Calculate arrow momentum for deeper penetration assessment. Better than KE for predicting performance on heavy game and bone impact.
Momentum in slug-feet per second equals total arrow weight in grains multiplied by velocity in feet per second, divided by 225,218. That divisor is exact: 7,000 grains per pound times 32.174 pounds per slug. Enter your chronographed speed and finished-arrow weight and the tool returns both the slug·ft/s figure and its metric equivalent in newton-seconds (multiply by 4.44822).
This tool sorts results into four bands. Below 0.40 slug·ft/s is rated small-game class, 0.40 to 0.50 is medium game, 0.50 to 0.65 is large game, and 0.65 and above is dangerous-game territory. A worked example: a 480-grain arrow at 270 fps gives 480 × 270 ÷ 225,218 ≈ 0.575 slug·ft/s, which lands in the large-game band suitable for elk.
Momentum is a better predictor of penetration through bone and heavy tissue, while KE measures the total energy. Because momentum scales linearly with both mass and speed (KE scales with speed squared), a heavy, moderately fast arrow often out-penetrates a light, fast one even when their kinetic energy is similar. Experienced hunters prioritize momentum for large game.
Increasing total arrow weight is the most effective way to gain momentum. Even if it slows the arrow down slightly, the increased mass carries more 'punch'. Because the speed lost to extra weight is roughly proportional, while momentum gains the full mass increase, adding grains almost always raises the slug·ft/s figure. Adding front-of-center weight with a heavier point also pushes penetration force forward.