Estimate Bullet Ballistic Coefficient (BC)
Use this calculator to estimate ballistic coefficient from bullet weight, diameter, and form factor.
What Is Ballistic Coefficient?
Ballistic coefficient (BC) is a number that describes how well a bullet resists air drag in flight. In plain language: higher BC bullets slow down less, drift less in wind, and generally retain velocity better over long distance.
BC is not the only thing that matters for precision shooting, but it is one of the most useful numbers in external ballistics. Shooters use it for trajectory tables, wind corrections, hunting setups, and long-range match preparation.
Formula Used in This Calculator
This tool estimates BC using sectional density (SD) and form factor (i):
Weight in pounds = Weight in grains / 7000
BC = SD / i
The drag model (G1 or G7) tells you what standard projectile your BC is referenced to. The math in this estimator is the same, but the correct form factor must correspond to your chosen model.
How to Use the Calculator
- Enter bullet weight in grains.
- Enter bullet diameter in inches (for example, 0.224, 0.264, 0.308).
- Enter form factor i.
- Select G1 or G7 to match your data source.
- Click Calculate BC to see the estimated coefficient.
If you already have a manufacturer BC value, use that for trajectory solvers. This calculator is most useful when doing manual estimates, comparing design changes, or checking ballistics homework.
G1 vs G7: Which Should You Use?
G1 BC
G1 is widely published and historically common. It references a flat-base, blunt-ogive standard projectile. Many commercial bullet boxes still list G1 BC values.
G7 BC
G7 often models modern long-range boat-tail bullets better, especially at extended range. For very-low-drag bullets, G7 can produce more stable ballistic predictions across velocity bands.
Example Calculation
Suppose you have:
- Weight = 175 grains
- Diameter = 0.308 inches
- Form factor i = 1.05
Convert weight to pounds: 175 / 7000 = 0.025 lb.
Compute SD: 0.025 / (0.308²) = 0.2635 (approx).
Then BC: 0.2635 / 1.05 = 0.2510 (approx).
Your estimated BC would be 0.251 in the selected reference model.
What Changes BC in Real Life?
- Bullet shape: ogive profile, meplat size, and boat-tail angle.
- Mass and diameter: heavier-for-caliber bullets can increase sectional density.
- Velocity regime: published BC can shift at different speed bands.
- Atmosphere: temperature, pressure, and altitude affect drag behavior.
- Manufacturing variation: lot-to-lot changes can slightly alter effective BC.
Practical Tips for Shooters
1) Confirm with real data
Even good BC estimates should be trued against actual impacts at distance. Chronograph data and real drop/wind observations are valuable.
2) Keep model consistency
If your solver is set to G7, use G7 BC. Mixing model types creates avoidable errors.
3) Track velocity zones
Some bullets have stepped BC values by velocity band. That can improve trajectory accuracy compared with a single constant BC.
Limitations and Safety Note
This calculator is an educational estimator, not a substitute for pressure-tested load data or professional ballistic software. Always follow safe reloading practices, verify ranges, and handle firearms responsibly.