I know 1 Kb = 1024 bytes. Why 1000 bytes now?
I use Electrum and I messed up with the fees option in settings.
Can you explain this to me in simple terms?
The size of the TX = 180b*inputs + 32b*outputs +/- 1*inputs for uncompressed keys
The size of the TX = 134b*inputs + 32b*outputs +/- 1*inputs for compressed keys
1Kb is 1000bytes while 1KB is 1024bytes. Kilo is 1000 units, so 1000bytes.
The public key needs to be included in the TX when spending coins from that address. For uncompressed keys, the size of the public key would be longer and hence it would take up more space in the transaction compared to compressed ones. [1] Recent versions of Bitcoin Core uses compressed key and IIRC, most if not all SPV clients generates compressed keys now.
For the calculation part, you basically times the 180byte or 134byte by the amount of UXTO you are going to include and 32 byte by the output accordingly. Add them up and you would have a fairly good estimate of the transaction size.
[1]
https://bitcoin.org/en/glossary/compressed-public-key