Ballistic Coefficient Calculator — G1 BC & Sectional Density

Formula

How this is calculated

The G1 ballistic coefficient is the ratio of a bullet's sectional density to its form factor. Sectional density (SD) is the bullet's mass in pounds divided by the square of its diameter in inches — it measures how much mass is packed behind each unit of frontal area. A heavier, narrower bullet therefore has a higher SD and, all else equal, a higher BC.

The form factor (i) compares the bullet's actual aerodynamic shape to the G1 reference projectile, a flat-based spitzer designed in the 19th century. A value below 1 means the bullet is more aerodynamic than that reference; above 1 means less. Flat-base spitzers typically have i ≈ 1.0, boat-tails around 0.9, and round-nose or pistol bullets 1.4–1.6. Modern very-low-drag (VLD) boat-tail bullets can reach i ≈ 0.7–0.85, yielding a BC well above what SD alone would suggest.

Because the G1 model was designed for flat-base bullet shapes, long-range shooters increasingly prefer the G7 drag function for modern boat-tail bullets — G7 BCs are more consistent across velocity ranges. G1 BC is still the most widely reported figure on ammunition boxes and remains useful for comparison. Typical G1 BC ranges: pistol and round-nose rifle bullets 0.1–0.2, hunting spitzers 0.25–0.4, long-range match bullets 0.4–0.65, and extreme-range competition bullets up to 0.75+.