I am looking to take in a uint256 (in hex notation) and convert it to bytes32. For some reason I am not getting the same value after converting.

function test(uint256 genes1) public returns (bytes32) {      
    bytes32 matron = bytes32(genes1);
    return matron;

genes.test.call(0x000063169218f348dc640d171b000208934b5a90189038cb3084624a50f7316c) Return value ----- 0x000063169218f348dd1c457e09cc8e86bd11d156019d255fbe00000000000000

2 Answers 2




is JavaScript, your problem is probably that the parameter is getting interpreted as a JavaScript number, which does not have enough precision to handle numbers as grand as the one you are using. The solidity call therefore gets fed a less precise number, which it correctly casts to bytes32 and returns.

Try quoting the parameter, eg


Alternatively you could pass a BN or BigNumber instance, eg

genes.test.call(new BigNumber("0x000063169218f348dc640d171b000208934b5a90189038cb3084624a50f7316c"))

The exact syntax you need varies a little bit depending on the web3.js versions and BigNumber / BN.js libraries you're using, but since native JavaScript numbers can't handle the full 256 bits you always need to handle them either as BigNumber/BN objects or as strings of hex beginning with "0x".


I think it's been answered before, but try this bad boy:

function toBytes(uint256 x) returns (bytes b) {
    b = new bytes(32);
    assembly { mstore(add(b, 32), x) }
  • 2
    Solidity can cast a uint256 to a bytes32 just fine, you shouldn't need to resort to assembly clevers. Commented Dec 2, 2018 at 2:30

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