Well, tell ya what....set your MTU size to 64Bytes (minimum ethernet packet size) then take the packet count, multiply by 64 and you got bytes (there's some minor overhead...of no consequence). That's the only way. Ethernet packets can vary in size from 64bytes to 1518bytes (or 1516 depending on your religion)...so ten packets could be 640 bytes, or 1,518,00 bytes.
If you set your machine so that the ONLY packet size you send & receive is 64Byte packets, then the math gets easy.
JFC.....I though O/C stood for "OverClocking."
Just connect, enjoy the surf, ignore the bits/bytes/packet stuff...
FWIW
Scott