# Sectional Density

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This Sectional Density calculator computes the factor used in the computation of the ballistic coefficient called sectional density

INSTRUCTIONS: Choose units and enter the following:

• (M) - This is the mass of the projectile
• (d) - This is the diameter of the projectile

Sectional Density (SD):  The value is returned as a real number

#### The Math / Science

The mass is converted to pounds and the diameter to inches.  The formula for sectional density is

SD = M/d²

where:

• SD is the sectional density
• M is the mass of the projectile or bullet in pounds
• d is the diameter of the projectile or bullet in inches.

This calculator automatically converts grains, grams and other mass/weight units to pounds, and millimeters and other length units to inches.  The resulting answer is provided with no units.

The sectional density as applied to a bullet is slightly different than the sectional density definition as applied in a general physics context.  Sectional density in a general physics context is defined as mass per cross-sectional area.  Sectional density is approximated as mass per diameter squared.

The ballistic sectional density is used to compute the ballistic coefficient (See equation: Ballistic Coefficient).  The ballistic sectional density is also used in computation of the penetration characteristics of terminal ballistics.

#### References

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