Finding gas to be used by a transaction

Question

Is there a way to find the total gas that will be used by the transaction before a tx actually takes place?

Does the gas used depend on the amount of tx size in kb?

Answer 1

Yes, there is a way to estimate the gas consumption using web3. For transactions involving contract functions execution you can use this:

myContract.methods.myMethod([param1[, param2[, ...]]]).estimateGas(options[, callback]), see here for more information.

For simple ether transactions you can use:

web3.eth.estimateGas({from: address, to: address, value: web3.toWei(1, "ether")})

See this discussion for more information

Answer 2

At least Metamask has some functionality to estimate the gas usage, but it's not perfect. More info about that: What is the general cause for out-of-gas with MetaMask?

The amount of gas required depends on what you are trying to do. There is no "kb" size of transactions as such - the gas requirements are calculated differently. If you are just sending some Ether around, 21000 is default and it's fine. If you are creating a contract, it takes a lot more gas. Some ideas about contract gas requirements: How to minimize gas consumption during contract creation