Ballistic Coefficient Calculator

Analyze a projectile's aerodynamic efficiency by calculating its BC and SD.

Determine a bullet's ability to overcome air resistance by calculating its Ballistic Coefficient (BC) and Sectional Density (SD) from its physical dimensions.

Examples

Explore these real-world examples to understand how the calculator works with different types of projectiles.

6.5mm Creedmoor Match Bullet

Example 1

A popular choice for long-range target shooting, known for its high BC.

W: 147 gr, d: 0.264 in, i: 0.496

.308 Winchester Hunting Bullet

Example 2

A common hunting round with a bonded-core, flat-base bullet.

W: 180 gr, d: 0.308 in, i: 0.98

.223 Remington Varmint Bullet

Example 3

A lightweight, high-velocity projectile designed for varmint hunting.

W: 55 gr, d: 0.224 in, i: 0.85

.50 BMG Match Grade Bullet

Example 4

An extreme long-range bullet with a very streamlined, VLD (Very Low Drag) design.

W: 750 gr, d: 0.510 in, i: 0.45

Other Titles
Understanding the Ballistic Coefficient: A Comprehensive Guide
Delve into the science of projectile flight and learn how the ballistic coefficient impacts accuracy, range, and performance.

What is a Ballistic Coefficient?

  • Defining the Concept
  • The Role of Sectional Density
  • The Importance of Form Factor
The Ballistic Coefficient (BC) is a fundamental measure of a projectile's ability to overcome air resistance during its flight. In simple terms, a higher BC value indicates that a bullet is more aerodynamically efficient. It will slow down less, be less affected by crosswinds, and maintain a flatter trajectory over long distances compared to a bullet with a lower BC. It's one of the most critical factors for long-range shooting accuracy.
The Two Pillars of BC: SD and Form Factor
The BC value isn't arbitrary; it's derived from two key physical properties of the bullet: its Sectional Density (SD) and its Form Factor (i). The relationship is straightforward: BC = SD / i. Understanding these two components is key to understanding the whole picture.

Key Takeaways

  • Higher BC = Better aerodynamic efficiency.
  • BC is directly proportional to Sectional Density.
  • BC is inversely proportional to the Form Factor.

Step-by-Step Guide to Using the Calculator

  • Gathering Your Bullet's Data
  • Inputting the Values
  • Interpreting the Results
Our calculator simplifies the process of finding a bullet's BC. Here's how to use it effectively:
1. Bullet Weight (W)
Enter the bullet's weight in 'grains'. This is a standard unit for projectile weight and can be found on the ammunition box or the manufacturer's website. (1 pound = 7000 grains).
2. Bullet Diameter (d)
Enter the bullet's diameter (also known as caliber) in 'inches'. For example, a .308 Winchester bullet has a diameter of 0.308 inches.
3. Form Factor (i)
This is the most technical input. The form factor compares the shape of your bullet to a 'standard' reference projectile (the G1 model is the most common). A lower form factor means the bullet is more streamlined than the standard. You can often find this value from the bullet manufacturer, or use these typical values: Flat-base, simple spire point: i ≈ 1.0; Spitzer boat-tail: i ≈ 0.7-0.9; Very Low Drag (VLD) match bullet: i ≈ 0.45-0.6.

Calculation Steps

  • First, the calculator computes Sectional Density: SD = W / (7000 * d²).
  • Then, it calculates the Ballistic Coefficient: BC = SD / i.

Real-World Applications of Ballistic Coefficient

  • Long-Range Target Shooting
  • Ethical Hunting Practices
  • Ammunition Development and Selection
Understanding and using BC is not just an academic exercise; it has significant practical applications.
Precision and Accuracy
For long-range shooters, a high BC is paramount. It means the bullet will arrive at the target faster and with a higher velocity, minimizing the time it's exposed to environmental factors like wind. This results in less 'wind drift' and a smaller 'bullet drop', making it easier to hit distant targets.
Energy on Target
Hunters rely on a bullet's kinetic energy to ensure an ethical and humane harvest. A bullet with a higher BC retains more of its velocity downrange. Since kinetic energy is a function of mass and velocity squared (KE = ½mv²), a higher retained velocity means significantly more energy delivered to the target, especially at extended ranges.

Application Scenarios

  • A competition shooter choosing a 147gr 6.5mm bullet (high BC) over a 120gr one (lower BC) for a 1000-yard match.
  • A hunter selecting a VLD-style bullet to ensure sufficient energy for taking an elk at 400 yards.

Common Misconceptions and G-Models

  • Is Higher BC Always Better?
  • The G1 vs. G7 Drag Models
  • BC Can Change with Velocity
While BC is a powerful tool, it's often surrounded by misconceptions.
The G1 and G7 Models
This calculator uses the G1 drag model, which is the industry standard and has been for decades. It's based on a flat-based, blunt-nosed projectile. However, most modern long-range bullets more closely match the shape of the G7 standard projectile, which has a boat-tail and a more streamlined ogive. For this reason, a bullet will have both a G1 BC and a G7 BC. The G7 BC value is generally lower than the G1 value, but it can provide more accurate trajectory predictions for modern bullets. Always ensure your ballistic solver app and BC value use the same model (G1 or G7).
BC is Not Constant
A bullet's BC value is not fixed. It actually changes as the bullet slows down and passes through different velocity zones (supersonic, transonic, subsonic). The BC value provided by manufacturers is an average for a typical range of velocities. This is why some manufacturers provide multiple BCs for different velocity brackets.

Important Notes

  • Always match the BC model (G1/G7) with the model used by your ballistic calculator app.
  • The published BC is an average; for ultimate precision, it must be verified in real-world conditions.

Mathematical Derivation and Examples

  • The Sectional Density Formula
  • The Ballistic Coefficient Formula
  • Worked Calculation Example
Let's break down the math behind the calculations.
Formula for Sectional Density (SD)
SD = Weight (lbs) / Diameter (in)². Since bullet weight is given in grains, we must convert it to pounds by dividing by 7000. Formula: SD = (W / 7000) / d².
Formula for Ballistic Coefficient (BC)
BC = Sectional Density (SD) / Form Factor (i). This shows the direct relationship between the mass/diameter ratio and the final aerodynamic efficiency.

Walkthrough: 6.5mm Creedmoor (Example 1)

  • Given: W = 147 gr, d = 0.264 in, i = 0.496
  • 1. Calculate SD: SD = (147 / 7000) / 0.264² = 0.021 / 0.069696 ≈ 0.301
  • 2. Calculate BC: BC = 0.301 / 0.496 ≈ 0.607