Airgun Ammo · Explainer
FPS vs FPE: how airgun power is really measured
Researched by James · Updated September 26, 2026 · Editorial policy
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The FPE numbers on this page are computed with the AAFTA formula (grains × fps² ÷ 450,436), the same one behind the site's FPE calculator, not measured on a chronograph for this specific page. Full methodology & editorial policy →
Why FPS alone can lie to you
Velocity is only half the energy equation; weight is the other half, and manufacturers know a big FPS number sells rifles. The fastest FPS figures on any spec sheet almost always come from the lightest pellet the gun will shoot — often a thin alloy pellet built to post a number, not to hunt with. Swap in a normal lead hunting pellet and the same rifle's real velocity drops noticeably, because it's now moving more mass with the same air charge, even though the rifle itself hasn't changed at all.
This isn't a new trick, either. Spring guns and gas-piston rifles have advertised eye-catching "magnum" FPS figures for decades, almost always using the lightest alloy pellet on the market, and PCP marketing does the same thing with fill pressure and shot count. The fix is the same in every case: ignore the headline number and calculate the energy of the pellet you'll actually load.
The formula that tells the truth
| Variable | What it is |
|---|---|
| Grains | Projectile weight (7,000 grains = 1 pound) |
| FPS | Velocity, squared in the formula |
| 450,436 | 2 × standard gravity (32.174 ft/s²) × 7,000 grains/lb — the AAFTA-specified divisor |
FPE = weight (grains) × velocity (fps)² ÷ 450,436. Run any load through the site's FPE calculator, or do the math by hand — it's the same three numbers every time.
Worked example: the marketing trap
Take a rifle advertised at a flashy 1,400 fps using a 5.25-grain alloy pellet: 5.25 × 1,400² ÷ 450,436 ≈ 22.8 FPE. Now take a plain 14.3-grain lead pellet at a far less exciting 950 fps: 14.3 × 950² ÷ 450,436 ≈ 28.7 FPE. The "slower" rifle actually delivers more real power. This is exactly why a big FPS number on a box tells you almost nothing about how hard a rifle actually hits.
| Load | Velocity | Weight | Energy (computed) |
|---|---|---|---|
| Alloy pellet, "magnum" marketing number | 1,400 fps | 5.25 gr | 22.8 FPE |
| Plain lead pellet, modest number | 950 fps | 14.3 gr | 28.7 FPE |
| .177 lead pellet, "high-power" claim | 1,250 fps | 7.33 gr | 25.4 FPE |
| .22 lead pellet, ordinary claim | 800 fps | 18.13 gr | 25.8 FPE |
| My .22 hunting load (ACE FRAG slug) | 950 fps | 40.1 gr | 80.3 FPE |
All computed: FPE = grains × fps² ÷ 450,436. My own 40.1gr slug load is the heaviest here by a wide margin, which is exactly why it clears small-game energy needs several times over — see the full setup.
What number should you actually chase?
Not the biggest FPS on a spec sheet — the FPE that clears your quarry's energy floor, delivered at the animal, with some cushion for distance. Chasing raw power past that point buys range and wind resistance, not a deader animal, and it usually costs noise, air efficiency, and shot count. See the full quarry-by-quarry targets in the airgun FPE chart, and remember that .22 and larger calibers generally trade some FPS for more FPE at a given power level — the tradeoff explained in .177 vs .22.
How to get your rifle's real number
- Chronograph your actual hunting pellet, not the lightest one in the drawer.
- Use the average of a 5-10 shot string, not a single reading, since velocity varies shot to shot.
- Run weight and velocity through the formula (or the FPE calculator) to get your real FPE.
- Compare that to your quarry's energy floor in the FPE chart, not to another rifle's advertised FPS.
FAQ
What's the difference between FPS and FPE?
FPS (feet per second) measures how fast a projectile is moving. FPE (foot-pounds of energy) measures how much energy it's carrying, which depends on both velocity and weight. A rifle can advertise an impressive FPS number using an ultra-light pellet while actually delivering less real hunting power than a slower rifle shooting a heavier one.
How do you convert FPS to FPE?
FPE = projectile weight in grains × velocity in fps², divided by 450,436, the divisor specified in the official AAFTA Field Target Rules. Example: a 40.1-grain slug at 950 fps works out to 40.1 × 950² ÷ 450,436 ≈ 80.3 FPE.
Why do manufacturers advertise FPS instead of FPE?
A big FPS number is easy to market and easy to compare at a glance, and it's usually measured with the lightest, thinnest alloy pellet the gun can shoot, which posts the highest velocity but not the most energy. FPE requires knowing the pellet's weight too, which is why it's a more honest but less commonly advertised number.
Can a high-FPS air rifle have less power than a slower one?
Yes, easily. A 5.25-grain alloy pellet at an advertised 1,400 fps carries about 22.8 FPE, while a plain 14.3-grain lead pellet at a modest 950 fps carries about 28.7 FPE — more energy at a much lower headline speed. FPS alone can't tell you which rifle actually hits harder.
Is a higher FPE always better for hunting?
No. Past the energy your quarry actually needs, delivered at the animal, extra FPE mostly buys range and wind resistance, not deader animals. Matching energy to quarry matters more than chasing a bigger number; see the FPE chart by quarry.
Should I trust the FPS number printed on the box?
Only as a rough starting point. Box numbers are usually measured with a specific lightweight pellet under lab conditions, not the pellet you'll actually hunt with. Chronograph your own hunting load and calculate its real FPE rather than relying on the printed spec.
What FPE do I need for small game?
Roughly 9-12+ FPE delivered at the animal for rabbits and squirrels, which in practice usually means a rifle chronographing about 15+ FPE at the muzzle to leave cushion for field distances. Full quarry-by-quarry numbers are in the FPE chart.
Does caliber affect the FPS-vs-FPE gap?
Yes. Because .22 and larger pellets are heavier than .177 pellets, they generally run slower for a similar FPE, which is exactly why comparing calibers by FPS alone is misleading. See .177 vs .22 for the caliber-specific numbers.