The ballistic coefficient (BC) of a projectile is a measure of its ability to overcome air resistance in flight. It is given by the mass of the object divided by the diameter squared that it presents to the airflow divided by a dimensionless constant i that relates to the shape. It is inversely proportional to the deceleration—a high number indicates a low amount of deceleration, so a bullet with a high BC will travel further than one with a low BC. Since dense materials give more mass for a given volume or cross-section, bullets often employ lead in their construction.

Ballistic coefficient has units of lb/in² or kg/m².

The formula for calculating the ballistic coefficient is as follows:

$(1)\; C\; =\; SD/i\; =\; w/id^2$

where:

• C = ballistic coefficient
• SD = sectional density
• i = form factor
• w = weight of object, lb or kg
• d = diameter of the object, in or m

Alternately:

$C\; =\; M/Cd\; A$

where:

• C = ballistic coefficient
• M = Mass
• A = cross-sectional area
• Cd = Cd factor

Projectiles | Aerodynamics