Foot-pounds of energy (FPE) is how airgun and archery equipment is usually rated. Many countries set legal FPE limits for airguns, and hunters use FPE to match equipment to game.
How to use the FPE Calculator – Foot-Pounds of Energy
- Velocity
- Weight
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Enter the velocity and the projectile weight in grains. The constant 450,240 combines the grains-to-pounds conversion, standard gravity and the ½ in the kinetic energy formula.
Inputs explained
| Field | What to enter |
|---|---|
| Velocity | Units: ft/s, m/s, mph, km/h |
| Weight | Units: gr, g, kg, oz |
What each result means
- Foot-pounds of energy = [Weight]×[Velocity]^2÷2
- Momentum = [Weight]×[Velocity]
Example values
How the main result changes when Velocity changes and everything else stays at the example values above:
| Velocity (ft/s) | Foot-pounds of energy (ft·lbf) |
|---|---|
| 450 | 6.429 |
| 675 | 14.465 |
| 900 | 25.715 |
| 1,125 | 40.18 |
| 1,350 | 57.859 |
| 1,800 | 102.86 |
Formula
FPE = weight (gr) × velocity² (ft/s) / 450,240
Worked example
Inputs — Velocity: 900 ft/s; Weight: 14.3 gr.
Results — Foot-pounds of energy = 25.715 ft·lbf; Momentum = 0.25419 kg·m/s.
Key terms
- Foot-pound
- Energy to raise one pound by one foot.
Frequently asked questions
What is the UK legal limit for air rifles without a certificate?
12 ft·lbf (6 ft·lbf for air pistols).
How do I convert FPE to joules?
Multiply by 1.3558.
Why 450,240?
It equals 2 × 32.16 × 7,000 (an older rounded g) – gravity times grains per pound times two.
Does a heavier pellet give more FPE?
Often slightly, at a lower velocity.