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I'm studying ERC20 specification and all seems to make sense. Well, all but allowance. I couldn't grasp to the concept and it's usefulness.

What should I use it for? (Should I at all?)

What are the drawbacks of setting an allowance for too high or too low?

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  • I believe allowance is used to allow a transfer from 1 address to another
    – user22075
    Commented May 19, 2018 at 22:27
  • 1
    You could also check out this cool website which explains the concept of allowance with graphical animations.
    – Paul Razvan Berg
    Commented Nov 12, 2018 at 13:37
  • Neither of these answers seem to justify the necessity of the allowance "mechanism". If you can just send tokens to an address, then of course, they won't know that the tokens are even there. That's true and that could cause a sink, but the problem I have is that this makes for an easy backdoor. ("The recipient might transfer all your approved tokens.") Are there any alternate solutions that don't violate compliance with ERC20?
    – JustKevin
    Commented Nov 13, 2021 at 1:21
  • Non of the answers are informational, read this to understand better the concept: medium.com/ethex-market/…
    – Davoud
    Commented Apr 21, 2022 at 10:33

2 Answers 2

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The typical use case is you want to buy something from a contract and pay with ERC20 tokens. Since contracts cannot listen for events so they will never be notified when they receive tokens.

A solution is for the payment operation to split in two parts:

  • approve: The user approves the contract for the value of the payment.
  • transferFrom: The user call the buy function of the contract, and the contract will call transferFrom.

An example contract of this technique is EtherDelta, when you want to deposit tokens you need to approve the EtherDelta contract with the amount you want to deposit and then call the deposit function that will move your token to EtherDelta contract.

This solution has its own issues:

  • There are two transactions to pay for.
  • There is no guarantee that both transactions will succeed. If the first succeed and the second fails then you have to manually revert the first.
  • If you approve a lower value the second transaction will fail.
  • If you approve a higher value then the recipient might transfer all your approved tokens.

The ERC223 has a proposal that allows contracts to be notified when they are the recipient of a transfer of ERC223 tokens. That should solve some of the issues found with the ERC20 approach.

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  • If you approve a higher value then the recipient might transfer all your approved tokens. -> Devs could simply check the contracts to see what's going on, but what about non-technical users? Do they have to trust dApps not to withdraw all the tokens they allowed them to transfer?
    – Paul Razvan Berg
    Commented Nov 12, 2018 at 13:46
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    @PaulRBerg For dapps an audit should prevent this type of errors, wallets can show a warning for values above certain threshold, perhaps a better standard that allows the single transaction...
    – Ismael ♦
    Commented Nov 13, 2018 at 6:54
  • If buy something from a contract and pay with ETH, it seems no allowance needed. Can you advise why?
    – Burt
    Commented Jun 26, 2021 at 8:27
  • @Burt ETH is the native currency of Ethereum and contract have a receive or fallback function that's called when ETH is transferred to them. ERC-20 token do not have such "callback" function. Check new standard like ERC-1155 that allows contract have a callback function. If you have a question it is better to ask by clicking on the Ask Question button.
    – Ismael ♦
    Commented Jun 26, 2021 at 16:10
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Explaining in very simple terms:

Let's say you are the Owner of a property and you have 1000 tokens. You also have 3 persons working under you as "Employee1", "Employee2", "Employee3".

Calling the approve function, the Owner of the property approves Employee1 to use 200 tokens for certain purpose ("address of owner" approves "address of Employee1").

This only approves that "who" is eligible but "how many tokens" is not specified. This is where the allowance function comes into picture: it tells which address is given what amount of tokens to be used (in our case: Owner allows Employee1 to spend 200 tokens).

So Employee1 is only allowed to use 200 tokens of the total of 1000.

Hope it helps!

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    But.. why not simply transfer 200 tokens to the employee's address? That way they will still only be allowed to use 200 tokens.
    – Alex Spurling
    Commented Oct 2, 2020 at 17:39
  • Like Alex me still does not gets the idea of allowance need maybe someone should describe real world scenario where it is already used?
    – BIOHAZARD
    Commented Mar 22, 2021 at 19:03
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    'This only approves that "who" is eligible but "how many tokens" is not specified' is incorrect, ERC-20 approve has the following signature: approve(spender, amount)
    – Steven Varga
    Commented Apr 4, 2023 at 16:16

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