Manual Calculation of Transaction Size and Fee

Question

I realize that the CLI has commands for calculating a minimum fee based on a transaction size, but I'm wanting to do a calculation manually. The documentation states that the formula for calculating the transaction minimum fee is a * size(tx) + b where a & b are protocol parameters, and size(tx) is the transaction size in bytes.

Is there a way to calculate the transaction size if I know the number of consumed and output UTxOs? Also, are a and b in the genesis file?

Answer 1

there are two options that do come to my mind to calculate a transaction size:

starting from the fees

since we know that the fees are calulated as a * size(tx) + b == fees using some simple algebra we can see that size(tx) == (fees - b) / a

this should be the most reliable way, but you need the fees in order to calculate the size, so it shouldn't fit you request

use the CBOR size

when builing a transaction using either cardano-cli transaction build or cardano-cli transaction build-raw you need to specify an --out-file which turns out to be a .json file with the following chema:

{
    "type": "Tx AlonzoEra",
    "description": "",
    "cborHex": "<some hex value>"
}

ultimately the cborHex value is what goes on-chain that value is an hexadecimal string and represents the bytes of the CBOR message

since we know that each byte can have a value from 0 to 255 the hex representation must take 2 hexadecimal digits per byte

this way you could calculate the tx size in bytes as half the length of the cborHex string

in nodejs

"<some hex value>".length / 2

however, this approach returns usually a value smaller by ~400/~500 bytes in contrast to the first approach, so I might be missing some data

P.S. cardano-serialization-library

using the cardano-serialization-library it should be enough to use the Transaction.to_bytes() method and get the length

e.g.

myTx.to_bytes().length

Answer 2

Here is my bash script

fee=$(cardano-cli transaction calculate-min-fee \
                    --tx-body-file tx.tmp \
                    --tx-in-count 1 \
                    --tx-out-count 1 \
                    --mainnet \
                    --witness-count 1 \
                    --byron-witness-count 0 \
                    --protocol-params-file protocol.json | awk '{ print $1 }') >> $log

In your case, it depends how many TX_in and TX_outs you will have.

Answer 3

The a and b fees parameters are protocol parameters, so you should probably check the current epoch, even they are unlikely to change suddenly.

$ curl -s  -H "project_id: $BLOCKFROST_PROJECT_ID" https://cardano-mainnet.blockfrost.io/api/v0/epochs/latest/parameters | jq '. | .min_fee_a, .min_fee_b'
44
155381

Then, all your need to do to calculate your fees is:

a × size (in bytes) + b

Read more on docs.cardano.org.