Calculate a bullet's ballistic coefficient (BC) from its weight, diameter, and form factor using the standard sectional density (SD) divided by i formula, for the G1 or G7 reference drag model.
Quick Facts
Formula
BC = SD ÷ i
Sectional density (weight ÷ diameter²) divided by the bullet's form factor i, relative to a standard reference projectile shape.
Typical range
0.12 – 1.10
Handgun bullets sit near the low end; sleek, boat-tailed long-range rifle bullets typically exceed 0.500.
Results
Calculated
Ballistic Coefficient (BC)
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Set inputs and press Calculate
Sectional Density (SD)
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Weight ÷ diameter², in lb/in²
Bullet Weight
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Converted from grains to pounds
Aerodynamic Rating
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Qualitative BC classification
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Understanding Ballistic Coefficient
The ballistic coefficient (BC) is a single number describing how efficiently a bullet fights air resistance in flight. A higher BC means the bullet retains velocity better, drops less, and drifts less in wind at distance. BC is not a measure of energy or accuracy — it is purely a measure of aerodynamic efficiency relative to a standard reference projectile.
The formula
The standard method for estimating BC from a bullet's physical dimensions uses two steps:
Sectional density (SD): SD = weight (lb) ÷ diameter² (in²). Weight in grains is first converted to pounds by dividing by 7000 (there are 7000 grains in a pound).
Ballistic coefficient (BC): BC = SD ÷ i, where i is the bullet's form factor — a dimensionless number describing how its shape compares to a standard reference projectile (blunter shapes have i > 1, sleeker shapes have i < 1).
Combined: BC = W ÷ (7000 × d² × i), where W is weight in grains and d is diameter in inches.
G1 vs. G7 reference models
Form factor is always relative to a chosen reference drag curve. G1 models an older flat-based, blunt-nosed projectile and is the number printed on most factory ammunition boxes. G7 models a modern boat-tailed, sleek-ogive bullet and tracks the drag of long-range match bullets more accurately at extended range. The same physical bullet has a different form factor — and therefore a different numeric BC — under each model, so always compare BC values computed under the same reference.
Why BC matters
Ballistic coefficient feeds directly into external-ballistics trajectory calculations: it is the key input that determines how quickly a bullet decelerates due to air drag, which drives bullet drop and wind drift downrange. A bullet with BC 0.600 retains velocity and resists wind far better past 500 yards than one with BC 0.200 at the same muzzle velocity.
Frequently Asked Questions
What is a ballistic coefficient?
Ballistic coefficient (BC) is a number describing how efficiently a bullet overcomes air resistance in flight. It is calculated as sectional density divided by a shape-dependent form factor: BC = SD ÷ i. A higher BC means the bullet slows down less, drops less, and drifts less in wind over distance. BC measures aerodynamic efficiency only — it says nothing about accuracy or terminal energy.
What is the difference between G1 and G7 ballistic coefficient?
G1 and G7 are reference drag models the form factor is measured against. G1 models an older flat-based, blunt-nosed projectile and is the number printed on most factory ammunition boxes. G7 models a modern boat-tailed, sleek-ogive bullet and predicts drag more accurately for long-range match bullets. The same bullet has a different numeric BC under each model, so always compare BC values computed under the same reference.
What is sectional density?
Sectional density (SD) is a bullet's weight divided by the square of its diameter: SD = weight (lb) ÷ diameter² (in²). Weight in grains is converted to pounds by dividing by 7000. SD describes how the bullet's mass is distributed relative to its frontal area, independent of shape — it is the first term in the ballistic coefficient formula, before the shape-based form factor is applied.
Does a higher ballistic coefficient mean a more accurate bullet?
No. Ballistic coefficient measures how well a bullet resists air drag, not how consistently it groups. Accuracy depends on manufacturing tolerances, barrel quality, twist rate matching, and shooter technique. A high-BC bullet still needs to be well-made and properly stabilized to shoot accurately; BC only predicts how the trajectory behaves once the bullet is in flight.