Equation forcalculating Sectional Density (SD):

7000 x (bullet diameter in inches) x (bullet diameter in inches) =X
Y= (bullet weight in grains)

Y divided by X = SD

ie: .308 180gr bullet

7000 x .308 = 2156
2156 x .308 = 664

X= 664
Y = 180

180
664

= .271 SD