Ballistic Coefficient Calculator
Sectional density, BC = SD / i, G1 & G7 drag models, missing-variable solver, and unit converter — all client-side.
- A ballistic coefficient is model-specific: a G1 BC and a G7 BC are not interchangeable and cannot be converted exactly.
- Typical form-factor values in the table are guidance only, not exact per bullet — use the manufacturer's published BC for the correct model.
- Educational math tool only — not for load development or safety-critical decisions.
BC from mass, diameter & form factor
SD = m / d² in canonical units (lb, in). Example: .308 150 gr → SD = 0.2259 lb/in² (158.81 kg/m²); with i = 1.05 → BC = 0.2151.
Solve for the missing variable
Enter any four values; the fifth is computed. Check: BC = 0.2259, i = 1.00, d = 0.308 in → mass = 150.0 grains.
Typical form factors — guidance only
Not exact per bullet. Pick a model, then “Use” copies the typical i into the BC calculator.
| Bullet profile | Typical i |
|---|
Rough G1 ↔ G7 cross-reference
Example: BC(G1) = 0.523 × 0.50 ≈ BC(G7) 0.2615 — rough guidance only.
Mass & diameter unit converter
Example: 100 g = 0.2205 lb; 7.62 mm = 0.3 in.
Related tools
Twist Rate Calculator — barrel twist (Greenhill/Miller) is about stabilizing the bullet, while BC is about drag in flight: different quantities.
Bullet Energy Calculator — muzzle energy ½mv², a different quantity from ballistic coefficient.
Frequently asked questions
What is the ballistic coefficient (BC)?
The ballistic coefficient measures how well a bullet resists air drag. It is computed as BC = SD / i, where SD is the sectional density (mass divided by the square of the diameter) and i is the form factor relative to a standard drag model. Example: a .308 bullet weighing 150 grains has SD = 0.2259 lb/in^2 (158.81 kg/m^2); with a typical G1 form factor of 1.05 its BC is 0.2151.
How do you calculate sectional density?
Sectional density is SD = m / d^2, using consistent units. In imperial units, mass in pounds and diameter in inches: a 150 grain bullet is 150/7000 = 0.02143 lb, and (0.308 in)^2 = 0.094864 in^2, so SD = 0.02143 / 0.094864 = 0.2259 lb/in^2. In metric that is 158.81 kg/m^2. This tool also solves the formula in reverse for mass, diameter, SD, i, or BC.
What is the form factor i?
The form factor i compares a bullet's drag to the standard reference projectile of the chosen model (G1 or G7). A value of 1.00 means the bullet matches the reference; lower means less drag. The typical i values in the table above are guidance only, not exact per bullet - always use the BC published by the bullet manufacturer for the correct drag model.
What is the difference between G1 and G7 BC?
G1 and G7 are different standard drag models (G1 is an older flat-base shape, G7 matches modern boat-tail bullets). A BC value is only meaningful together with its model: G1 and G7 BCs are not interchangeable and cannot be converted exactly. As a rough guidance-only estimate, BC(G7) is about 0.40-0.60 times BC(G1); for example 0.523 x 0.50 = 0.2615. Never treat such an estimate as exact.
What does a higher BC mean?
A higher BC means the bullet loses velocity more slowly, so it drops less and drifts less in wind at long range. Doubling BC does not double anything exactly - it is a relative efficiency rating, and it only applies within the same drag model (G1 or G7).
Which units can I use?
Mass: grains, grams, ounces, pounds, or kilograms. Diameter: inches or millimeters. The tool converts everything to canonical units (pounds and inches) before applying SD = m / d^2, and also shows SD in kg/m^2. Example: 100 g = 0.2205 lb and 7.62 mm = 0.3 in.
Example: .224 55 grain bullet?
Mass 55 grains = 0.007857 lb; diameter 0.224 in, squared = 0.050176 in^2. SD = 0.007857 / 0.050176 = 0.1566 lb/in^2. With a typical G1 form factor i = 1.08 (guidance only), BC = 0.1566 / 1.08 = 0.1450.
Can I solve for a missing value?
Yes. Enter any four of the five values - BC, sectional density, form factor i, bullet mass, bullet diameter - and the solver computes the fifth. For example, BC = 0.2259 with i = 1.00 and diameter 0.308 in gives mass = 0.02143 lb = 150.0 grains.
Is this calculator safe for load development?
No. This is an educational tool for understanding sectional density and ballistic coefficient math. It does not model pressure, stability, or terminal ballistics, and its typical form factors are approximations. Always follow published reloading manuals and the bullet manufacturer's data for any real ammunition decisions.
How does this relate to twist rate and muzzle energy?
Ballistic coefficient describes drag in flight, twist rate describes the barrel rifling needed to stabilize the bullet, and muzzle energy (1/2 m v^2) describes energy at the muzzle - three different quantities. See also our twist rate calculator and bullet energy calculator.
Free tool by NRTechStudio