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For example when I want to send ERC-20 tokens to a contract, I specify the amount in Wei
So I have to convert 34678393 tokens to Wei. But it only works ok with tokens that have 18 decimals

I'm using web3's toWei() function like this

  const sendTokens = async (amount)  => {
    const contract = new web3.eth.Contract("ADDRESS",ABI)
    await contract.sendTokens(web3.utils.toWei(amount))
  }

How can I convert tokens amount with different decimals to Wei?

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Actually, yes, you can.

Just make sure that you send the correct weis to the token contract. The token contract should have a function to get how many decimals it uses.

If you know that the token contract uses 3 decimals, then you can convert your amount toWei passing a kwei param. If it uses 6 decimals, then mwei, etc. Check the following code and read the comments:

// 34678393_000 the last 3 zeros could be used as decimals. If your token uses 3 decimals, then use `kwei`.
const result2 = web3.utils.toWei("34678393", "kwei");
console.log("34678393 to wei (kwei): ", result2);

// 34678393_000000 the last 6 zeros could be used as decimals. If your token uses 6 decimals, then use `mwei`.
const result3 = web3.utils.toWei("34678393", "mwei");

console.log("34678393 to wei (mwei): ", result3);

// 34678393_000000000 the last 9 zeros could be used as decimals. If your token uses 9 decimals, then use `gwei`.
const result4 = web3.utils.toWei("34678393", "gwei");

console.log("34678393 to wei (gwei): ", result4);

// 34678393_000000000000 the last 12 zeros could be used as decimals. If your token uses 12 decimals, then use `szabo`.
const result5 = web3.utils.toWei("34678393", "szabo");

console.log("34678393 to wei (szabo): ", result5);

// 34678393_000000000000000 the last 15 zeros could be used as decimals. If your token uses 15 decimals, then use `finney`.
const result6 = web3.utils.toWei("34678393", "finney");

console.log("34678393 to wei (finney): ", result6);

// 34678393_000000000000000000 the last 18 zeros are the decimals
const result = web3.utils.toWei("34678393");

console.log("34678393 to wei (ether): ", result);

// 34678393_000000000000000000000 the last 21 zeros could be used as decimals. If your token uses 21 decimals, then use `kether`.
const result7 = web3.utils.toWei("34678393", "kether");

console.log("34678393 to wei (kether): ", result7);

// Use `mether` for 25 decimals
// Use `gether` fo 26 28 decimals
// Use tether for 31 decimals

// For other decimals, you will probably need to implement your own logic or find a library that handles other decimals.

It prints something like:

34678393 to wei (kwei):  34678393000
34678393 to wei (mwei):  34678393000000
34678393 to wei (gwei):  34678393000000000
34678393 to wei (szabo):  34678393000000000000
34678393 to wei (finney):  34678393000000000000000
34678393 to wei (ether):  34678393000000000000000000
34678393 to wei (kether):  34678393000000000000000000000

Here is the docs: https://web3js.readthedocs.io/en/v1.7.5/web3-utils.html?highlight=toWei#towei

If you are calling a token that uses 3 decimals, then the last 3 digits of the number you send will be treated as the decimals.

So, if you want to send 34678393 tokens with no decimals, then you need to append 3 zeros, because if you don't, then the token will treat the number 34678393 as 34678.393. Which may not be what you want.

That's you you need to either append the 3 zeros or use the web3.utils.toWei("34678393", "kwei") function, so it returns 34678393000 and we can send it to the token, indicating that the amount does not have any decimal values.

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  • 1
    Thanks that's a great answer. I have a question though, What about tokens that have different decimals like 5 or one and 7? according to this answer ethereum.stackexchange.com/questions/118896/…. we could use any decimals as long as it is less than 77
    – Yaya
    Commented Sep 13, 2022 at 21:05
  • 1
    Simple add 5 ir 7 zeros at the end of it. Commented Sep 13, 2022 at 21:11
  • But what about double values like 0.3?
    – Yaya
    Commented Sep 13, 2022 at 21:23
  • When I add zeros according to decimals in 0.3, if I'm not wrong it throws an error like wrong input value
    – Yaya
    Commented Sep 13, 2022 at 21:25

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