M means bolt is of Metric Thread, and 20 refers to its diameter. It's cap diameter is 1.5 D (1.5D is for hexagonal bolt). Strength is determined from grade of bolt. Here grade 4.6 So ultimate strength, fu=4×100=400×N∕mm2 Yield strength, fy=0.6× ultimate strength =0.6×400=240N∕mm2