D^2 / 162

for 1 m length steel rod

I ts volume V =(Pi/4)*Dia x Dia X L
=(3.14/4)x D x D X 1 (for 1 m length)

Density of Steel=7850 kg/ cub meter

Weight = Volume x Density
=(3.14/4)x D x D X 1 x7850 (if D is in mm )

So = ((3.14/4)x D x D X 1 x7850)/(1000x1000)

= DxD/162.27