Shotgun Choke Calculator

Choke constriction controls how tightly a shotgun's pellet cloud stays together downrange, and this calculator starts from the standard reference point that different chokes put a known percentage of the payload inside a 30-inch circle at 40 yards — roughly 40% for a cylinder bore up through 75% for extra-full. From that percentage it derives a spread rate in inches per yard (tighter chokes spread more slowly) and scales it linearly by your entered distance to estimate pattern diameter, so a full choke's pattern at 20 yards comes out proportionally tighter than at 60. Pellet count is estimated from a standard pellets-per-ounce table indexed by shot size (a numerically larger shot-size number means smaller, more numerous pellets — #8 shot has roughly 410 pellets per ounce versus 72 for #1 buck-adjacent shot) multiplied by your payload weight.

The calculator then derates the choke's rated pattern percentage as distance moves away from the 40-yard reference, since patterns thin out faster than the linear diameter growth would suggest, and uses that adjusted percentage plus the pattern's circular area to estimate pellet density — pellets per square inch — which in turn drives an estimated effective range where the pattern gets too sparse for reliable hits. All of these figures are rule-of-thumb approximations built for comparing chokes and loads against each other, not a substitute for patterning your actual gun and load on paper at your intended distance, since barrel-to-barrel and lot-to-lot variation in real shotshells is significant. A frequent misread: "pattern diameter" describes overall spread, not pellet density inside it — a wide, sparse pattern and a tight, dense one can report similar diameters but very different reliability on game or clays.