Foot-pounds of energy, usually written as FPE or ft-lbs, is the single number that actually tells you how much punch a pellet is carrying when it leaves the muzzle. It's easy to fixate on velocity alone, a box that advertises 1,200 FPS sounds more impressive than one advertising 800 FPS, but velocity by itself is misleading. A very light pellet can post a big FPS number while carrying less real energy than a heavier pellet moving slower. FPE combines weight and velocity into one figure, which is why it's the number hunters, pest control shooters, and regulators actually care about.
If you already have a chronograph reading and just want the number, our FPE calculator does the math instantly. This guide is for the part before that, understanding what the number actually means and how to think about it.
FPE measures kinetic energy, the same physical quantity a moving car or a thrown baseball carries, just scaled down to pellet-sized numbers. The formula is FPE = (grains × fps²) ÷ 450,240. Because velocity is squared in that formula, small changes in speed affect energy more than the same-sized change in weight does. That's part of why two pellets with very different weights can end up carrying surprisingly similar energy if their velocities differ enough.
This is also why relying on FPS alone to judge a rifle's power is a mistake. A rifle pushing a very light pellet extremely fast might look impressive on a box label, but a heavier pellet moving slower through the same rifle can carry more actual energy, and typically retains it better over distance too.
The same FPE number means the same amount of energy no matter what powers the rifle, but how consistently a platform delivers that number shot after shot varies quite a bit.
Each shot compresses a fresh spring or gas ram, so FPE tends to be very consistent shot to shot once a rifle is broken in, but that consistency comes with recoil and a distinctive firing behavior that takes practice to shoot accurately.
PCPs draw from a compressed air reservoir, giving recoilless shots and typically higher, more adjustable FPE. The trade-off is that FPE drifts slightly as reservoir pressure drops over a shot string, which is why PCP shooters track their gun's usable shot count at a given power level.
CO2 platforms are simple and low-recoil, but FPE swings with ambient temperature since CO2's pressure output changes with heat and cold. A CO2 gun's FPE on a hot day can differ noticeably from the same gun on a cold one.
The most common mistake is treating FPE as a straightforward "bigger is better" scoreboard. It isn't. A pellet gun set well above what a task actually calls for often costs you accuracy, pellet stability, and noise, without any real benefit. Matching FPE to the actual job, small pest, squirrel, larger game, is what determines a clean, humane, effective result, not maximizing the number for its own sake. Our FPE calculator includes a general reference chart for typical ranges by use if you want a starting point.
It's also worth knowing FPE and joules measure the same thing in different units, not two different properties. If you see a rifle's power quoted in joules instead of foot-pounds, that's a unit conversion, not a different specification. One foot-pound equals about 1.356 joules.
A number of countries and states regulate airguns specifically by their FPE or joule output rather than by caliber or appearance. The UK is the most commonly cited example, setting 12 FPE as the threshold above which a rifle legally requires a firearms certificate. Other countries set their own limits, and in the US, airgun regulation is largely left to individual states and even municipalities, so there's no single national FPE limit to point to.
Because these rules change over time and vary so much by jurisdiction, treat any specific number you read, including the UK figure above, as a starting point for your own research rather than current legal advice. Always check your local and national regulations directly before assuming a given FPE level is legal where you are.
No. More energy than a task requires usually costs accuracy and adds noise without real benefit. Match FPE to the job, not the other way around.
They don't measure different things, they're the same energy in different units. Multiply foot-pounds by about 1.356 to get joules.
It depends entirely on your country, and in the US, often your specific state. There's no universal number, always check current local regulations directly.