Shotgun Shot Pattern Calculator
Calculate shotgun pellet spread at any distance.
Find pattern diameter and pellet density for different choke constrictions in clay and sport shooting.
A shotgun fires hundreds of small pellets in a spreading pattern. The pattern diameter at a given range determines the effective area and how many pellets hit the target.
Pattern spread: Pattern diameter (cm) ≈ range (m) × spread factor
What a choke actually is. A choke is defined by the share of pellets it puts inside a 30-inch circle at 40 yards, not by how wide the visible pattern looks. Those definitional percentages are the industry standard and they are what this calculator works from:
| Choke | Pellets in a 30" circle at 40 yd | Pattern width at 40 yd | Spread factor |
|---|---|---|---|
| Cylinder (no choke) | 40% | 40" (102 cm) | 2.8 cm/m |
| Improved Cylinder (IC) | 50% | 35" (88 cm) | 2.4 cm/m |
| Modified (M) | 60% | 30" (77 cm) | 2.1 cm/m |
| Improved Modified (IM) | 65% | 27" (69 cm) | 1.9 cm/m |
| Full Choke (F) | 70% | 24" (62 cm) | 1.7 cm/m |
The two right-hand columns describe the same pattern from different angles, and it is worth being clear that they are not the same measurement. The width is where the outermost pellets land. The percentage is about the core, because a pattern is not an even sheet of pellets: it is dense in the middle and thins toward the fringe. A full choke at 40 yards throws a 24-inch pattern and still only lands 70% of the shot in a 30-inch circle, even though the circle is wider than the pattern. The missing 30% is the fliers: pellets deformed on the way up the barrel that leave the swarm entirely.
Percentages rise as the range shortens, since the circle covers proportionally more of a smaller pattern. This calculator scales with the square of range, which reproduces the definitional figure exactly at 40 yards and caps out at 95%, because no gun puts everything in the circle.
Pellet count by shot size:
| Shot Size | Pellet Count per oz (28g) | Pellet Diameter |
|---|---|---|
| #9 | ~585 | 2.0 mm |
| #7.5 | ~350 | 2.4 mm |
| #6 | ~225 | 2.7 mm |
| #4 | ~135 | 3.3 mm |
| #2 | ~87 | 3.8 mm |
| BB | ~50 | 4.6 mm |
Pattern efficiency: A good hunting pattern puts at least 70% of its pellets inside a 30-inch (76 cm) circle at the intended range. Most modern guns with full choke achieve this at 40 yards (37 m).
Effective ranges for hunting
These are the maximums the calculator holds you to, and they are set by pellet energy rather than pattern. Small shot runs out of energy before it runs out of pattern: a #7.5 pellet at 45 yards may still be inside a dense pattern and no longer carry enough to reach anything vital.
| Shot | Game | Ideal | Maximum |
|---|---|---|---|
| #7.5 | Quail, dove, other small upland | 20 to 30 yd | 35 yd |
| #6 | Pheasant, grouse, larger upland | 25 to 35 yd | 40 yd |
| #4 | Duck, turkey | 30 to 40 yd | 45 yd |
| #2 | Geese, large duck | 35 to 45 yd | 50 yd |
| BB | Geese, predators | 40 to 50 yd | 55 yd |
Deer slugs are a different question entirely and sit outside this calculator, at 75 to 100 yards depending on whether the barrel is rifled.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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