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I see this question time and time again, but I only find answers for calculating the fees in creating raw transactions.
I am using sendtoaddress and would like to be able to calculate the tx fee needed prior to calling the api. I would like to avoid creating a raw transaction if reasonably possible.
how might i do this using the json-rpc api? (i'm using bitcoind)
If you call
bitcoin-cli settxfee <fee>
just before call
bitcoin-cli sendtoaddress "1LipeR1AjHL6gwE7WQECW4a2H4tuqm768N" <value>
Then the fee will be <fee>.
Important note: make sure in the graphic mode (when you start with bitcoin-qt command) the "custom" is selected, otherwise your command settxfee will be ignored (see image bellow).
Also, I'm not sure if this option is selected by default, and I am not sure as well if it you can set "custom" via API. If someone knows that, please edit this answer and fix it.
Note 2: I'm not sure if this is strictly necessary, maybe not. I didn't make many tests. The test I made was starting bitcoin with bitcoin-qt -txindex -server & and then letting the interface open. If "recommended" is selected the command settxfee will be ignored. I didn't tested e.g. what if let the "recommended" option selected, close the interface and then start with bitcoind -txindex for example.
A bit late but you might take a look at https://github.com/bitcoin/bitcoin/pull/5503 It's not merged (yet) but it would give you the opportunity to get the fee before sending the transaction (with 4 steps: createrawtransaction, fundrawtransaction [get fee], signrawtransaction, sendrawtransaction)
