Ballistic Coefficient (BC) Calculator

Q: Which mode should I use?

If you know shape factor i (G1/G7 contexts), use the form-factor route. If you have a measured/estimated Cd, use the drag-coefficient route.

Compute a projectile’s ballistic coefficient (BC) to quantify how efficiently it overcomes air drag. Use either the form-factor model $BC = \frac{m}{i d^{2}}$ or the drag-coefficient model $BC = \frac{m}{C_{d} A}$ with $A = \frac{π d^{2}}{4}$ .

Background

The ballistic coefficient increases with mass and decreases with cross-section and drag. A higher BC means less deceleration in flight (flatter trajectory, better energy retention). This tool supports practical units (kg/g/grains, m/cm/in) and shows clear steps.

Choose a mode and enter values:

Mode:

Use form factor i :

BC = \frac{m}{i d^{2}}

Use drag coefficient

C_{d}

BC = \frac{m}{C_{d} A}, A = \frac{π d^{2}}{4}

Projectile mass (m):

We convert to kilograms internally.

Diameter (d):

We convert to meters internally.

Form factor (i):

Dimensionless; depends on projectile shape/drag model (G1/G7, etc.).

Options:

Step-by-step working

Mini gauge: “higher BC → lower drag”

Round to sensible significant figures

Quick picks:

Chips prefill fields; you can tweak and recalculate.

Result:

No results yet. Choose a mode and enter values.

How this calculator works

Form-factor route: enter mass, diameter, and shape factor i → $BC = \frac{m}{i d^{2}}$ .
Drag-coefficient route: enter mass, diameter, and $C_{d}$ ; we compute area $A = \frac{π d^{2}}{4}$ → $BC = \frac{m}{C_{d} A}$ .
Units: we convert mass to kg and diameter to m internally. BC is returned in consistent SI-based terms.

Formula & Equation Used

Form-factor model:

BC = \frac{m}{i d^{2}}

Drag-coefficient model (with frontal area $A$ ):

BC = \frac{m}{C_{d} A}, A = \frac{π d^{2}}{4}

Example Problems & Step-by-Step Solutions

Example 1 — Form-factor route

Given: m = 9.72 g, d = 0.308 in, i = 1.05.
Convert: m = 0.00972 kg; d = 0.007823 m.
Then $BC = \frac{0.00972}{1.05 {0.007823}^{2}}$ ≈ 0.152.

Example 2 — Drag-coefficient route

Given: m = 4.20 g, d = 0.177 in, $C_{d} = 0.30$ .
Convert: m = 0.00420 kg; d = 0.004495 m; $A = \frac{π {0.004495}^{2}}{4}$ $\approx 1.59 \times 10^{- 5} m^{2}$ Then $BC = \frac{0.00420}{0.30 1.59 \times 10^{- 5}}$ $\approx 0.88$ .