Estimated fee and Used fee

Question

Subscan recently made a change to show estimated fee and used fee, while if you query data from sidecar, partial fee + tip = estimated fee. What is this used fee and how is it calculated?

Answer 1

You can guess the fee before call the function querying the information from the payment RPC methods.

A little script to query your example is:

    const wsProvider = new WsProvider('wss://kusama-rpc.polkadot.io')
    const api = await ApiPromise.create({ provider: wsProvider });

    const blockHash = '0x9646cfeddaf4afd297f8a904f9458871b7203e6922cff502b3588af4c6cd1f0d';
    const { block } = await api.rpc.chain.getBlock(blockHash);

    // The extrinsic from your example(0x436d39517244cd5fc7c3d35dd08de0ccb62cb620868b5428db831857d4a952eb) is in block.extrinsics[7]
    const extrinsic = block.extrinsics[7].toHex();

    const queryFeeDetails = await api.rpc.payment.queryFeeDetails(extrinsic, blockHash);
    console.log('queryFeeDetails:', JSON.stringify(queryFeeDetails.toHuman(), null, 2));
    const queryInfo = await api.rpc.payment.queryInfo(extrinsic, blockHash);
    console.log('queryInfo:', JSON.stringify(queryInfo.toHuman(), null, 2));

This will give return you:

queryFeeDetails: {
  "inclusionFee": {
    "baseFee": "33.3333 µKSM",
    "lenFee": "489.9995 µKSM",
    "adjustedWeightFee": "5.4706 µKSM"
  }
}
queryInfo: {
  "weight": "155,732,000",
  "class": "Normal",
  "partialFee": "528.8035 µKSM"
}

The estimated fee is partial fee, which is partial_fee = base_fee + len_fee + adjusted_weight_fee

But the adjusted_weight_fee might slightly change depends in which block the extrinsic is included. I am not 100% sure how subscan calculates it, but this is the Used Fee field.

Check this response to see an excellent explanation about how to calculate the final fee: https://substrate.stackexchange.com/a/4224/2762