Metric threads are called that way, because you measure the distance between the threads (at the highest point), in millimeters., As opposed to SAE (standard), which are measured by the number of threads in an inch.
Check in the Machinery's Handbook 17'th edition on page 1191 through 1193 for a description of all the metric threads as agreed to by the international congress for thread standardization at zurich...
Metric thread pitch is measured directly from crest to crest of the thread form. For example the distance from crest to crest of a M8 x 1.25 MM thread would be 1.25 MM. Another way to measure it...
For converting between two systems you need to know relations. For example 1 yard is 0.9144 meters. So every other number of yd you need to multiply by above number to get meter.