Contrary to popular belief, Bitcoin's proof of work is not actually based on the number of zeroes. Rather the block hash, when interpreted as a 256 bit integer, must be less than the target value. The target value is what actually determines the difficulty. The target value is represented in a compact form in the
nBits field of the block header.
nBits field of the block header compresses the target from 256 bits into a 32 bits. A description of the format can be found here.
nBits field, when represented in Big endian, is split into two parts: the first byte, and the last three bytes. The formula for decompressing the
nBits field is as follows:
(last three bytes) * 256 ^ ((first byte) - 3). This gives you a 256 bit integer that has the first 3 most significant bytes of the target.