Ballistic Coefficient Estimator
Estimate G1 BC from sectional density and a form factor, or true one from two chronographed velocities.
How this works
The "from weight, diameter and form factor" method starts from sectional density (see that calculator) and divides by a form factor — a number describing how efficiently the bullet's shape cuts through air compared to the reference G1 projectile. A form factor of exactly 1.0 means the bullet behaves exactly like the G1 reference shape; most modern boat-tail bullets score better (a lower form factor) than that.
Why "true" BC needs a chronograph
Manufacturer-published BCs are measured under specific test conditions and can run optimistic in the real world. The chronograph method instead measures how much velocity your actual bullet lost between the muzzle and a downrange distance, then works backward to find the BC that would produce exactly that velocity loss — a real-world number for your rifle and ammunition, not a catalog spec.
Frequently asked questions
Which method should I use — the form-factor estimate or the chronograph method?
Use the chronograph method whenever you can — it back-solves BC from your actual measured velocity loss, which is far more trustworthy than an estimate built from a guessed form factor.
Why does my calculated BC not match the manufacturer's published number?
Published BCs are measured under specific test conditions and can run optimistic for your actual rifle, ammunition lot, and local atmospheric conditions. A BC you solve for yourself from real chronograph data reflects your specific setup, not a catalog average.
Do I need to retest my BC often?
Not usually — but it's worth rechecking after a barrel change, a big shift in altitude or temperature, or if you switch to a different lot of the same ammunition, since any of those can shift the real-world number.
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