Hey is there a way to convert bytes to uint in Solidity?

  • Could you explain what conversion you would like? Best if you can give an example of a bytes value and the corresponding uint value you would like to see.
    – user19510
    Jun 14, 2018 at 14:52

4 Answers 4


You can use this function to convert bytes to uint:

pragma solidity ^0.4.23;

contract mycontract {

    function bytesToUint(bytes b) public returns (uint256){
        uint256 number;
        for(uint i=0;i<b.length;i++){
            number = number + uint(b[i])*(2**(8*(b.length-(i+1))));
        return number;

Updated function for solidity ^0.8.11

function bytesToUint(bytes memory b) internal pure returns (uint256){
        uint256 number;
        for(uint i=0;i<b.length;i++){
            number = number + uint(uint8(b[i]))*(2**(8*(b.length-(i+1))));
    return number;

The amount of gas used depends on the length of your bytes variable, but this is cheap. Converting a bytes variable of length 20 uses about 420 gas.

Hope this helps

  • Getting an error in Remix: TypeError: Return argument type bytes memory is not implicitly convertible to expected type uint256
    – arete
    Jun 14, 2018 at 15:02
  • Can you show me your code?, this works for me well in remix, I put the whole code so you can just copy and paste it. Let me know
    – Jaime
    Jun 14, 2018 at 15:13
  • Oh there you go. I think your first post had the return value as "b". This works. Thanks!
    – arete
    Jun 14, 2018 at 15:25

Slicing an uint out of bytes using inline assembly

function sliceUint(bytes bs, uint start)
    internal pure
    returns (uint)
    require(bs.length >= start + 32, "slicing out of range");
    uint x;
    assembly {
        x := mload(add(bs, add(0x20, start)))
    return x;
  • 2
    Gas efficient solution here
    – LampShade
    Oct 29, 2020 at 19:45

A fully working modern solution can be found in this repository, authored by ConsenSys:

function toUint256(bytes memory _bytes, uint256 _start) internal pure returns (uint256) {
    require(_bytes.length >= _start + 32, "toUint256_outOfBounds");
    uint256 tempUint;

    assembly {
        tempUint := mload(add(add(_bytes, 0x20), _start))

    return tempUint;

The accepted answer is not working in Solidity ^0.5 anymore; it produces a type conversion error.

  • nice nice nice!!!!
    – Howard
    Sep 11 at 4:59

If you know the bytes length(for instance address has 20 bytes), you can convert by casting

uint256 time = uint160(bytes20(0x2E4e72EDC83053F8ADE4a525191Ba7aBA086c067));

  • Any risk for this?
    – Howard
    Sep 11 at 4:58
  • I don't see any risk. uint160 is 20 bytes long. And uint256 fits uint160.
    – tonisives
    Sep 12 at 5:52

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