Free Tool
Airgun FPE calculator
By James · Updated July 24, 2026 · Editorial policy
The exact formula I use at the bench: FPE = weight (grains) × velocity (FPS)² ÷ 450,436 — the divisor specified in the official AAFTA Field Target Rules (2026), derived from 2 × standard gravity (32.174 ft/s²) × 7,000 grains per pound. Enter your load — ideally the velocity off your own chronograph, not the box.
Quick presets: .177 · 8.44gr @ 900 · .22 · 18.1gr @ 880 · .25 · 33.95gr @ 880 · 40.1gr slug @ 900
How to read the number
Muzzle energy is the start of the story, not the end — pellets bleed FPE every yard downrange. The humane working floor for rabbits and squirrels is 9–12+ FPE delivered at the animal, which is why a rifle chronoing ~15+ FPE at the muzzle is the practical small-game minimum at field ranges. Full context in the caliber guide and the pest-control field guide.
One warning that saves hunts: never use advertised velocity. It's measured with featherweight alloy pellets nobody hunts with. Chronograph your actual hunting load — my strings all come off the radar unit below — and then this calculator tells you the truth.