Crossbow & Arrow Kinetic Energy Calculator
Enter your bolt weight and speed to get kinetic energy in foot-pounds and momentum. The formula and worked examples are below the calculator.
Calculator
Kinetic energy
142.1 ft-lbs
Momentum
0.710 slug-ft/s
The formula
Kinetic energy (ft-lbs) = grains × FPS² ÷ 450,240. Momentum (slug-ft/s) = grains × FPS ÷ 225,400.
Speed is squared in the energy formula, so speed drives energy more than weight does. Momentum rises in a straight line with both, which is why heavier bolts gain on it. Weigh the bolt with its tip or broadhead fitted; see grain weight and kinetic energy in the glossary.
Worked examples
| Bolt | Speed | Energy | Momentum |
|---|---|---|---|
| 400 gr | 330 FPS | 96.7 ft-lbs | 0.586 |
| 400 gr | 400 FPS | 142.1 ft-lbs | 0.710 |
| 425 gr | 380 FPS | 136.3 ft-lbs | 0.717 |
| 400 gr | 450 FPS | 179.9 ft-lbs | 0.799 |
| 400 gr | 515 FPS | 235.6 ft-lbs | 0.914 |
My measured speeds for each reviewed crossbow are collected on the test results page, so you can plug in a real number rather than the box rating.
What the numbers do not tell you
- Energy at the rail is not energy downrange; bolts slow with distance.
- Broadhead design, sharpness and shot placement matter as much as the energy figure.
- Some states set minimum draw weight or bolt requirements for hunting. Check your state wildlife agency's current rules; see crossbow hunting laws by state.
What to do with your result
Read nextBest crossbows for deer huntingCompare the kinetic energy I measured on six tested hunting crossbows.Read nextBest crossbow boltsHeavier bolts raise energy; see which tested bolts suit your crossbow.Frequently asked questions
What is the formula for crossbow kinetic energy?
Kinetic energy in foot-pounds equals bolt weight in grains, times speed in feet per second squared, divided by 450,240.
Does a heavier bolt give more kinetic energy?
From the same crossbow, a heavier bolt leaves slower but usually carries slightly more energy and noticeably more momentum, because the crossbow transfers its stored energy more efficiently into a heavier projectile.
Should I use the rated or measured speed?
Use a speed measured with your own bolt if you can. Ratings are best-case figures, usually with a specific bolt weight, so a heavier bolt will be slower than the box number.