What is a ballistic coefficient?

A ballistic coefficient, or BC, measures how well a projectile keeps its speed against air drag. A higher BC means the bullet slows down less, drops less and drifts less in the wind. It equals the sectional density divided by the form factor relative to a standard drag model such as G1 or G7.

What is sectional density?

Sectional density is the bullet weight in pounds divided by the square of its diameter in inches. Using grains, it is the weight in grains divided by 7,000, then divided by the diameter squared. It describes weight relative to frontal area, before shape is taken into account.

What is the form factor?

The form factor compares a bullet's drag shape to the standard reference projectile for the chosen model. A sleek, streamlined bullet has a form factor below 1, so dividing sectional density by it raises the ballistic coefficient. Manufacturers usually publish the BC directly, and you can back out the form factor from it.

Ballistic Coefficient Calculator

This ballistic coefficient calculator works out how well a bullet holds its speed against air resistance, the single number shooters use to predict drop and wind drift at range. You enter the bullet weight in grains, its diameter in inches, which is the calibre, and a form factor for the drag model you are using, and the tool returns the sectional density and the ballistic coefficient, or BC. Sectional density is the weight relative to frontal area: the weight in grains divided by 7,000 to reach pounds, then divided by the diameter squared. The ballistic coefficient is that sectional density divided by the form factor, which compares the bullet's shape to a standard reference projectile such as the G1 flat-based model or the sleeker G7 boat-tail model. A higher BC means the bullet slows, drops and drifts less, so long-range and target shooters favour heavy, streamlined bullets with a high figure. Reloaders and hunters use these numbers to compare projectiles, to sanity-check a maker's published BC, or to back out a form factor when only the BC is given. Remember that a BC is tied to its drag model, so a G1 value and a G7 value for the same bullet look very different and are not interchangeable. Enter your bullet's figures below and the sectional density and BC update as you type.

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Bullet weight (grains)

Diameter / calibre (inches)

Form factor (i)

0.533

ballistic coefficient (BC)

Sectional density0.253

Form factor (i)0.475

Bullet weight0.024 lb

BC = sectional density / form factor. Sectional density = (grains / 7,000) / diameter squared, in lb/in². A BC is tied to its drag model (G1 or G7), so values from different models are not interchangeable. Estimate only.

How it works

First find the sectional density: convert the bullet weight from grains to pounds by dividing by 7,000, then divide by the square of the diameter in inches. That gives a figure in pounds per square inch describing weight relative to frontal area. Then divide the sectional density by the form factor, which measures how the bullet's drag shape compares to the standard projectile for your chosen model. The result is the ballistic coefficient. A streamlined bullet has a form factor below 1, which lifts the BC above the sectional density.

Worked example

Take a 168 grain match bullet of 0.308 inch diameter with a G1 form factor of 0.475. The sectional density is 168 divided by 7,000, which is 0.024 pounds, then divided by 0.308 squared, or 0.094864, giving about 0.253. Dividing that by the form factor of 0.475 gives a ballistic coefficient of about 0.533, a typical G1 value for a bullet of this style. Feed that BC into a ballistics solver to predict drop and wind drift at range.

Related calculators

Kinetic Energy Calculator: energy from mass and speed.
Momentum Calculator: mass times velocity.
Projectile Motion Calculator: range, height and flight time.
Drag Force Calculator: air resistance on a body.
Terminal Velocity Calculator: steady falling speed.

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