Volume of square bar = cross sectional area x length
= (side)2 x length
Let us consider a rectangular bar of length 1m, side length 20mm = 0.020m
Hence, Volume = (0.020)2 x 0.0004 m3
So, the mass of square bar = density x volume = 7900 x 0.0004 = 3.16 kg
Similarly, we can calculate the mass of a square bar of different dimensions.