Ballistic Coefficient Calculator
Enter your bullet mass, diameter, and form factor to get the G1 or G7 ballistic coefficient, sectional density, and a step-by-step breakdown of the calculation. Switch to drag-coefficient mode to solve BC from mass, cross-sectional area, and Cd directly. Metric and imperial units are both supported, and all results update as you type.
What is the ballistic coefficient?
The ballistic coefficient (BC) is a dimensionless number that describes how efficiently a projectile overcomes air resistance in flight. A higher BC means the bullet loses velocity more slowly, drops less, and drifts less in the wind over a given distance. BC is derived from the ratio of a bullet's sectional density to its form factor, both of which depend on the bullet's physical dimensions and shape. In everyday use, BC is referenced against one of several standard drag models -- most commonly G1 (referenced against a flat-base spitzer) and G7 (referenced against a boat-tail tangent ogive) -- because real bullets are not perfect replicas of any single reference shape.
Form-factor model: the standard method
For the vast majority of published bullet BCs, the form-factor model is used: BC = SD / i, where SD is the sectional density (bullet mass in pounds divided by the square of the calibre in inches) and i is the form factor (the ratio of the bullet's drag to that of the reference projectile at the same velocity). Sectional density alone tells you how much mass is packed into the bullet's frontal area -- a heavy, narrow bullet has a high SD and inherently better BC potential. The form factor adjusts for shape: a sleek boat-tail bullet with a low tangent ogive will have a form factor below 1 relative to the G7 standard, meaning it is more aerodynamic than the reference. Typical G1 form factors for rifle bullets range from about 0.8 to 1.2.
Drag-coefficient model: when you know Cd
If you have aerodynamic test data -- from a wind tunnel, Doppler radar, or computational fluid dynamics -- you may know the drag coefficient Cd directly. In that case, BC = m / (Cd x A), where m is the projectile mass in kilograms and A is the maximum cross-sectional area in square metres. This gives BC in kg/m^2. To compare against published G1/G7 values (which are in lb/in^2), divide by 703.07. Note that Cd is velocity-dependent: a subsonic projectile has a substantially different Cd than the same projectile at supersonic speeds, so a single Cd value is an approximation valid near the velocity at which it was measured.
G1 vs G7: which drag model should you use?
G1 is the legacy standard. Most bullet manufacturers publish G1 BCs because the G1 reference projectile (a Mayevski/Ingalls 1-inch, 1-lb flat-base spitzer from the 1870s) was used when the vast majority of ballistics tables were first built. The practical disadvantage is that modern boat-tail bullets, which retain velocity differently from that old reference shape, show a BC that changes significantly with velocity under G1, making G1 less accurate at very long range. G7 was developed to address this: the reference is a boat-tail tangent ogive that matches modern long-range rifle bullets far more closely. A bullet's G7 BC is lower in absolute value than its G1 BC (roughly 0.45 to 0.55 times), but it stays nearly constant across the velocity range, which is why ballistic software increasingly favours G7 for anything past 600 yards. Use G1 for standard published data comparison and for pistol/short-range rifle bullets; use G7 for long-range work and modern boat-tail match bullets.
Typical ballistic coefficients for common bullet types
| Bullet type | Caliber | Weight (gr) | G1 BC | Category |
|---|---|---|---|---|
| Round nose / FMJ (pistol) | 9 mm | 115 | 0.140 | Low |
| Hollow point (pistol) | .45 ACP | 230 | 0.195 | Low |
| Soft point (rifle) | .30-30 | 150 | 0.186 | Low |
| Flat base spitzer (rifle) | .223 Rem | 55 | 0.243 | Moderate |
| Boat-tail spitzer (rifle) | .308 Win | 150 | 0.397 | Moderate |
| Match BTHP | .308 Win | 168 | 0.447 | Good |
| VLD boat-tail | .308 Win | 175 | 0.505 | Good |
| Long-range boat-tail | 6.5 mm | 140 | 0.626 | Very High |
| ELD-X / Hybrid VLD | 6.5 mm Creedmoor | 143 | 0.625 | Very High |
G1 BC values from manufacturer data. Actual BC varies with muzzle velocity, bullet lot, and atmospheric conditions.
Frequently asked questions
What is a good ballistic coefficient for a rifle bullet?
For general hunting at ranges under 300 yards, a G1 BC of 0.25 to 0.40 is perfectly adequate. For long-range target shooting beyond 500 yards, shooters typically look for G1 BCs of 0.45 or higher, with top competition bullets (6.5 mm Creedmoor, .338 Lapua) reaching 0.60 to 0.80. Pistol bullets rarely exceed 0.20 because their blunt nose and short length limit both sectional density and form factor.
Why is the G7 ballistic coefficient lower than G1 for the same bullet?
Because G1 and G7 reference different drag curves. G1 is referenced against a less streamlined 1870s spitzer, so a modern boat-tail bullet looks very aerodynamic by comparison and gets a relatively high number. G7 is referenced against a boat-tail projectile that is already quite streamlined, so the same modern bullet looks less exceptional by comparison -- hence a lower absolute value. When using G7, multiply the result by roughly 2.0 to 2.2 to get the approximate G1 equivalent.
How does altitude and air density affect ballistic coefficient?
Ballistic coefficient itself does not change with altitude -- it is a property of the bullet. What changes is air density, which directly affects drag force. At higher altitudes where air is thinner, bullets experience less drag and fly flatter, which is equivalent to behaving as if they had a higher BC. Some ballistic solvers let you enter altitude (or temperature and pressure) and adjust trajectory calculations accordingly, but the BC value entered should always be the sea-level reference value from the manufacturer.
What is sectional density and how does it relate to BC?
Sectional density (SD) is the bullet mass in pounds divided by the square of its diameter in inches. It represents how much mass is concentrated in the bullet's frontal area: a heavy, narrow bullet has high SD. Higher SD is good for both ballistic coefficient and penetration. BC is SD divided by the form factor, so two bullets of the same calibre and weight can have different BCs if their shapes (and therefore form factors) differ.
Is BC constant across all velocities?
No. BC is defined at a specific velocity (or velocity range) and changes as the bullet transitions between supersonic, transonic, and subsonic flight. This is especially pronounced for G1 BCs, where the reference curve does not match the actual drag curve of modern boat-tail bullets well. G7 BCs are more stable for such bullets but still vary near the transonic zone (roughly Mach 1.0 to 1.4, around 340 to 475 m/s at sea level). Advanced ballistic solvers use custom drag functions measured by Doppler radar to account for this variation precisely.