I'm trying to use 0x to buy tokens with ETH. I'm following the sample at https://0x.org/docs/guides/swap-tokens-with-0x-api#swap-eth-for-1-dai.

I got the token address, and the decimals. let's say USDC and 6. And let's say I want to buy 1 USDC. How do I calculate the value I need to pass to buyAmount? I tried several things: passing '1', passing '1000000' (1 and 6 0s). I tried 5 0s. When it hits MetaMask it's always the wrong amount: 10 or 100 etc. And the fee is crazy ($39 in gas to buy 10 USDC?).

It's even worse with other tokens and amounts. Can anyone clue me into how to use the decimals to translate the amount I want to buy to the amount I need to send to the quote endpoint?


Let's call the decimals a token has d and the amount you want to buy a. Then the value you are looking for can be calculated by a * 10^d.

Example: You want to buy 50 USDC (a = 50) and USDC has 6 decimals (d = 6). The amount is 50 * 10^6 = 50000000.

  • Great! Does this hold for float values as well? If I want to buy 10.675 USDC, it's 10.675*10^6? – Traveling Tech Guy Sep 11 '20 at 17:28
  • Didn't work - still shows 10x the value I've been trying to buy (i.e 1 USDC is 0.02 ETH == $10 in MetaMask, before gas fees). – Traveling Tech Guy Sep 11 '20 at 17:50
  • Yes, the whole point of this conversion is to avoid floats. So 10.675 USDC would be 10675000. Unfortunately I'm not sure why there is a problem with 0x as I have never used it. But the conversion is definitely correct. – Bobface Sep 12 '20 at 7:37
  • Thanks for your help! Had it figured out after chatting with someone from 0x. For the record, it was the slippage parameter in their API that added to the total. – Traveling Tech Guy Sep 12 '20 at 16:05

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