When going over a binary file in c, do I go by bits or bytes?

When going over a binary file in c, do I go by bits or bytes?

I have a long binary string which contains a number of bytes, I need to
use fseek to get to a specific byte in the string.
I know I need to calculate the offset but I'm not sure if the offset is
calculated by bits or bytes. If I need to get to the 3rd byte for example
I need to advance the indicator by 3 or by (3*8=)24?