Foot Pounds of Energy Calculator
Foot-pounds (ft-lbs) measure the kinetic energy of a moving projectile. A 150-grain bullet at 2,800 fps carries about 2,611 ft-lbs of energy. This calculator works for bullets, arrows, pellets, or any projectile where you know the weight in grains and the velocity in feet per second.
The formula:
KE (ft-lbs) = (weight in grains × velocity in fps²) / 450,240
The constant 450,240 equals 2 × 7,000 (grains per pound) × 32.174 (gravitational acceleration in ft/s²). It converts the standard KE = 1/2 mv² formula into grain and fps units.
Common reference values:
- .22 LR (40gr @ 1,080fps): ~104 ft-lbs
- 9mm (124gr @ 1,150fps): ~364 ft-lbs
- .223 Rem (55gr @ 3,240fps): ~1,282 ft-lbs
- .308 Win (168gr @ 2,650fps): ~2,620 ft-lbs
- .30-06 (180gr @ 2,700fps): ~2,914 ft-lbs
- Crossbow bolt (400gr @ 400fps): ~142 ft-lbs
Energy vs. momentum:
Energy grows with the square of velocity, so doubling speed quadruples energy. Momentum grows linearly with velocity. Both matter for terminal performance, but in different ways. Energy relates to tissue damage potential, while momentum relates to penetration depth.
Archery note: Arrows and crossbow bolts typically range from 250-500 grains at 150-400 fps, producing 30-150 ft-lbs. Even though this is much less than firearms, arrows achieve penetration through a different mechanism (cutting rather than expansion).