Understanding FPS and Joules
"What FPS does your gun shoot?" is the most common airsoft question — yet safety rules are written in energy (joules), not FPS. That's because FPS is relative: it changes with BB weight, while energy doesn't.
The energy formula
Kinetic energy is E = ½ × m × v², with m in kg and v in m/s.
A 0.20g BB at 100 m/s (328 fps): E = ½ × 0.0002 × 100² = 1.00 J.
FPS ↔ joule table (0.20g)
| FPS (0.20g) | m/s | Energy |
|---|---|---|
| 250 fps | 76.2 | 0.58 J |
| 280 fps | 85.3 | 0.73 J |
| 300 fps | 91.4 | 0.84 J |
| 328 fps | 100.0 | 1.00 J |
| 350 fps | 106.7 | 1.14 J |
| 400 fps | 121.9 | 1.49 J |
| 450 fps | 137.2 | 1.88 J |
| 500 fps | 152.4 | 2.32 J |
Why does FPS change with BB weight?
Your gun delivers (nearly) constant energy to the BB. At equal energy, velocity
follows v₂ = v₁ × √(m₁/m₂). A gun shooting 400 fps on 0.20g shoots:
- 0.25g → ~358 fps (still 1.49 J)
- 0.28g → ~338 fps (still 1.49 J)
- 0.30g → ~327 fps (still 1.49 J)
The key point: lower FPS, same energy. "Heavy BBs make your gun weaker" is a myth — heavier BBs actually retain more energy at impact.
Check rules in joules
Most fields set energy limits per role (e.g. ~1.14–1.5 J for rifles, ~2.3 J for bolt-action snipers), and chrono checks are usually done with 0.20g BBs. To find your equivalent FPS on any other weight, use the ballistics calculator.