A convenient calculator for total bytes written. In short, it is calculated by multiplying Total_LBAs_Written with the sector size (512B or 4KB). Both info can be retrieved using smartctl.

~# smartctl /dev/ada0 --all |grep "Sector Size"
Sector Size: 512 bytes logical/physical
~# smartctl /dev/ada0 --all | grep Total_LBAs_Written
241 Total_LBAs_Written 0x0032 099 099 000 Old_age Always - 25351376107