pitch" in millimeters from thread to thread, or equivalently, how far the screw or bolt advances after one complete turn. The English system instead measures the number of threads per inch.
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Convert the diameter from millimeters to inches by dividing it by 25.4. For the example of a 2-millimeter diameter "M 2" screw, divide 2 by 25.4 to yield a 0.0787-inch diameter.
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Convert pitch to threads per inch by dividing 25.4 by the pitch value. For a pitch of 0.5 mm, divide 25.4 by 0.5 to yield a threads-per-inch (tpi) count of 50.8.
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Recognize that metric and English threads, however close sometimes, are not perfectly interchangeable. Still, in the field of screw-on photographic filters and lenses, these are some common substitutes: M-.5 for 48 tpi, M-.75 for 32 tpi, M-.9 for 30 tpi and M-1.0 for 24 tpi.