A trailing/leading zero counter includes a plurality of hardware logic blocks, each of which calculates one bit of the output value (i.e. the number of trailing/leading zeros depending on whether it is a trailing/leading zero counter). Each hardware logic block includes two blocks of section hardware logic which each receive a section of an input string and generate one or two outputs from this section of bits. Combining logic then combines the outputs of the section hardware logic to generate the bit of the output value. For hardware logic blocks which calculate bits other than the least significant bit of the output, the hardware logic blocks also include one or more OR reduction stages which reduces the length of the input string by pairwise combining of bits using OR gates before the resultant string is divided into two sections and input to the section hardware logic.