Ballistic Coefficient Calculator
Estimate BC using bullet weight, diameter, and form factor. Optional fields can provide a rough retained velocity estimate.
What Is a Bullet BC?
BC (ballistic coefficient) is a number that describes how well a projectile resists drag in flight. In plain terms, a higher BC generally means the bullet slows down less quickly, carries velocity farther, and is less affected by wind drift compared with lower-BC projectiles of similar caliber.
BC is always tied to a drag model, most commonly G1 or G7. Because each model uses a different reference shape, the same bullet can have different BC values depending on which model is used.
How This Calculator Works
This page uses the standard relationship between sectional density and form factor:
Bullet Weight (lb) = grains / 7000
BC = SD / i
Where i is the form factor. Lower form factor means a more efficient shape and therefore a higher BC.
Optional Velocity Estimate
If you provide muzzle velocity and range, the calculator also gives a rough retained velocity and energy estimate. This is intentionally simplified and should not replace verified trajectory software, chronograph data, or published manufacturer curves.
G1 vs G7: Which Should You Use?
- G1 is very common and widely published by manufacturers.
- G7 often better matches modern long-range boat-tail projectile shapes.
- If comparing bullets, always compare BC values from the same drag model.
As a rough conversion in this calculator’s output, we show an approximate relation of G7 ≈ G1 × 0.52. Treat this as a convenience estimate only.
Typical Input Ranges
| Input | Typical Example | Notes |
|---|---|---|
| Weight | 55 to 230 grains | Use actual projectile weight, not loaded cartridge weight. |
| Diameter | 0.224, 0.264, 0.308 in | Must match bullet caliber diameter. |
| Form Factor (i) | 0.8 to 1.2 | Lower values indicate a more aerodynamic projectile. |
Step-by-Step Example
Example Inputs
- Weight: 175 grains
- Diameter: 0.308 inches
- Form Factor: 1.00
- Model: G1
Calculated Values
The tool computes sectional density first, then divides by form factor to estimate BC. If you add muzzle velocity and range, it also provides an estimated downrange velocity and energy.
Important Notes for Real-World Use
- Published BC can vary by velocity band.
- Temperature, altitude, and pressure significantly affect external ballistics.
- Chronograph your setup whenever possible.
- Use manufacturer data and validated ballistic solvers for precise dope.
This calculator is best used as a quick comparison and learning tool for understanding how projectile geometry and mass influence drag performance.