Metric bolts differ from American bolts in the way that they are measured. The most significant difference is that American bolts use the number of threads per inch as a measurement while metric bolts use the distance between threads. Metric bolts use the millimetre as the standard reporting unit.
Measure the diameter of the bolt. Most bolts use the shank diameter, which is the distance from one side of the bolt to the other through the centre, measured at the top of the bolt, just below the head.
Count the number of threads and measure the length the threads take up. Divide the distance by the number of threads. For example, if 10 threads took up one centimetre, the thread length would be one millimetre.
Measure the length of the bolt from the end of the bolt to the start of the head.
Report steps one through three in the standard format which is diameter x distance between threads x length. For example, a bolt with a diameter of 2 centimetres, 1 millimetre between threads and a length of 25 millimetres would be denoted "2x1x25."
Make sure you take your measurements in metric measurements rather than standard measurements. Many, but not all, metric bolts start the measurements with "M" to signify the use of metric measurements.