Compute ballistic coefficient (BC) using the definition that matches your inputs: Small-arms (bullet weight, diameter, and form factor i) or Physics (mass, drag coefficient Cd, and cross-sectional area A). Your result is shown with units and the G-type label you selected so you can record it correctly.
Advanced options
How to use our Ballistic Coefficient Calculator
- Choose Calculation method: Small-arms (SD / i) for bullet specs, or Physics (m / (Cd x A)) for a drag-physics setup.
- Select Unit system for inputs and BC output (US or Metric). This determines which fields you should use and what BC units will be displayed.
- (Optional) Set BC reference model (G-type) for labeling only (G1 or G7). This is a label to help you keep your data organized.
- If needed, open Advanced options to override how you enter weight/mass, or to choose whether Physics-mode area is entered directly or computed from a diameter.
- Small-arms mode: enter one bullet weight (grains or pounds), one bullet diameter (inches or mm), and Form factor i (dimensionless).
- Physics mode: enter one mass (kg or lbm), enter Drag coefficient Cd (dimensionless), then choose an Area entry type and provide either a diameter (to compute area) or the area A directly.
- Choose Number display (Auto, Plain, or Scientific) so the result is easy to read and copy.
- Click Calculate.
- Sanity-check: read Inputs used (summary). Make sure it shows the definition (Small-arms vs Physics) and the exact fields you meant to use (for example grains vs pounds, inches vs mm). If not, fix the inputs and recalculate.
Common mistakes and quick fixes
Mistake: Calculating Small-arms BC when your data is actually mass, Cd, and area from a physics problem.
Fix: Switch Calculation method to Physics and enter mass, Drag coefficient Cd (dimensionless) , and area (directly or from diameter) before calculating.
Mistake: Entering grains into Bullet weight (pounds) (example: typing 168 as pounds).
Fix: Put 168 in Bullet weight (grains) , or convert to pounds first (grains divided by 7000), then recalculate and confirm the weight field used in Inputs used (summary) .
Mistake: Typing a millimeter diameter into Bullet diameter (inches) (example: 7.82 typed as inches).
Fix: Enter 7.82 in Bullet diameter (mm) , or convert to inches (about 0.308) and use the inches field. Recalculate and recheck Inputs used (summary) .
Mistake: Using form factor i like a percent, or entering i as 0 or a negative number.
Fix: Enter Form factor i (dimensionless) as a positive ratio (example: 1.00). If you do not know i, you cannot compute Small-arms BC from weight and diameter alone.
Mistake: Leaving Physics-mode area blank, or entering Cd as 0/negative, or confusing Cd with BC.
Fix: In Physics mode, make sure Drag coefficient Cd (dimensionless) is greater than 0 and you provide area using your chosen Area entry type (diameter-for-area or cross-sectional area A).
Mistake: Comparing BC values that come from different definitions or mixing a G1-labeled number with a G7-labeled system (or vice versa).
Fix: Only compare or reuse BC when Inputs used (summary) shows the same definition and you keep the same G-type label all the way through your workflow.
Definitions
Ballistic coefficient (BC): A number used to describe how strongly air drag slows a projectile under a specific definition. This calculator supports a Small-arms definition (from sectional density and form factor) and a Physics definition (from mass, Cd, and area). They are not interchangeable. [1]
Sectional density (SD): In Small-arms mode, SD is weight divided by diameter squared (using consistent units). SD goes up when a bullet is heavier for its diameter. [1]
Form factor i (dimensionless): In Small-arms mode, i compares your bullet to a standard reference projectile. BC is SD divided by i, so a smaller i usually means a higher BC. [1]
Drag coefficient Cd (dimensionless): In Physics mode, Cd is a shape-and-speed factor used in drag equations. It can change with speed and flow conditions. [2]
Cross-sectional area (A): The frontal area facing the airflow. If computed from diameter, it is the area of a circle (units like m^2 or in^2).
BC reference model (G-type): A label such as G1 or G7 used with published small-arms BCs. Here it is for labeling only so you do not mix data sources. [1]
Methodology
Step 1: Choose the definition. "Ballistic coefficient" is used in more than one way. This calculator makes you pick between (A) Small-arms BC based on sectional density and form factor i, and (B) Physics BC based on mass, drag coefficient Cd, and cross-sectional area A. These give different numeric values and units, so do not treat them as the same thing. [1]
Small-arms method (SD / i).
BC = SD / i
SD = w / d^2
Here w is bullet weight and d is bullet diameter. In the classic US form used for many bullets, w is in pounds and d is in inches, so BC comes out in lb/in^2. If you enter grains, the calculator converts grains to pounds first. [1]
Physics method (m / (Cd x A)).
BC = m / (Cd * A)
If you choose to compute area from diameter, the calculator treats the projectile front as a circle:
A = π * (d/2)^2
Units follow your unit system: kg and m^2 produce kg/m^2, and lbm and in^2 produce lbm/in^2.
Conversion used (US grains to pounds).
w_lb = w_gr / 7000
This uses 7000 grains per pound. [1]
Worked mini-example (Small-arms, US). Use Bullet weight (grains) = 168, Bullet diameter (inches) = 0.308, and Form factor i = 1.00.
w_lb = 168 / 7000 = 0.024
SD = 0.024 / (0.308^2) = 0.253
BC = 0.253 / 1.00 = 0.253 lb/in^2
Assumptions and limits. (1) The G-type (G1 or G7) is a label only in this tool; it does not generate drag tables, and you should not mix a BC labeled for one model with calculations expecting another model without a proper method. [1] (2) Cd and effective BC can change with speed, so a single BC value can be an approximation over a speed range. [2] (3) The calculator does not guess units from the size of a number, so always use the field that matches your units and confirm the Inputs used summary after you calculate.