If a donation includes an input with Witness data we do some calculations and generate the following message.
Here are the calculations in a bit more detail.
const satsPerByte = Math.ceil(utx.payload.transaction.fee_int / utx.payload.transaction.vsize);
const savedBytes = Math.ceil(utx.payload.transaction.size - utx.payload.transaction.vsize);
const savedBytesPct = Math.round((savedBytes / utx.payload.transaction.size) * 100);
const satsSaved = Math.ceil(satsPerByte * savedBytes);
One example with 10 inputs and
fee_int = 16235
vsize = 951
size = 1757
results to
satsPerByte = 18
savedBytes = 806
savedBytesPct = 46
satsSaved = 14508
Kudos to Smartbit.com.au for providing a free API that includes the witness data.
Note: up until today most block explorer display the fee rate (sats/byte) using the old size instead of vsize, so the fee rate for SegWit TXs is wrong. But Trezor’s bitcore instances seem to have the correct functionality.