Muzzle Energy Calculator (FPE)
Get foot-pounds of energy from the two numbers on every ammo box. Enter bullet weight in grains and muzzle velocity in feet per second — the tool returns FPE from the standard formula (weight × velocity² ÷ 450,240), plus joules and power factor.
Example: with Bullet weight (grains) 115 · Muzzle velocity (fps) 1180 → Muzzle energy: 355.6 ft-lb.
- In joules482 J
- Power factor (gr × fps ÷ 1,000)135.7 — makes IPSC minor (125), under major (165)
Computed by the calculator below using its default values. Change any input to see your own numbers.
FPE = weight (gr) × velocity² (fps) ÷ 450,240. The constant is 7,000 gr/lb × 2 × 32.16 ft/s² — the classic ½mv² with ballistics-convention gravity baked in.
Where 450,240 comes from
Muzzle energy is plain kinetic energy, KE = ½mv², dressed in shooters' units. Bullet weight comes in grains (7,000 to the pound) and velocity in feet per second, so to get foot-pounds you divide the weight by 7,000 to reach pounds, then by 2 × 32.16 — twice the traditional ballistics value of gravitational acceleration — to convert pound-weight into mass units (slugs) and apply the ½. Multiply those divisors out: 7,000 × 64.32 = 450,240. One formula, no unit juggling: FPE = gr × fps² ÷ 450,240.
Using today's standard g = 32.174 ft/s² would give 450,436 — about 0.04% different, far less than shot-to-shot velocity spread. The 450,240 convention is what ammo makers' published tables use.
What the number is good for — and what it isn't
Because velocity is squared, speed dominates: a 55-grain .223 at 3,240 fps carries 3.5 times the energy of a 230-grain .45 ACP at 850 fps despite weighing a quarter as much. FPE is the standard yardstick for comparing loads, meeting state hunting minimums (many states require ~1,000 ft-lb for deer rifles), and checking airgun regulations. It is not a complete measure of effectiveness — bullet construction, expansion, and shot placement matter more, and momentum (which favors heavy bullets) tells a different part of the story. Treat FPE as one clean, comparable number, not a verdict.
How it’s calculated
FPE = w × v² / 450,240, with w in grains and v in fps. The constant is 7,000 grains per pound × 2 × 32.16 ft/s² (the long-standing SAAMI-era ballistics convention for g), equivalent to KE = ½mv² with mass in slugs. Joules at 1 ft-lb = 1.3558179483 J. Power factor = grains × fps ÷ 1,000, with IPSC floors of 125 (minor) and 165 (major).
Energy at the muzzle only — bullets shed velocity downrange, so energy on target is lower and depends on ballistic coefficient and distance.
Muzzle energy of common loads
| Cartridge | Load | Muzzle energy |
|---|---|---|
| .22 LR | 40 gr @ 1,200 fps | 128 ft-lb |
| 9mm Luger | 115 gr @ 1,180 fps | 356 ft-lb |
| .45 ACP | 230 gr @ 850 fps | 369 ft-lb |
| .223 Rem | 55 gr @ 3,240 fps | 1,282 ft-lb |
| .308 Win | 150 gr @ 2,820 fps | 2,649 ft-lb |
| 12 ga slug | 437 gr (1 oz) @ 1,600 fps | 2,485 ft-lb |
Computed with FPE = gr × fps² ÷ 450,240 from typical factory-load specs; individual loads vary.
Common mistakes
- Entering bullet weight in grams instead of grains — 1 gram is 15.43 grains, so the error is 15× in the result.
- Using the whole cartridge weight; only the projectile counts.
- Comparing loads by muzzle energy at different ranges — energy falls with distance, so compare at the muzzle or at matched yardage.
- Reading small FPE differences as meaningful: shot-to-shot velocity spread of ±20 fps moves a 9mm's energy by about ±12 ft-lb.
Frequently asked questions
What is the muzzle energy formula?
FPE = bullet weight in grains × velocity in fps, squared, divided by 450,240. A 115 gr bullet at 1,180 fps: 115 × 1,180² ÷ 450,240 ≈ 356 ft-lb.
Why divide by 450,240?
It bundles the unit conversions of ½mv²: 7,000 grains per pound times 2 × 32.16 ft/s² (the traditional ballistics figure for gravity, used to convert pounds-weight to slugs of mass). 7,000 × 64.32 = 450,240.
Does a heavier bullet always mean more energy?
No — velocity is squared, weight is not. Heavy bullets usually leave slower, so a light, fast load often out-energies a heavy, slow one. Heavy bullets win on momentum and penetration instead; this is the classic 9mm-vs-.45 confusion.
How many foot-pounds do I need for deer?
A common guideline — and a legal minimum in some states — is about 1,000 ft-lb remaining at the distance you shoot deer-size game with a rifle. Check your state's regulations, since several set explicit energy or caliber floors.
What is power factor?
Grains × fps ÷ 1,000, the momentum-based score competitive shooting uses. IPSC/USPSA floors are 125 for minor and 165 for major scoring; this tool reports yours alongside the energy.