The IPv4 base header is 20 bytes long without options, and can extend up to 60 bytes if options are included.

The IPv4 base header has 10 mandatory fields, and together they add up to 160 bits.
1 byte = 8 bits then 160 bits / 8 bits = 20 bytes.

So, IPv4 base header = 20 bytes (160 bits) without options.