell them apart in the same contract. For unknown selectors use the public databases 4byte.directory and openchain.xyz.

Function Selector & Event Topic Calculator

What is a function selector?

Common selectors

Worked example

ERC-20 selectors you will see most

Common mistakes

Accepted input formats

Frequently asked questions

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